Why do hot water tanks have such a bad rep?

I recently came across a thread here talking about tankless hot water heaters, which I think are great for certain circumstances. I was surprised by how many people had a negative opinion of hot water tanks.

So... in defence of the tank, I would like to share some things that people may not be aware of, or may not have thought about. Also, I am not defending your standard 30-50 gallon electric tank with hardly any insulation. It works, but inefficiently. Kind of like a normal fireplace.

When you need a lot of hot water (large family, large gathering, heating a house, etc), a large, well insulated tank can be much more efficient and use less fuel than a tankless. I am speaking of large tanks in the multiple hundred gallon range that are super insulated. These sorts of tanks can keep water hot for multiple days without running the heater. One test by American Solar Technic 180gallon tank, showed that by starting with 180F water (which is common for boilers), and using a configured ant-scald valve, they were able to get 224 gallons of steady 110F water. And at the end of that, the water in the tank was still close to 110F.

In order for most boilers to work efficiently, they need to run at full capacity for a period of time. When an oil or propane boiler needs to turn on and off frequently, it is less efficient. When you need to do this with a wood fired furnace it's almost worse, because wood fires don't like to start and stop. This leads to heat being wasted. Solar hot water heaters are the same, in that they need a place to dump their heat, and don't work well starting and stopping.

You need a large "battery" to store the heat you are making so efficiently. A heat battery. Much like the mass in a Rocket Mass Heater, we can use the tank to capture heat for later. By using the large tank, you can run whatever heat source you want, at full efficiency, until the tank is heated, and then shut it off for a while. The unpressurized tanks that I am a fan of, just use heat exchangers for inputs and outputs, and allow different sources and destinations.

I can't find the video now, but one family I saw, had installed a large tank, I believe it was 300 gallon. They heated the tank with a wood fired boiler and were able to heat their house in Vermont for 2 days, plus have hot water for showers before they had to heat it again. I would argue that these sorts of hot water storage tanks are to the rocket mass heater, what those 50 gallon electric hot water heaters are to a fireplace.

At the moment our 'water heater' is a kettle on our gas stove since the on demand limed up too badly to use and we weren't able to descale it enough...and we could never decide on the right water heater for us so I'm interested in this discussion.

One of the downsides, that I hear, to a tank water heater is the energy used to hold it at a temperature that does not grow bacteria.  That temp is 135F according to this article https://www.hotwatersafety.org/blogs/turning-a-water-heater-up-to-kill-bacteria

Water heater tank temperatures in the neighborhood of 120 degrees create a supportive environment for the presence of Legionella and other bacteria. The temperature setting on a water heater thermostat actually only represents a maximum temperature when the tank is full and completely heated. Temperatures in the tank often fall below the indicated maximum as hot water is used and cold water circulates into the heater. It is at these lower temperature ranges of 95 degrees to 115 degrees Fahrenheit that Legionella bacteria multiply and become established inside a water heater. A study by the American Society of Sanitary Engineering showed that the bacteria cease to multiply when water temperature reaches 120 degrees. They are not destroyed at that temperature, however, and can survive to be conveyed to an outlet such as a shower head or faucet.  

 The American Society of Sanitary Engineering recommends setting the temperature of home water heaters to 135 degrees to 140 degrees Fahrenheit, a range shown to destroy bacteria such as Legionella. At those temperatures, bacteria can neither thrive or survive to contaminate fixtures downstream from the heater. Adjusting a water heater to a higher temperature must always be accompanied by the installation of anti-scald devices in the home by a licensed plumber to prevent potential burn injuries, if such safeguards are not already present.

Valid points are made here, and valid encouragement for us all to super insulate whatever heaters ate used at home or business.  I feel that hot water tanks are maligned, rightly or wrongly, because of our collective wastefulness in the USA. For one example, my military upbringing and general desire to save the planet has me still taking "Navy showers", that is, 3 minutes max with water turned off while lathering. My dad got me on this kick the day my diapers came off.  But my dear long suffering wife and teenager like luxuriant longer showers.  Different strokes , etc. Everyone has their own style, thank God.  

A perusal online shows an average household in 'Merica...about 4 humans....uses around 64 gallons per day of  heated water. Here in sunny Georgia we keep the tank at about 120 F, a bit lower than average.  It is a basic name brand heater, insulated but soon to be super insulated.  Thanks for the nudge my friend.  It is 16 years old so up for replacement, great time to try a more comprehensive plan for incorporating DIY solar heating.    Also on demand water heating if I can diy a solution.

Many folks sing the praises of tankless,and I hope they sing loudly in this case.  In the meantime, I see it's time to look in the mirror.  Although with a face like mine that could be a fright.  My two drops,
Rico

This was an interesting topic for me since I have only ever had hot water tanks in my houses, albeit not large super insulated ones as Matt suggests. I suspect that this is something that people who take showers rather than baths need to be a bit more aware of, since it is inhaling the infected water seems to be the greatest risk.

Judith Browning wrote:One of the downsides, that I hear, to a tank water heater is the energy used to hold it at a temperature that does not grow bacteria.

There are ways around that apparently. If you want a read, see how the UK gets around this: HSE technical guidance on hot water systems. It depends a lot of how the system is plumbed, and I don't know what is normal in the US. The temperature of incoming mains water is a factor, as well as whether the water is recirculated.
Legionella isn't a big issue in the UK in domestic settings, usually only in more complicated municipal buildings I believe.

For the house I'm going to build, I only took a quick look into "best water heaters."  During that quick look, I did see more plumbers recommending tankless over tanks.  But, that's as far as I got.

