Faulty reasoning? Heating water electrically...

Don't know if this is the appropriate place for this query, but here goes. Have gotten good comments on this forum already for some house retrofits for more efficient energy use. For various reasons that I won't go into here, we wish to stick with electrical hot water production. First, it's just the two of us and we already are pretty lean on hot water usage (short showers, no dishwasher, washing machine is warm/cold cycles), and even have the thermostat on the behemoth tank heater set at 110 degrees. The well-water is an average of 42 degrees F. upon entering the house. Current heater is a 50 gal tank that is about 16 years old. I was thinking of replacing it when it dies with a 2.5 gal tank heater, followed in-line by a lower-wattage, thermostatically controlled (!!!) tankless water heater. The idea would be that the mini-tank would be 120V and provide enough hot water for the infrequent, low volume use that we typically use. Since this warm water would be going through the tankless heater (220V), it would be signalling to the tankless unit that little or no additional heating is required of the water and so the tankless heater for many demand situations would not even turn on. But during higher demand situations, when the mini-tank was temporarily exhausted of warm/hot water, the tankless heater would now assume full heating duty until the demand was no longer needed. Then the mini-tank would heat another tank-full of water until demand was again needed.

The conundrum here is that I have not seen anywhere a good discussion on combining tank and tankless (along with electrical and tank sizing) for efficiency, only advice for maximizing hot water for when an army platoon descends on your home and you need showers for all of them at once. Anyone come across a discussion of this type on the configuration of tank and tankless combinations for maximum efficiency in a low-demand household? Comments and criticism welcomed. Thanks!

I've no experience with dual system heating. I too prefer electricity for water heating.

I did learn one thing from Paul, if your water is set under 140 you can breed legionella in your water heater. That can cause pneumonia, so it could be a real issue in the making. You may know that already but thought it might be helpful.

One thing I was told was that if your water is high in minerals, a tankless heater can clog up very quickly. Never had one, just what I was told when looking into them the last time my water heater took a dump. a topic to look into when shopping for a tankless.

My guess would be that it is extremely dependent on the situation. If you set up the tank and tankless water heaters in series, they would both be running when the temperature drops below the threshold set by the thermostats. Even assuming that threshold is set higher on the small tank and lower on the tankless heater, once that lower point is reached then both would be consuming energy. Without any solid facts to back it up, I would say it would consume more energy simply due to the fact that there is a loss of energy when you convert it to a different form of energy (in this case electricity to heat). The 220V heater will probably be more efficient than the 120V heater.

Having them in parallel to perform different tasks could potentially save much more energy. This seems especially suited to your situation since you are already conscious of your hot water usage and have actively minimized waste. Places that may demand more water at times, such as perhaps the bathroom, could benefit from the 'on demand' nature of the tankless heater. Other places that would normally use more consistent levels of hot water, perhaps the kitchen sink, could use the small tank. Once it is depleted you either stop and wait or source hot water elsewhere. The biggest benefit of this that I see would be to use two small heaters as opposed to one large 'whole house' solution which could be much more costly.

I don't think the two in series really compliment each other as a whole house solution. With one or two people conscious of the water usage and understanding the benefits and pitfalls of the situation then this may not be a large issue. Hosting guests, even after being informed, would likely lead to someone ending up less than happy. If I had to guess I would say most people used to electric hot water want hot water now and that may be part of why you don't see tank and tankless systems used together. That and the tankless system being designed to not use a tank and sized for a given amount of flow. Once the tank runs out you are at the mercy of the limitations of the tankless heater. Trying to flow more water than it can effectively heat will result in less than appropriately hot water.

Is the current location of the water heater in a heated space? Or perhaps a basement or other place of relatively stable temperature? Given the cold temperature of the water supply I would use the old water heater as a holding tank. Disconnect the electricity and strip the insulation from around the tank and it could be used to hold and warm up the water prior to going into the new water heater. This is similar to how some solar water heating systems work, except you would be removing the insulation instead of adding to it. If you heat your home with electricity then this wouldn't be efficient. If this is in a basement, or in a living space heated by wood then it should be able to save electricity. This goes back to my first sentence about depending on the situation.

