Instant water heating?

My wood burner presumably isn't that great at capturing all the heat from the burn as I've measured the flue temperature at over 230 degrees Celsius. Now I've seen people wrap copper coils around their flues to heat a water container, and that's not feasible where my stove is. However, is there any reason one can't use such a coil to provide instant hot water? Just pour in cold at the top and extract hot at the bottom? Or even let it drip into coffee grounds?

Also, why is copper always used? Is there better heat transfer to copper vs stainless steel?

Yes copper is better than steel at transferring heat.

They do have instant tankless hot water heaters. And it will cut your energy usage by 33% compared to the regular hot water heater with a tank.
Here is one that is only $800, 8gpm, indoor
https://www.amazon.com/Takagi-Indoor-Tankless-Heater-Natural/dp/B0057X221K


$600, 5gpm, outdoor
https://www.amazon.com/Noritz-NR501-OD-NG-Outdoor-Tankless-Natural/dp/B00MBBH1OE

Graham Chiu wrote:My wood burner presumably isn't that great at capturing all the heat from the burn as I've measured the flue temperature at over 230 degrees Celsius. Now I've seen people wrap copper coils around their flues to heat a water container, and that's not feasible where my stove is. However, is there any reason one can't use such a coil to provide instant hot water? Just pour in cold at the top and extract hot at the bottom? Or even let it drip into coffee grounds?

Also, why is copper always used? Is there better heat transfer to copper vs stainless steel?

There is more too all this then what their first seems, because if you get too much heat, it can flash to steam and BOOM! It can, and has killed people. It can be done, but there are safety valves that must be employed.

Also, some heat up the chimney is not a bad thing because you need a warm enough temperature to produce draft. Most woodstoves are only 50% effecient, BUT because the cost of fuel is so low, it makes it an overall better use of fuels then anything else, like say a propane heater to heat a home. Firewood is cheaper then propane, so wasting 50% of it is not a big deal.

If that is not the case with your situation, a rocket mass heater, or rocket stove possibly could work.

Myself, I just use my coffeemaker to make my coffee, then shut it off and place the decanter on the stove to keep it warm. It saves a wee bit in electricity costs.

I do however refute the claims that tankless water heaters are more effecient; that depends on the size of the household. I have a family of (6), with 5 being women. That means we consume a lot of hot water in a day, so after reviewing my options, it was determined that I would save more money by having a tank type heater then tankless. That is because while a tankless does not have to keep a tank of water hot 24/7, it takes a lot of BTU's (fuel consumption) to make instant hot water. Effeciency also comes from how far the unit is from sinks, showers, appliances, etc. So it is not clear cut. It really depends on teh application.

That gives me an idea...

First I'll second the assertion that copper transfers heat much better than stainless steel or regular steel.  And I'll robustly second the warning from Travis about steam.  I think in the coil system it just starts to splutter and the water above the steam could shoot back out at a high speed.  If it's contained in a vessel though, watch out.

My initial idea was to pour water down the exterior of the stove pipe to get it hot without any risk of boom.  Of course that would be messy, dirty and not give you clean hot water.  But the idea of running water over a hot surface that is open to air is a possibility.

Here's the idea.  If the water is trickling down a hot surface, it will heat up.  If your coil of copper is 1" diameter and very short (12") and you pour water down it slowly, there is a lot of room in the pipe for steam (if it happens) to escape.  If your coil was the equivalent of a small copper gutter that coils around the pipe, it's even more open to the air.  Any boiling is uneventful.

So I'm not sure what material would work best but I'm imagining a piece of 1" soft copper tubing (if they even make it in 1").  Coil it around the pipe and trickle water through it.  The hotter the stove pipe, the faster you can pour.  If it's not hot enough, run the water through again.  If it's consistently not hot enough, make a longer piece of tubing.  I'd try to make sure the tube never had more than 2' in contact with the stovepipe and keep the water flow at a trickle (pencil sized water stream at most).  The tube could be connected to a funnel at the top and extend to the side at the bottom to fill a pot easier.

If you patent it and get rich, send me some $  

Graham Chiu wrote:Also, why is copper always used? Is there better heat transfer to copper vs stainless steel?

S Bengi wrote:Yes copper is better than steel at transferring heat.

Copper has an unbelievably good thermal conductivity. Table of values here.
Copper - 401 W/(m K)
Aluminium - 205 W/(m K)
Stainless Steel - 16 W/(m K)

Copper is also very malleable so it's easy to bend into coils.