I just want what works best for me, I don't care if that means a chimpanzee to boil the water for me.  I have no skin in the game one way or the other.  I'll be on well water but, depending on the water quality, I may go with a whole house water filtration system after the initial sediment filter.  I also figure a whole house water filtration system would be easier on the plumbing, washing machine, and water heater.  If I'm wrong, please correct me!  

My requirements are (at least right now):

- Propane, because I want to function fully off-grid as well as on the grid when needed.
- Non-smart (invasive) technology.
- Reliable.
- Durable.
- Reputable.

And chimps aren't as cheap as you think, so I need to settle this matter before I start building...

Matt McSpadden wrote:I recently came across a thread here talking about tankless hot water heaters, which I think are great for certain circumstances. I was surprised by how many people had a negative opinion of hot water tanks.

So... in defence of the tank, I would like to share some things that people may not be aware of, or may not have thought about. Also, I am not defending your standard 30-50 gallon electric tank with hardly any insulation. It works, but inefficiently. Kind of like a normal fireplace.

When you need a lot of hot water (large family, large gathering, heating a house, etc), a large, well insulated tank can be much more efficient and use less fuel than a tankless. I am speaking of large tanks in the multiple hundred gallon range that are super insulated. These sorts of tanks can keep water hot for multiple days without running the heater. One test by American Solar Technic 180gallon tank, showed that by starting with 180F water (which is common for boilers), and using a configured ant-scald valve, they were able to get 224 gallons of steady 110F water. And at the end of that, the water in the tank was still close to 110F.

In order for most boilers to work efficiently, they need to run at full capacity for a period of time. When an oil or propane boiler needs to turn on and off frequently, it is less efficient. When you need to do this with a wood fired furnace it's almost worse, because wood fires don't like to start and stop. This leads to heat being wasted. Solar hot water heaters are the same, in that they need a place to dump their heat, and don't work well starting and stopping.

You need a large "battery" to store the heat you are making so efficiently. A heat battery. Much like the mass in a Rocket Mass Heater, we can use the tank to capture heat for later. By using the large tank, you can run whatever heat source you want, at full efficiency, until the tank is heated, and then shut it off for a while. The unpressurized tanks that I am a fan of, just use heat exchangers for inputs and outputs, and allow different sources and destinations.

I can't find the video now, but one family I saw, had installed a large tank, I believe it was 300 gallon. They heated the tank with a wood fired boiler and were able to heat their house in Vermont for 2 days, plus have hot water for showers before they had to heat it again. I would argue that these sorts of hot water storage tanks are to the rocket mass heater, what those 50 gallon electric hot water heaters are to a fireplace.

While those are fair points, how many people have one in their house?  I know exactly zero people that have a huge boiler-type water heater in their home, so if you want to compare tank vs tankless, I think it's fair to use the type of tank people normally have in their home.  I believe that is why people prefer tankless to tank.

Judith Browning wrote:One of the downsides, that I hear, to a tank water heater is the energy used to hold it at a temperature that does not grow bacteria.  That temp is 135F according to this article...

Keeping the heat in, is simply a matter of insulation, like a house. It doesn't matter how efficient the heater is, if you don't have insulation, then you will lose heat quickly and need to use a lot of energy to keep it hot.

The article was interesting. It seems that legionella is the one everyone talks about. While I believe that it can survive at 120F... I also don't think it is too big of a risk. Keeping the water at 120F has been standard for a lot of years... and if there were huge outbreaks, I suspect they would have changed this recommendation long ago. I suspect it is a legitimate, but minimal concern.

Trace Oswald wrote:While those are fair points, how many people have one in their house?  I know exactly zero people that have a huge boiler-type water heater in their home, so if you want to compare tank vs tankless, I think it's fair to use the type of tank people normally have in their home.  I believe that is why people prefer tankless to tank.

A fair point. But I wanted to let people know that there are other options in the "hot water tank" category. Sort of like someone who is comparing an old wood stove to a modern efficient wood stove. They are both in the wood burning heater category... but wouldn't it be nice to know that there is something even better known as a Rocket Mass Heater? In this case, while it is not widely adopted, there is a much better option for hot water storage. It takes up space, and people who install plumbing are often not familiar with it.


***Edit - Also, boilers are very common here. Anyone who heats their house with hot water (baseboard, radiators, radiant, etc) most likely has a boiler of some kind. Most are simply connected to a small tank, which means the boiler has to kick on and off much more frequently, losing efficiency.

One advantage here to having a well-insulated quite-large (only 101 gal for us) tank heated by grid-electricity is that you can put it on a separate meter that the coop can turn off during peak-use times and get a 50% discount on any power it does draw.

Matt McSpadden wrote:Anyone who heats their house with hot water (baseboard, radiators, radiant, etc) most likely has a boiler of some kind. Most are simply connected to a small tank, which means the boiler has to kick on and off much more frequently, losing efficiency.

My 1972 built apartment complex has baseboard heat and a huge boiler.  We have three problems with this system:

1.  Heat goes out periodically due to air in the pipes.  It requires going into an apartment and bleeding the air out of the pipe, then going down to the boiler and adding more water.

2.  Freezing pipes that burst and flood the apartment, ruining the carpeting (and tack strips and even damaging the older concrete subfloor) or cheap plank flooring (in some of the "renovated" apartments).  If it's not discovered immediately because it's only in a small area, then we're dealing with mold issues.

3.  Having to check the water temperature daily.  (This could just be specific to this boiler though.)

Disclaimer:  Like I said, I have no skin in the game.  I'm just mentioning my experiences and small amount of research.  I'm also saying that I would never have baseboard heat in my house.  Ever.

EDITED TO ADD:  I should mention that, in my new house build, I'm having a full masonry heater built to heat it.  (That's also why I'm leaning towards tankless for my hot water needs - laundry, shower, doing dishes (no dishwasher).