A 50 gallon electric water heater is just sitting there and wasting electricity as the water tank losses heat and has to turn on to maintain the temp even though no water is being used. So a step down to a 2gallon tank will be a huge saving due to less surface area throwing off heat and due to the smaller size it is in theory easier to insulate better R-60 (12 inch of insulation around it creating a 2.5ft ball)

All that said, you can save even more energy if you went totally tankless. Because you wouldn't have to fight 24/7 heat loss from a tank. And if you use 1 heater or 2 heater it doesn't make a difference in the temp of the water. The heater can easily handle the ramp up from 42F to 110F.

Now the only reason why you would want to use the small tank heater in series with a tankless heater is if you are off-grid. And you want to slowly pump energy into the small tank heater while it is sunny and then once it gets dark you only need to send a trickle of energy into it to maintain the temp. And once it is empty you just slowly refill and bring the temp back up, that way you can use a low wattage inverter. And if you really need alot of hot water due to family coming over for the holiday you start up the generator and plug in the tankless hotwater heater. Without crashing your battery bank/inverter.

"it would consume more energy simply due to the fact that there is a loss of energy when you convert it to a different form of energy (in this case electricity to heat)."

In this and most cases, the lost energy is in the form of heat, so it really is 100% efficient at this step, provided the heating element is fully enclosed by water.

Thanks for all of these excellent replies.....a lot of food for thought that has spurred additional ideas. In general, it sounds as though there is no real advantage to a series-connected tank + tankless configuration, except as Daniel S. noted, each situation has its unique aspects. The current tank is in the basement and that location would be the easiest in which to install any new appliances, tank or tankless. Yet with our cold winters, that basement is really quite cold....won't go into why because that scenario is not likely to change soon, but the cold temperature of the basement really is a factor here. In that regard, I began over the weekend to examine other locating possibilities for either tank or tankless and where the most efficient locales would be. The shower/bathroom is upstairs, so from the basement this is the longest run of pipes that will be cooling down the water on its journey.

Actually, under the kitchen sink (main floor) would not be a bad new location, even though it would involve a bit of creative pipe re-configuration. In that regard it may be possible, by re-doing some of the cabinetry, to locate a new small 10 gal. tank system (220V or 120V, is there a preference here?) or 220V tankless unit in that space which would offer a number of advantages--(1) warm kitchen, so reduced heat loss to the cold basement, (2) against south facing wall, so if a model was purchased that could integrate with a solar heating system with an exterior wall-mounted panel, additional reduced energy usage, and (3) closer proximity to demands.....kitchen sink (top demand), laundry room washer and sink (second top demand. adjacent to kitchen), and upstairs shower/bathroom (dead last demand). As alluded to above, I've seen small tank heaters offered in either 120V (single element) or 220V (double element): With the exception of the rate of re-heating the water, is there an ultimate energy use advantage with going with the single 120V element or vice versa? (My mind may not be working on this properly since I'm imagining a Cadillac Escalade versus a Toyota Echo, both reaching the same mountain pass but one having used more gas.)

There is some time to wait here because these would likely be retirement or near-retirement projects which would be nice to have completed within the next 3 - 5 years. But continuing thanks for your current and any additional input, time, and experience.

If there is an existing circuit for an electric range that is no longer being used then 240V would be more efficient. The higher voltage device will draw less amps for a given amount of watts. It allows you to use smaller diameter conductors, or in the case of reusing larger diameter conductors it will have lower resistance. This is one portion of why it isn't really feasible to get 100% efficiency as the resistance of conductors will lose heat, likely in places where the heat can't be utilized as effectively. If the kitchen is near the point where power enters the house then it may be easy and cheap enough to run a short 240V line. If you have to do a long run plus buying new breakers plus hiring someone to help with installation and it can quickly become more expensive than the savings of reusing an existing 120V circuit.

Any use of solar that can warm the house or water may be beneficial in a cold climate. The tankless system would probably be more efficient on grid, but is dependent on usage. Having multiple points drawing hot water simultaneously will need a considerably more powerful (expensive) unit. As mentioned above, a tank system could work well in an off grid situation. Using excess energy being produced while it is being produced, such as sun shining on a PV system with full batteries, means you can make use of energy that otherwise isn't being stored.Missouri Wind and Solar has a charge controller designed for wind, but I believe can be used for solar and hydro, that sends energy to a dump load when the batteries are full. This dump load can also be a water heating element. His youtube channel has videos about this and tons of other off grid and energy saving techniques.