My concern with copper is that, if you look around a typical home, you rarely ever see drinking water being heated or stored in copper vessels. Washing water is often heated and stored in copper, but not drinking water. Makes me wonder if there is some health issue.

Mike Jay wrote:
Here's the idea.  If the water is trickling down a hot surface, it will heat up.  If your coil of copper is 1" diameter and very short (12") and you pour water down it slowly, there is a lot of room in the pipe for steam (if it happens) to escape.  If your coil was the equivalent of a small copper gutter that coils around the pipe, it's even more open to the air.  Any boiling is uneventful.

Sounds good.  Maybe controlled release from a container rather than pouring it out.  And an air gap from the water source to the opening of the copper coil.

One should be able to calculate the rate of heat transfer, and the rate of water falling through the pipe to get the optimal length of pipe.

Mike Jay wrote:

If you patent it and get rich, send me some $  

Too much prior art!

Matt Coston wrote:
Copper is also very malleable so it's easy to bend into coils.

It also bends easily so to prevent that, prefill the copper pipe with sand before bending it.

Matt Coston wrote:
My concern with copper is that, if you look around a typical home, you rarely ever see drinking water being heated or stored in copper vessels. Washing water is often heated and stored in copper, but not drinking water. Makes me wonder if there is some health issue.

I suspect it's a cost issue these days.  All the pipe to my house was copper but when there was a leak, they replaced it with plastic.

But there is this http://www.health.state.mn.us/divs/eh/water/factsheet/com/copper.html

So, if your child is under age 1, no added copper.
That's why I was also wondering about using stainless steel instead of copper, but it just might need too long a run of coil?

Wood cook stoves often had a resevoir so you didn't have to worry about the steam. Here is an amazon flue style: https://www.amazon.com/Attachable-Kettle-Teapot-Camping-Heater/dp/B077ZDDQQY/ref=sr_1_4?ie=UTF8&qid=1543277157&sr=8-4&keywords=wood+stove+water+heater. small pipe size but perhaps there is a 4-6 incher out there

Graham Chiu wrote:But there is this http://www.health.state.mn.us/divs/eh/water/factsheet/com/copper.html

Thank you for the link. However, there's this import part lurking at the end:

In addition, hot water dissolves copper more quickly than cold water; as a result, water to be used for drinking or cooking should not be drawn from the hot water tap.

Not sure if this statement is true in isolation. Earlier in the article it says copper pipes in new homes have not yet developed the "coating" (I assume this is copper oxide) to stop leeching into the water. The article doesn't explicitly state if hot water from old copper pipes is safe.

I was taught that the reason we don't drink the hot water from a kitchen tap is because the hot water tank is not hygienic. Maybe both are true.

Matt Coston wrote:

Graham Chiu wrote:But there is this http://www.health.state.mn.us/divs/eh/water/factsheet/com/copper.html

Thank you for the link. However, there's this import part lurking at the end:

In addition, hot water dissolves copper more quickly than cold water; as a result, water to be used for drinking or cooking should not be drawn from the hot water tap.

Not sure if this statement is true in isolation. Earlier in the article it says copper pipes in new homes have not yet developed the "coating" (I assume this is copper oxide) to stop leeching into the water. The article doesn't explicitly state if hot water from old copper pipes is safe.

I was taught that the reason we don't drink the hot water from a kitchen tap is because the hot water tank is not hygienic. Maybe both are true.

I was told that people used to teach not to drink from the hot water because "back in the day", lead pipes were sometimes used for water and hot water made more lead in the water.  No idea if that is accurate.  I do wonder how long until they discover something in PEX that is bad for us as well.

Most houses I've lived in have had hot and cold water pipes made from copper.  I believe that's been the standard for decades until PEX and/or cpvc came along.  I've never heard that hot water through copper pipes is bad.  If so, are you supposed to make soup starting with cold water?  Copper pots, tureens and other copper cookware would be suspect as well.  

I'm thinking we're getting away from the original subject...

One of my "Yard Art stoves actually has a manifold of pipe in the fire box. Dikenson heaters that are common on sail boats and in my bus conversion have a coil either one or two loops in the box and it appears to be stainless. Beer chillers are out there in the brewing community that are stainless and some apperar to be flue sized.  Some copper cook ware is tinned  to prevent acidic foods from leaching copper into the food. After seeing the gunk that accumulates in a hot water tank I just don't drink from a hot water tap, but for bathing or cleaning I don't think there is a problem.