We briefly considered a tankless water heater in our last house, but the installer (who did both) encouraged us to go with a tank. His argument was that the tankless ones need more (annual) maintenance, and missing it voids the warranty. You also need to make sure your water is soft, or a tankless heater will scale up and die much more quickly than a tank heater. I learned later that the water in that area was hard on heaters in general.

I've always had tank water heaters (both growing up and as an adult) and we've never had issues with bacteria; this is the first time I've ever heard that could be a concern. I don't know what they were set at, but they weren't super hot.

Being as this is a rather contextual topic and it's in a general gear section, I'll add my penny in.

Single person, no electric and living tiny (seperate bath house is where the only "plumbing" is and it has drainage valves/hoses hooked to freeze proof hydrant) coupled with work that takes me off farm for occasional 3 day outings, led me to getting a tankless on-demand propane setup that uses 2 D cell batteries to ignite.

I'm sure I could tinker with it to light with a match if need be.

When it's below 20F for 2 weeks straight, sometimes I only use a gallon or so of water a day to live - and cold is fine, until Im ready for a shower, then I do the dishes at the same time. It instantly makes steamy water for 20 min straight to fill up the tub for baths or laundry if needed as well.

Seems pretty good to me for a couple hundred bucks as it really doesnt use much propane at all, look up the efficiency specs but it takes over a month for me to empty a 20lb tank at my useage. Of course Im not doing ALL my laundry (especially in winter since I cant hang out to dry).

My other option is a pot on the wood burner or propane cooktop in the house, or use another propane heater I have, placed under the tub while filling it up (which is not very efficient but does work in a pinch...which has led me to wanting to modidy the whole setup to accept a woodburner under the tub for options.

Thank goodness for running water at least :) with this setup I dont waste much water since in the winter I still need everything quickly useable but also drainable to prevent freeze.

Also, no idle energy consumption or worry about any of it, the water stays down in the ground in a pipe until I need it. I've created some little ponds to bucket water if ever need be, but the heater does require some pressure to operate (I think 20psi).

Other types of tankless water heaters run as low as 5psi which would be nice.

Did I miss mention in this thread of heat-pump based tank water heaters?  I've been considering this for our next water heater as our electric coop is promoting them for energy savings.  Is the efficiency much better than typical resistance-rod based water heating?  Additional consideration now is that we switched from fuel oil to propane for home heating--- If we go tankless, it seems like gas tankless water heaters may be a valid choice...?

Also, with respect to shortened life-span of tankless due to scale build-up and forgotten maintenance:  What is the experience of those in countries where point-of-use water heating is the norm?  Are they more failure prone or do those locations build cleaner water into the whole system?  I recall seeing many gas-based systems in Germany and my wife recalls electric versions in Brazil.  Thoughts and opinions?

We have dishes and 7 bodies to rinse. We have a 6 gallon electric/propane hot water heater (RV size).

It is off most of the day. We turn the electric element on before dinner, do the lunch dishes and what we have so far from dinner prep. After dinner, we turn the propane on and everyone showers. Once it heats back up from the showers, we turn it off and use the hot tank full to do the remaining dinner dishes.

In the summer months, it is only used for dishes as the groundwater temp feels very nice for showering then. It is on for around 2-3 hours a day (cold season) and we use 1 or 2 20# tanks of propane per year. I am not sure what the electric element heating amounts too. This is a very inexpensive solution that meets our needs.

In my experience I have seen a direct relationship to "user input" and "resource conservation". Resources being financial and material. The more user input you are willing to take part in, the more resources you will conserve. I have to flip 2 switches twice a day = User input. I save electricity and money and water = Resources.

Judith's situation of a kettle on the stove is probably one of the best examples.

I personally know many plumbers who would be thrilled to sell you a very expensive solution to a "problem" that can be "solved" with your existing tank heater by insulation and flipping a breaker twice a day.

However, if I were starting from scratch, I would locate all my hot water needs in one place and buy a $75 wall mount tankless propane heater that ignites with batteries. Or I would do what we are doing now.

John Weiland wrote:Did I miss mention in this thread of heat-pump based tank water heaters?  I've been considering this for our next water heater as our electric coop is promoting them for energy savings.  Is the efficiency much better than typical resistance-rod based water heating?

I don't think anyone has brought it up. Just keep in mind that most commercial solutions sell the heating element as well as the tank combined. They can be separated and you can have a lot more flexibility on what fuel you use to heat the water.

We have a heat pump hot water heater than can be just heat pump, hybrid, or electric. They are more efficient, but only in certain circumstances. The colder it is, the hard it has to work to heat the water (e.g. the more electricity it uses). In our case, ours is in an unheated basement with a fieldstone foundation connected to a crawl space that is practically connected to the outside. Even in hybrid mode it does not keep up well most of the year. We spoke to tech support about it not keeping up, and they informed us that if the air temp was over 60F, then the heat pump alone could recover 6 gallons of water an hour. If you have low hot water needs and a warm place to put it... it really will sip energy. Ours does not, because the only place we could put it... is too cold and wet for it to work efficiently.

Matt McSpadden wrote:...... If you have low hot water needs and a warm place to put it... it really will sip energy. Ours does not, because the only place we could put it... is too cold and wet for it to work efficiently.

Thanks for the comments and experiences, Matt.  I think we have the same issue.  The basement where the hot water heater sits is really cold and drafty to the Minnesota winter air, so it sounds like a heat pump might not be the best solution.   I'm at a stalement with my wife on the possibility of reducing the size of a new tank heater and placing it upstairs in the kitchen, effectively taking up lower cabinet space near the kitchen sink.  Cons of the idea are loss of pot and pan storage space, but the pros are many:  warmer location, reduced pipe distance from hot water production to all destinations, easier install if PEX tubing used over older copper.  Alternatively, if we route a propane line to the back utility room for a tankless, that too would be closer to destination locations for hot water use.  Decisions......decisions......