Thanks for the additional information, Daniel. As luck would have it, I may have 2 additional un-obligated 220V breakers in our main breaker box, one for a clothes dryer and the other for a under-sink disposal unit, neither of which exist in the house any more. Especially if the sink disposal unit was 220V, then the wiring for sure is still there, but even the line for the dryer is only one room over so it could be pretty easily re-strung in the basement raftering. Then it would just be deciding if additional cabinet mods would need to be done or if there is enough room for a the small tank heater as is below the sink. Definitely will be looking into some form of supplemental solar water heating using the south exposure of that wall. Educating myself more on 220/240V will be good as I'm thinking of a 220V-producing solar array grid-intertied with the rural cooperative power utility in the short term, with possible battery addition as they become more plug-n-play and the price per Ah drops (....hopefully). Great info....thanks much!

Seems to me that a tankless electric will find ways to break even though it is a simple machine. If you are willing to go without hot water for a few days now and then a couple of solar panels, some sort of small inverter and another 50 gallon electric would be more reliable and less money than something designed provide hot water 24/7. DOE requires electric water heater to have more insulation since sometime in 2015 so there won't really be that much heat loss from the tank- I don't know the specifics, but cost of eating water with 240 volts is going to be more related to how much water you use and less to the size of the tank.

Leaving cold/ warm water in a metal tank for weeks will make the water smell quite brackish.

I see that you can now buy a couple of 600 watt DC water heater elements and a 12VDC thermostat for less than 100 bucks. Now if some one can do the math on how much hot water a couple of solar panels will heat up on a average day. I know this will not provide the same amount of hot water as other methods, but it is cheep, reliable and simple

@Alex S.: ".....a tankless electric will find ways to break even though it is a simple machine. If you are willing to go without hot water for a few days now and then a couple of solar panels, some sort of small inverter and another 50 gallon electric would be more reliable and less money than something designed provide hot water 24/7."

I like knowing the fact that the insulation may have gotten better in recent years in the tank heaters. As attractive as the tankless heaters are, if they are more prone to breaking (not only in the heating per se, but in temperature regulation), then we'll probably stick with a small tank heater. With some extra time on my hands coming up, was also considering designing a solar water heating unit with one or more heat exchangers as shown below serving as the core of the unit....the idea being that 90% of our hot water use is done in ~2-3 gallon (US) blocks or less of usage. With each use, we would draw from a small electric tank heater in the house, which itself would become replenished from water coming through the solar unit. Could be used for 2.5 - 3 seasons per year, but drained and left unused for the cold winter....don't know, still brainstorming here.

Tankless electric heaters use a huge amount of power, something like 40 amps at 240v.
A small tank can use an electric element quite efficiently. If you were to put a gas or propane tankless heater after a very small electric tank heater it would only kick in when the water from the tank became cool enough to trip the burner on.

There are hybrid units that combine a tank with a tankless. The only one I've seen has a 40 gallon tank. A 5 gallon tank would seem to make much more sense to me.

Thanks for additional comments on this thread. I can see where tank water heaters still have many advantages for home use. Just a quick thought experiment, however, to see if my thinking is clear on this.

For argument's sake, and please correct my math and logic if off-base, if I'm looking at a 240V/18 kW tankless water heater, that unit will draw 75 amps when activated if I'm thinking about this right. If run for a hypothetical 0.1 hr per day (low end, obviously), this would be 1.8 kWh devoted to this use. Let's assume comparison to a hypothetical tank heater with a 3600 W element. In order to equal the same amount of power use, the tank element would have to be called by the thermostat to heat for a total of 0.5 hr. per day. In both cases, 1.8 kWh were used. This is where I get somewhat lost, because I'm not so sure, even given a 0.1 hr of actual hot water draw-down, how often your average tank unit is requesting for the element to turn on across a given day. It seems like something that would be very dependent on tank size, tank location, degree of insulation, etc., and yet further seems like the thermostat in the tank heater would be calling for element activation rather frequently (?)....somewhat frequently?... I can't believe a comparison like this has not been done and published somewhere.

Now it does begin to seem like a properly-located small tank unit for serving a low-use household would be an efficient solution, but I'm not sure how much more or less efficient than tankless, *assuming* the tankless unit is as robust in performance as the tank version.

The best way i can think of using a tank in combination with a tankless heater, is to heat the tank (storage) with solar thermal hydronics and send that water from the tank to the tankless heater (electric, gas, biomass, co-gen) in your case, electric.

If a solar domestic hot water system has pv power for pump(s) and controller, the coefficient of performance is infinite. Also in most regions of the u.s., SDHW has the shortest payack of the active solar energy systems.

The system in the photo is one of our installs, pv pumped and controlled (direct pv, no battery) and feeds a Takagi/A.O. Smith 200kbtu tankless heater. Its a monster! 90° rise at 8gpm! The tank holds 120 gal. Of purified water as thermal storage (it never leaves the tank) with two stainless coil type heat exchangers, one solar, one feeding potable water to the heater. The storage is heated by two 4'X10' Heliodyne collectors.

A system for you could be a 15-20 gallon tank-electric water heater (short and fat has better thermal resistance/more gallons to surface area) and a 60-80 gallon solar storage tank or vat. Additional insulation on the tank and a timer for the tank heater, rounds out the machine.

Electric water heaters are commonly used as solar storage tanks as they have a great price-point, this usually requires an external heat exchanger and two pumps, not always.

Some tanks for boilers and solar will have one or more coil heat echangers built in and electric or gas backup, then you only need one pump and one tank (except large systems) and immersed exchangers are more efficient. You can build a vat type tank with immersed coil exchanger at a good cost to performance ratio, the same cannot be said for collectors.
Either way, a storage tank in series with a heater is usually more efficient than a single for residential use

This is brief but i hope it helps. I have been designing and installing sdhw and solar space heat since 2004. We mainly install pv today, but i hate to see solar electric get all the glamor. Heat is precious and these systems make gobs of it, even in mild overcast. They are hard to stop and your great grand kids may inheirit it!

Alex Kosmicki wrote:Seems to me that a tankless electric will find ways to break even though it is a simple machine. If you are willing to go without hot water for a few days now and then a couple of solar panels, some sort of small inverter and another 50 gallon electric would be more reliable and less money than something designed provide hot water 24/7. DOE requires electric water heater to have more insulation since sometime in 2015 so there won't really be that much heat loss'

1°F per hour is the benchmark for high performance storage tanks and heaters, they are not common.

The rough conversion for btu to watts, is btu ÷ 3.4121 = watts For 1200w pv and 30 gallons hot water per day avg., 30 gallons is 250lbs and we need to raise the water by temp 80°F.

One btu per pound, per degree. So (i know, the mathmaticians will be 'all up at arms') 1200wp=1020w X 4 hour solar avg.(Variable)= around 14,000 btu and the hypothetical requirement is around 20,000 btu/5,860w/hr plus thermal losses. A small tank will probably not have better than 2° or 3° loss per hour at room temp, unless additional insulation is added and or the location is warmer than room temp. 1,200w-1,600w pv and a backup

With a pv, direct-drive reverse cycle chiller, pv sizing could be 1/3rd to 1/4 or about 600w pv, if a heat pump machine that small is available.

Hi,

I read some of this string and I think I may be able to assist. You have a very interesting approach.

I wrote about hot water heaters on my website in late 2012 (In the conservation>heat>hot water heater section). There is a lot of information about garbage, efficiency, money and return on investment (for water heaters). I even created 5 different calculators (just for water heating) for you to make calculations on your own (so you can see what is right for you). There is a lot of information there and you might spend an hour or two going through the information.

Let me take this time to summarize what you might find there for the type of system you want to have.

Interesting approach. Pre-heating the water with a very small tank. What I have found that with this kind of mixing problem, (my testing and confirmed in literature) when 70% of the water is removed quickly from the tank it will begin to loose temperature. That means in your set up, when 1.75 gallons is removed the water temperature will fall from that 110 you mentioned. At that point, your tank-less might engage.

Regulation: Each unit is separately regulated. You may have a little difficulty getting approval on a joint unit.

Electrical or Gas connections: A tank-less system requires a higher flux of electricity (or gas) than a normal tank system. By my calculations it is like turning 70 burners on on your stove at the same time. That is a lot of power all at once. You may need to run an extra line (electrical wiring or gas pipe) creating extra labor and garbage. I am not a big fan of how these tax the power grid when operating either.