We use a tankless Toyotomi water heater. Should I want I can add a tank, pumps and manifolds for hydronic under floor heating. Have never run out of hot water.
https://toyotomiusa.com/product/om-122dw-toyotomi-semi-on-demand-water-heater/

I use a gas water heater at my house.
Our water heating needs are lowered, since we wash our clothes in cold water only.
Most of of our dishes go in a dishwasher that uses its own coil to bring water up to temperature.


Setting aside the means of heating the water for a moment, I think a large tank of water could be an excellent thermal mass for any home.
In situations where the home is already built a tank like this, often referred to as a buffer tank, allows the mass to be located in one place but moved to another as needed.
That allows heating the mass(water) in an outdoor or low access location  (roof, stand alone green house or basement), then moving it to storage in the buffer tank, to be distributed later.

New homes can be designed around the solid mass of a masonry heater, like a rocket stove, but a buffer tank could still be good as a pre-heater for domestic hot water.
If your space is warmed to a comfortable degree with efficient wood heat, but your water supply is at  a low ground temperature, a buffer tank could be an effective way to pre heat your domestic host water and also have a supply of water on hand for emergencies.
Even if you are using an electric, gas or oil fired water heater  with a tank, I think a buffer tank would be useful.

I'm not sure I follow the turning off and on being a problem and creating inefficiencies. It may create more wear and tear or need more servicing, but shouldn't create inneficiency unless it causes heat to go somewhere other than into the water. If we want to compare apples to apples, heating water to desired temperature at time of use will always be more efficient than 1. heating water to a higher temperature and using it as it cools, or 2. heating water to the desired temperature and holding it there for any amount of time. There's just no way around that. There are drawbacks to every type of system including tank, tankless, boiler, water stove, etc, but as far as energy efficiency goes, it's going to go to the one that heats water to desired temperature at moment of need.  

All discussion here informative and varied. It's great weigh the opinions and countervailing views on a crucial topic, so thanks to everyone.

I have two options roiling in my mind ....i should note its a small mind without much usable space....bout creating a DIY on demand water heater.  Has anyone given this a try, and if so, how is it performing?   Any thoughts or warnings will be graciously accepted

If I was going to build my own on demand hot water heater here would be my process and thoughts:

First, I'd set up a solar collector so that the water entering would need minimal heating. An on demand heating system with any type of hoped for efficiency is gonna need to run on propane or natural gas, as these build heat immediately and have no trouble being lit and unlit often. An insulated metal box would be ideal, maybe something along the size of a larger ammo can. The 50 caliber cans would be ideal in my opinion. It will need a vent pipe in the top as well. Build a copper coil to fit inside the box with the inlet closer to the top, coiling its way down toward the bottom where I would fit a burner, and the pipe would exit heading towards the intended use point. To automate the firing process, I would use a flow switch on the water line, probably a paddle type to actuate a gas control solenoid. This way, when water was demanded at the point of need, the flow would trigger the gas to begin to flow. If you are super handy and seeking maximum efficiency, you could ignite the propane each time it was needed with some type of electric arcing igniter, but a simple pilot light set up would use minimal gas and simplify the entire process. Of course I'd also include a pressure relief valve somewhere in the system, post burner, just in case. If you want to build your own burner, they are not too hard and can be done in most home garages pretty easily with readily available parts, but could be scavenged from things like grills and gas stoves, fish cookers, etc.

Just my thoughts on how to begin the process, if I need to elaborate on anything let me know and I'll be glad to.  

If you have source of cheap heat that isn't always available like PV solar, thermal solar, a tended woodfire or simple grid tied electricity in off peak hours, heating and storing water might be energy inefficient, but financially effective.

If it takes an expensive tankless water heater to duplicate the effective output of a much cheaper water heater with a tank, and the tankless water heater has a lower effective lifespan, then we are financially incentivized to choose what maybe the less energy efficient option.

I think  point of use  tankless  water heaters potentially  have advantages over whole house tankless water heaters, like price point, required infrastructure(110 vs 220), and ability to closely match the capabilities to the demands.

Ironically, some of the point of use water heaters have tiny tanks, so the hot water can be instantly available.

Rusty speaks the truth here, on demand is always the most efficient and cost saving method in theory.  It's also the best choice in most, not all, situations.  But like many of our discussions on Permies, the true path leads to:   it depends ....we all have to be pragmatic in our assessments and therefore in our practice.  

Speaking of pragmatic,  William has posted the most introspective and clever summary of the "tank warfare" that could ever be crafted.

My wife and I have seriously considered buying two 120 heaters ...bathroom and kitchen .... to replace our outdated single water heater using a 240 circuit.  Our hesitancy stems from the exact reason William describes.  Would we just be trading a 40 gallon tank for two smaller ones?  

Thomas Rubino has had a great experience with his selected brand, Ecotemp I think it is, but is our well water going to cause maintenance problems over time? I can faintly hear a familiar voice whisper in my ear:

It depends

It can be confusing, because efficiency depends on how it is used.

Just looking at stats, most on demand hot water heaters are about 80-85% efficient at converting fuel to hot water. Boilers are generally 90-95% efficient at converting fuel to hot water. (Electric hot water heaters are in the 90% efficient range.)

Boilers
By stats, the boiler wins... but they are only that efficient when burning at full capacity. When they are starting up and cooling down, they are WAY less efficient. So the more they start and stop the less bang for the buck you are getting. Having a small tank means the fire is starting and stopping a lot and reducing efficiency. Having little to no insulation on your tank means it has to start and stop more to keep it hot. If you are using a lot of water, with a large tank, with lots of insulation, then it does not need to fire up as much and can be very efficient.

On Demand Heaters
On demand heaters are less efficient when looking at the stats, but are designed to start and stop. When used for small water usage, they are actually better because they do not need to keep a tank hot. If you try to heat a house with one, or have a large family, it will be used more, and you will notice the lower efficiency. If you only need a little hot water, these are probably the best option, because heating 10 gallons at 80% efficiency is better than heating 500 gallons at 90%.

I have never had an `On Demand`water heater though I feel these are misunderstood and causing the Bad Rap.

I have owned a lot of hot water heaters over the years.

We only had to replace one in all those years.

Hot water heaters for me do what they were designed to do.  Heat water.

Get an electric anode. Only cost a few cents a year in electricity and it protects plumbing and faucets. I live in Ohio and have extremely hard water that I have to treat with several sediment filters and a water softener system.

Powered Anode Rod for Water Heater, 20-Year Warranty, Eliminates Rotten Egg/Sulfur Smell within 24 hours, Stops Corrosion and Reduces Limescale, Electrical Anode Rod Made of Titanium

I went through all the above musings because I am in the middle of the tank wars myself and find I am on all sides of all the issues. The praises of tankless here fit some situations. But certainly not mine. I am off grid, all solar. I chose years ago to shuck the propane tank for what I thought were multiple good reasons so solar electric is my preferred choice. Tankless is a massive electrical draw. Much too massive for my 600AH Lithium bank. I am wanting the HPWH but do not have a warm space for it, and to provide that would run the total costs well over 4 thousand.  Currently using a 20 gallon standard which only heats when the batts are satisfied, but it is nearing life end.  As mentioned above  a couple of under counter 7 gallon ones (Bosch) would cost a fraction of that. What few realize is that the HPWH saves on heating (only when in heat pump mode) by using the heat in the space they occupy. If you are also providing that heat you are saving little. Only when the earth for example provides that heat freely is the HPWH a real savings.

The heat pumps seem to do well at taking a lot of warmth and concentrating it.
A low mass space sum space(like hoop house) gets hot during the day,but freezing at night.
A heat pump in that space could eliminate the need to vent such a sun space during the day, cooling the space down to  while concentrating the heat into hot water for usage.

An air to water heat pump that could be added to any water water tank would be great.
There are cheap easily available heat pumps are water to air (the office water cooler, chest freezers) but I don't know of any air to water heat pumps that are equally unbiquitous.

Has anyone heard of or played with an electric tank with two elements, one is to AC element (grid or otherwise) and the other is to a DC element ( https://windandsolar.com/search/?q=water%20heater%20element )  you can connect direct to solar panels? I have an old existing electric water tank in my barn that I would love to let the solar do most of the heating to save money. I bought one with the built in thermostat but have not yet set it up.  I like the idea of direct DC as it cuts the batteries which I don't have. I've been wanting to do this for years but have never pulled the trigger.

Michael S. New wrote:What few realize is that the HPWH saves on heating (only when in heat pump mode) by using the heat in the space they occupy. If you are also providing that heat you are saving little. Only when the earth for example provides that heat freely is the HPWH a real savings.

The only way I'd consider implementing a heat pump hot water unit would be a split system with the evaporator outdoors. I guess there's also the option of putting the whole thing outside if you're in a mild climate.

Ed Lewis wrote:Has anyone heard of or played with an electric tank with two elements,

A typical 40 gallon residential electric water heater has two elements.  They are not used at the same time.  They actually employ a trick that permits more hot water to be drawn from the water heater during peak usage (usually as the family is taking showers in turn) before the drop in temperature becomes noticeable.  There are (typically) two thermostats inside the water heater at two different levels.  The lower heating element is controlled by the lower thermostat, which is low in the tank; and the upper element is controlled by another thermostat that is about half-way up the tank.  There is a switch that prevents the lower element from drawing power whenever the upper element is on, in order to prevent overloads; because each element is capable of the full rated power of the whole system.  Cold water enters the top of the tank, but is directed to the bottom via a tube; while hot water is drawn from the top of the tank.  This is done to prevent 'mixing' of the thermocline.  As hot water is drawn off from the top, the lower element turns on almost immediately, trying to heat the water that is being drawn into the bottom of the tank.  As the hot water demand continues, the thermocline rises until it hits the upper heating element, which turns on and continues to heat the cold water in turn.  The reason is that, as it takes time for people to take showers; most tend to out run the ongoing heating capacity of a typical residential water heater, but a 40 gallon unit can actually produce about 60 gallons of "hot" (about 110 F to the skin) water when set to 135 degrees, and drawn over about 60-90 minutes.  My personal problem with on demand type water heaters is that if they are large enough to supply the real & practical hot water needs of a family of 5 at peak demand; then they are large, very expensive & probably not cost effective over a lifetime of service.  Keep in mind, these on demand heaters are much more complex machines than a typical 40 gallon electric  water heater, and are much more likely to require service.  I've had my electric water heater for well over a decade without any service necessary; and it was installed when I bought the house.

Yes, electric heat is the most expensive form of heat energy; but can be very effective overall if done correctly.  There are tricks that can be added to an electric water heater to make things a bit more sensible.  For example; one of the great complaints with a standard electric water heater (versus an electric on demand water heater) is that the heat that is lost to the environment during the time periods that the unit doesn't have any demand is simply "lost heat".  Well, that's not really true when one lives in a predominately cold climate, as every heat source inside your insulated home envelope contributes to your heat demand.  But if the heat lost when you are asleep or at work bothers you, buy and install a power supply timer unit.  You could tell it when your family is most likely to be using water heat, and simply set the timer to switch on power and hour or two before your family would need it, and off an hour after the last shower; and your wife might never notice.