Garbage: In general, switching your current system to another type of system will create more garbage. You will need to cut holes in sheet rock, add wiring or gas lines, upgrade your electrical panel and so on. That would be for any kind of switch in heat source (electric, solar, gas) and tank type (tank, tank-less).

Cost: At the time I wrote the stuff, the tank-less system was expensive. So was the contracting to tear out sheet rock, re-pipe, re-wire.

More: Yea, you will find a lot more there.

My suggestion: I have noticed it is common to plum for peak. "What would be the need if 2 people are taking a shower at the same time doing a hot load of laundry, with running the dishwasher?" Even if that were to happen at your house, it may only be 1% of the time. People do not have to operate under that assumption, they can do one thing at a time. So then it becomes, "How much hot water do I need for a single operation". That could be the 70% size mark of your hot water heater need. What happens is the size requirement of your tank shrinks. You get a tank, much smaller than 50 gallons. You might even realize that the tank-less, is folly in cost and garbage. Maybe just getting a hot water heater of 10 or 20 gallons (an RV) model might just do the trick.

What happens if you get a house full of visitors? Make the tank bigger. Yea, I said that. Well at least for showers you can make the tank bigger. Realize, a person will not take a shower hotter than what they are comfortable with. Usually it is slightly above body temperature (say 100 deg F). I say this in the web site. You say you keep the water at 110 deg F. If you were to change the temperature setting to 120 deg F, you have made the tank 20% bigger for showers. Setting the tank to 140 makes the tank about 50% to 60% bigger. (Same applies if you do dishes in the sink instead of a dishwasher). When the company leaves, you can dial the temperature back on your heater. One point about legonela. I an not 100% certain but It might be best to run the hot water heater at a higher temperature a couple weeks a year to reduce the threat of legonela.

So what do you get? There are small tanks, but I might suggest you get the most durable 15 gallon or 20 gallon tank and forget about pluming in a tank-less after the main tank. A tank-less is an expense that will be used so infrequently you will not see a return on your investment in 100 years. The smaller tank, on the other hand, will show savings from the moment you install. You should wait till the 50 gallon is dead. You might need to make a very slight adjustment on when you do things (shower, laundry, dishes) but both of you will be able to take a hot shower, wash a load of laundry and also do a load of dishes every day on a 15 gallon heater (try one of the calculators).

Edit: Oh yea, I forgot. You talked about a thermostat to control times to heat. I am guessing you would turn it on and off at different times of the day. Well, that is fine, however, may have a long time for a return on investment (decades), especially if you are controlling a very small water heater. I might just get a little higher R-value (insulation) and leave the unit on all the time.

I just installed a tankless heater.    I am very happy with it.    It was installed during the house construction,  do not problem.    I ran 3 2-8 with a ground.    3 double breakers at 40 Amps each.   Wow,  I woukd not like doing that in existing homes.    My water is said to be average 44 degrees.    

I have a very large pressure tank,  120 gallon to be exact.   I can tell when it has been sitting all day,  takes very little heat.    

I will later put a solar system as primary and this for backup.  

Surprisingly I can get a lot of hot water flow with my tank,  I was concerned due to low incoming Temps.     I would not install any less if I was planning to shower with it.   42 degrees will take a lot of power to get to 110.  

I also like the idea of Parallel for different areas.    

For anyone considering this issue, the most efficient and cost effective way to heat water with electricity is to use a heat pump water heater, sometimes called a "hybrid" water heater.  Currently there is a guy on e-Bay selling heat pump add-ons for normal electric water heaters, for $365 delivered.  These plumb into the normal inlet and the drain connector on the water heater and use the existing thermostat to control the heat pump.

I bought one and hooked it up to my water heater and the electrical consumption on the water heater dropped to 1/3 what it previously was.  

Note: these require 240V AC, but they max out at around 600 watts (probably a bit more briefly when they start up).

Here is the listing if anyone is interested:
http://www.ebay.com/itm/NEW-Heat-Pump-Water-Heater-SAVE-50-or-more-on-your-Hot-Water-/282584753572?hash=item41cb5d25a4:g:RMQAAOSwj0NUa3~6

standard disclaimer: other than being a very satisfied customer, I have no commercial interest in these.