Another trick worth mentioning is installing a heating element at the bottom of the unit that is of a lower power rating than the original.  This will affect your ability to draw large amounts of hot water during a peak time, but if you live alone, who's to care?  Don't mix this one with the timer trick above, you would end up with cold showers.  As Ed already noted, this is how one can use an electric water heater as a diversion load for a solar array or wind turbine; by swapping the lower element with a DC element.  In this case, you'd want a high temp cutout switch in line with that DC element; in order to prevent your water from getting too hot.  You'd also want to add a water mixer to keep the tap temps from getting to dangerous levels for human skin.

And a final trick is to add a "jacket" to the electric water heater, to slow down passive heat loss.  This doesn't work as well for gas water heaters, because there's a chimney right down the middle; but it works just fine for electric tank water heaters.  I haven't checked in a while, but once upon a time I could just order a "jacket" for a  water heater from Amazon.

As Benedict mentioned, a major downside to tankless water heaters, in my eyes, is that they need annual maintenance. Mostly descaling, which isn't a big job, but should be done. Kinda like changing the oil in your car. I've actually suggested to some younger folks that it would be a good marketable service to offer. That and checking on folks septic tanks. Surprising how many folks don't think about home maintenance items often enough.
If your tankless was installed best practice, it should have bypass valves to make descaling easier. You also should be cleaning the burner.

Although it was in the seasonal camp, our 40 gallon electric water heater hit the end of its reliable life span after only 16 years.  
It still heated water, but leakage around the outlet pipe last spring was a concern. I considered changing out the fitting.
After closer inspection, it was obvious that the steel rust fitting would not depart without inflicting serious damage on the remainder of the tank.

I had previously purchased an on-demand water heater for a good price (found a retiring plumber with spare stock).
While it had really good operational reviews, more digging revealed that the longevity was rather limited on well water.
And it required three 50Amp, 240V breakers.  That's a 4 conductor #6 copper wire for each breaker.
It's 30+ feet from breaker panel to heater control, so at 5 bucks or more per foot locally, the wire costs more than the heater!

At the same time, Maine was sponsoring efficiency discounts for "better" water heaters.
Turned out I could pay $ 800 for a new electric WH with a 7 year warranty or $ 900 for a heat pump WH with a 12 or 15 year warranty.
That extra hundred bucks seemed like a darn good deal even with the minor changes needed for power and plumbing connections.
Stock was limited so I had to wait until the fall to acquire one.
I always drain the pipes for winter so the install will have to wait until the upcoming spring thaw.

An added benefit of the HPWH is that the unit is also a de-humidifier.
It will be located in the basement/enclosed crawlspace so the noise will be no problem and the removal of moisture a definite plus.
I had not considered super insulating it, but I guess I will now.  Thanks Matt!

Ed Lewis wrote:Has anyone heard of or played with an electric tank with two elements, one is to AC element (grid or otherwise) and the other is to a DC element ( https://windandsolar.com/search/?q=water%20heater%20element )  you can connect direct to solar panels? I have an old existing electric water tank in my barn that I would love to let the solar do most of the heating to save money. I bought one with the built in thermostat but have not yet set it up.  I like the idea of direct DC as it cuts the batteries which I don't have. I've been wanting to do this for years but have never pulled the trigger.

I'm in Europe and have found a consumer ready setup like you describe - unfortunately it wouldn't work for me in winter as my electric supply is totally solar and my battery bank wouldn't cope with the load.
Maybe youcan source it from your end?
https://termoselectricos.es/producto/aparici-100-litros-sol-hybrid-conectado-paneles-y-230vac/?utm_source=Google%20Shopping&utm_campaign=TermosShopping&utm_medium=cpc&utm_term=2404

You are talking my dream system so I can probably help with links etc.  The main difference is the goal at least starting out of having the tank as a preheater for the main water heater.

Here are the downsides on my list

1.  Space taken.  I am looking at mounting the preheat tank in the basement.  Which also means I need to be able to get it down a stairs and around a corner.  The house is designed to get a large chest freezer down but that is about all.  A box tank that big eats major space in the basement.

2.  Danger during a major tank failure.  If I have 300+ gallons of water sitting in that and it suddenly ruptures in for example an earth quake suddenly I could have 6 inches of boiling hot water to wade thru to escape the house.  If it is less than 140 degrees F probably survivable without horrible damage if I stepped fast but hotter still and I don't know if I can make it or not.  If the leak is slower the concrete of the floor will eat enough heat to moderate it so the only real risk is a tank rupture.  If doing a built in as the house was constructed would want a sub-basement within the basement capable of holding all the liquid in the tank if it ruptured with a bit of margin for error.

Now here is the video on the dream system.  Start at one hour and 9 minutes in and watch the next 45 minutes.  



Notice some things.  By having 5000 gallons in this he has 2 months of heat storage.  That would beat any run of gray days here I have had since watching close as the longest run was 17 dark gray days in a row with only about 6 hours of sun in that entire time.  By having the ability to augment it with wood heat it would get by that.  Notice a neat math fact.  every time you double the dimension of tank you increase the surface to lose heat thru by the square but the heat storage capacity by the cube.  Now I had been stuck on the word "tank" for years after seeing this because there was no way to retrofit that in my existing house.  

Then I got to this video and now suddenly I am looking at parts I can carry down the stairs and assemble.  Talking about a day and half of heat storage for a family of 4.  Betting with just me I could easily double that at hot water levels and probably double that again if I let showers etc get down to 70 or 75 degrees



The above channel as well as many others gets me combined information

The goal is to combine this with a spiral solar collector laid down flat with the tube climbing 1" per 20 feet of pipe from the outer edge to the middle.  Information suggests one 4 foot diameter spiral should almost produce enough heat to maintain the tank temperature steady most of the time.  In good times it would heat it for use some.  So thinking I need tubing to equal 2 4 foot diameter spirals for the base run.  Now flat double pane window collectors top out at roughly 140 degrees occasionally pushing towards 150.  So a 2nd collector doing evacuated tube system would eventually be added to heat the water on up giving potentially a 3 fold increase in heat storage capacity.  If the systems are built to function as drain back collectors there goes the complexities of the heat exchangers and the need for antifreeze.  While more careful design implementation is needed if all the tubing is PEX without any outdoor connectors it should hopefully be able to take freezing even if something goes wrong.  Next question PEX for absorber?  This article shows the PEX because of its thin wall and steady heat should perform nearly as good as copper.  Had 2 other articles found showing very similar results but lost the bookmarks for them in a hard drive crash and haven't found them again yet so this information isn't a one off.

tubing heat conduction for solar collector use

Notice the materials can all mostly top out right at 200 degrees F.  The EDPM is 200, the PEX is 200 and at my elevation maximum water temperature is 202 degree so worst case is within bounds is my thinking for a non pressurized system.  Figuring to build the box out of 3 layers of half inch plywood and some steel support structure.  3 layers of insulation 2 inches each.  Line the box with plastic sheet for leak prevention.  Outer layer standard extruded polystyrene where it is cooler as it won't take the inner heat.  Middle layer polyiso because it is good to 160 degree with another poly plastic layer for leak prevention.   Inner layer rock wool so as to provide a wicking layer for any leakage(rock wool will no grow mold and provides thermal mass.)  Since my box goal is just over 6 feet deep in water for 3 PSI at the bottom and rock wool is rated for just under 5 psi crush I am good that way too.  The only pressurize part of this is cold water line going in and thru the preheater water tank and back to the cold inlet of a standard water heater begin with.  An anti scald / tempering valve will need to be added to keep the outlet temperature safe.  All of the rest of the water would be open to atmospheric pressure and thus no risk.  Since the pressurized line is being heated by atmospheric hot water it could never get hot enough to be a problem.

The dream for backup for this is to build the barrel for the RMH slightly bigger than a normal barrel and spiral say 1 1/4" pipe up it in about 3 to 4 wraps with flat plate welded between to create a barrel.  Most of the squish boom arguments seem to agree pipe that large is not dangerous in a convective flow system because it can't flash enough of the water to steam at one time to be a threat.  Worst case it will sound like a percolator.  If the DYI "barrel" is 3 feet in diameter and you did 4 wraps that over 36 feet of pipe to gather heat from the RMH.  By adding the additional cooling it should actually rocket better slightly.  To catch rust flake want magnets at both inlet and outlet in a side passage trap.  To reduce deep blow back into the tank if there is a problem run the water inlet flow through a tesla valve.  No moving parts and nothing to plug off.  

The water tank gets 5 stratifiers.  1. Plate collector return, 2. evacuated tube collector return, 3. ground cooling/heating collector return 4. PV panel cooling return. 5 RMH return.

I have more links and thinking stored if wanted but this is already way too long.

Here's a good cost comparison of water heaters including initial cost, fuel, maintenance, etc. I've queued it up to the comparison chart:

https://youtu.be/wt95SE2ogNI?t=625

Also, here's another one discussing the pros and cons of tankless:

https://www.youtube.com/watch?v=8RFuFBI3r2c

Caleffi provides a good discussion of Legionella and domestic hot water in Idronics 22:

https://www.caleffi.com/sites/default/files/media/external-file/Idronics_22_NA_Safe%20and%20sanitary%20hot%20water%20delivery%20systems.pdf

Our new house (off grid, primarily solar electric with some propane appliances) will have radiant heat from a wood gassification boiler. There will be about 800 gallons of hot water storage which will run from about 120F - 185F. This thermal battery will heat the floor and will also preheat the domestic hot water. In order to make it as seamless as possible, the stored hot water will be pumped through a heat exchanger to preheat cold water going into a traditional tank-style propane water heater. If there is stored heat from a fire it will reduce (or eliminate) the use of propane. In the summer, propane will heat the water or we could light a fire as needed. The output of the propane water heater will go through a mixing valve to provide a safe temperature at the fixtures since the boiler output can be far too hot for safe use.

The heat storage tanks and a bunch of pipes need to be fully insulated and then the heated floor will be fully operational. Performance of the domestic water preheating won't be known for a while yet. So far, the boiler is working great except for one last leak in the piping. It's not a rocket mass heater but the burn chamber looks plenty hot to me as you can see below.

I've used on-demand in gas and electrical form, as well as electric element/oil boiler storage tank systems on 2 continents and that Island in the North Sea. I cant weigh in on the heat pump option but my experience has shown me that it's a "horses for courses" scenario.
I grew up in South Africa (before solar) and everyone had "geysers"- a fibeglass-insulated double skinned steel tank tank with a heating element and a thermostat. Legionella was unheard of. Solely for hot water, no heating system. They worked on mains water in urban areas and were gravity fed from storage tanks filled from a borehole in rural settings. Later on, homesteads had a booster pump fitted to the water supply coming from the tank. No filters, very occasionally an element or thermostat would need replacing. Maybe they were more economical/durable because they were only for hot water?
Off grid, I used a gas on-demand gravity fed system: a 1000L tank 7meters above the ground with an outside mounted boiler. Serviceable, was plumbed in to the kitchen and bathroom so had the luxury of hot and cold water from taps! Could only use 1 hot water outlet at a time. Probably could have done with more pressure or a lower L/m boiler. Also, building a wind guard would have helped efficiency a lot - and prevented cold water down your back in the middle of a shower!

In the UK on mains water, in older houses I have only ever had oil fired "boilers" attached to an insulated storage tank in the loft/airing cupboard for hot water and radiators. Some had a back up electrical element in the tank to supply emergency hot water (but no heat) should the boiler break/need maintainence.  Newer or refurbished houses had electric/gas boilers with a storage tank, supplemented with an on-demand shower. Never any Legionella problems in single houses although there was a scare 10-15 years ago in a complex type living arrangement that shared the same heating system. I think the distance the water had to travel was an issue but don't quote me.
I've also used the really small 5-8Litre electric tank systems common in rural sheds and I find them very hungry and very inefficient. The amount of hot water you get is minimal because it's filling up as you use it -as all tank systems do- so the small amount of water in the tank cools really quickly. You may get a hand wash or half a basin of warm water. In winter, even with insulated pipes, the water is COLD so takes longer to reheat. Again, no filters that a householder had to maintain.

In Spain, because I am completely on solar with a small battery bank, high load appliances are out so that rules out both instant and storage electric systems. And no spacial heating but my Walker cookstove provides that in the winter, along with hot water. But in the summer a mass heater is out of the question! It's so hot I cook and heat water outdoors on a gas range. It's the most economical, if not particularly convenient, way for my situation in my climate. I only have cold water plumbed to my taps via a pump in a cisterna catching rainwater. This is fitted with a filter to remove debris, another to filter particles up to a certain- nano-whatever- range, and our drinking water is further filtered at the kitchen tap. Dish water is easy- boil a kettle, tip into the sink, not as fast as on demand or tank stored but reasonably timely. Hot showers is a bit more complex! Water is boiled, tipped into a 20L bucket, topped with cold water and I use an electical float controlled pump in the bucket to the shower head. I could have a gas boiler but as I can't vent it due to the house layout it would have to be outside. Here I am influenced by 3 major factors
1)wind- I've lived elsewhere in my region and had problems with flame blowout and low water temps as the flame is not constant. Even with the boiler indoors- the wind roars back through the vent and you get a cold blast in the shower. Coupled with
2)cold in winter, sub zero C temps chill the water so the max temp acheiveable is lower. My relatively small (6000L) storage tank is half above ground, directly in the prevailing wind. Also butane turns to sludge at a certain temp. You could use propane or build an insulated bax for your tank.
3) Perhaps my biggest bugbear- water and wasted energy. Unfortunately, due to the internal layout of my house, hot water would take a long route to get from the boiler to the taps which wastes energy -it cools as it travels. Also, for short usage- wash hands, rinse a glass etc. the tap is turned off before the hot water reaches it so water is heated for no reason. Water is wasted while waiting for the hot water to reach the tap. Now I would save the cold water in a bowl for rinsing when washing dishes and use the cold water to wash my hands/ rinse a cup, but unfortunately, the rest of the household hasn't worked that out yet! It used to put my teeth on edge to hear the boiler kick in, only to turn off 5 seconds later, and then again and again! I still havent found a solution for  using the cold water prior to showering.....So fot the sake of my sanity, in this situation, on demand gas is out.

I was looking at a hybrid solar/electric tank system referred to in an earlier post but in winter (when I use the most hot water) I have the least sun for DC supply and lower AC power storage potential. I am trying to find out if I can attach  double the amount of panels to make up for the reduced solar imput in the winter, but I'm concerened about overload in the summer-I don't understand enough about where DC current goes when it's input exceeds load. It's a completely separate system and not connected to my inverter at all. The electric element is wired to the CB.

Ideally I would a storage tank directly above my kitchen which stores heated, thermosyphoned water from my Walker Stove in winter and the DC system/AC system or, even better, a solar thermosyphon in the summer for year round, efficient access to hot water.

Each of these situations is unique to the environment, usage, financial constraints, building codes any many other factors so I don't think there is a one-size-fits -all scenario. But I would definitely think outside the box and not conform to social "norms" in any situation.

We’ve been off grid since 2014, family of 4. Our best water heating has proven to be a 50 gallon electric hot water heater plumbed to our old kitchen queen cook stove heater coil, then a mixing valve, then to a propane fired on demand hot water heater.

In the deep winter the propane never kicked on because the incoming water was already hot. Shoulder seasons it would heat minimally until the summer when it did all the work.

We have since built a walker continental rocket stove and LOVE IT. The only setback is no water coil. Since we only run it a couple hours a day on average it doesn’t pay to put one in. OUR PROPANE USAGE IS CRAZY.

Next step is to hook up the panels we just got directly to the water heater coils in the tank (which have never been used) and crank up the temperature. Hopefully that will help with the propane usage.


Our two cents.

Her is an example of an old HW heating arrangement.  The electric hot water heater is located behind the wood stove where it baths in a very warm environment 8 mos of the year.  The pipes run up and along the ceiling where they stay warm, at least for everything in the kichen and laundry.  

I have use of an offgrid house that switched from solar powered well pump + propane tank heater, to
solar powered well pump + propane tankless heater.

The tankless heater only fires when water is flowing at sufficient pressure, and there is a lag of cooled water in the pipes.
Open a tap, the pressure drops, well pump starts, pressure recovers, tank heater starts, eventually hot water flows.  Repeat.
We might save some propane, but we use more (battery stored) solar electricity cycling the pump more often, which is our biggest draw.
And heat loss from the previous tank helped heat the house a bit.
So as others write I think the advantages one way or another are situational, and in a heating climate, a tank may be better overall.

Funny aside, I once lived with a tankless heater in France.
When it ignited with a gout of flame, it sent a compression shock wave through the apartment, and then after a lag of cold water, sent scalding water to the shower.  Fun times.