Trace Oswald wrote:I was told that people used to teach not to drink from the hot water because "back in the day", lead pipes were sometimes used for water and hot water made more lead in the water.

I'm pretty sure the Flint, Michigan water crisis was caused by lead pipes still being used. - https://www.theguardian.com/environment/2016/jun/28/lead-drinking-water-level-nrdc-report-flint-crisis

Mike Jay wrote:If so, are you supposed to make soup starting with cold water?

I always start with cold water when cooking.

Mike Jay wrote:Copper pots, tureens and other copper cookware would be suspect as well.

Yes I agree, and that is what I'm unable to find a trustworthy answer to.

Mike Jay wrote:

I'm thinking we're getting away from the original subject...

Agreed.  

On the builditsolar site, they have plan for a lot of batch-type heaters.  I would think some of them could be adapted to use the stove pipe as the form of heat, rather than the sun, and with that, you could solve a lot of the problems some people have had of pipes freezing and the like, because you don't have any piping outside.

Rather than thinking water in to the top and out the bottom for your drip coffee maker how about from bottom out the top using thermosiphoning to take the water to the ground basket? That would eliminate the closed loop steam problem.

I was imagining open copper piping at both ends.  

I've seen thermosiphons and some seem to spurt water out.   But maybe it was that one.

How would one get the siphon to develop?  By capillary action?

I wouldn't advise using copper.

https://articles.mercola.com/sites/articles/archive/2010/02/11/copper-pipes-in-your-home-may-cause-heart-disease-and-alzheimers.aspx

http://www.health.state.mn.us/divs/eh/water/factsheet/com/copper.html

https://www.cdc.gov/healthywater/drinking/private/wells/disease/copper.html

https://www.ncbi.nlm.nih.gov/books/NBK225400/

Copper stills are used in the distillation process to make Scotch whiskey.

The average daily intake of copper in the USA is 1-1.6 mg/day. The daily upper tolerable limit is 10 mg/day.
If one were to lose 10 mg/day from the copper tubing, it would soon disappear I suspect.

https://www.ncbi.nlm.nih.gov/books/NBK222312/
Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc.

Beer brewing vats were copper too, still call them coppers. As far as a thermosiphon it wouldn't be capillary action, once the water is heated it travels up the tubing (think Mr Coffee) and then would be directed into a ground basket. The water would be in a reservoir below the last run of your coil and your receiving vessel would be of the same size as the resevoir. It would make a pretty cool Steampunky coffee maker.

I still can't picture how this would work.  A thermosiphon needs the siphon to be present before it works to recirculate the water.

How about this?  There's a spiral copper pipe wrapped around the flue. The open top end points downwards into a feeding vessel, a cup which is empty.
The bottom end is above the coffee grounds filter.
Fill the top vessel up.  The copper pipe is about 200 deg C and starts to heat the water up.  Now we already have a copper element heating up the top container.
Have a metal plunger/syringe that then sucks the water down from the bottom opening just enough to start a normal water siphon with some preheated water.  If it's too hot it's going to evacuate at the top end since there's less pressure there ( have the top container closed off to contain any explosion of water).
Otherwise the siphon completes and the top vessel empties breaking the siphon. Drink.

A drip coffee maker heats water that travels up a tube (thermosiphon) and dispenses the hot water over the grounds and then into a catch vessel. Once the coffee maker is empty the heating coil is still hot but all the water has been transferred to the receiving vessel. That's why they burp and fart at the end of the brew cycle all the water is being sent to the carafe. No moving parts, no need to hover, sit back and have a cup of joe.

Ok, I sort of understand how these dripolators work.

I wonder though if you immerse a copper pipe which is 250 deg C into water whether you might just get steam instead rising up the tube.  Perhaps that could be condensed and then let drip like a dripolator?

Travis Johnson wrote:Most woodstoves are only 50% effecient, BUT because the cost of fuel is so low, it makes it an overall better use of fuels then anything else, like say a propane heater to heat a home. Firewood is cheaper then propane, so wasting 50% of it is not a big deal.

We use a little Morso Squirrel to heat the main house.



Because it utilizes current burn technology it's rated at 76% net efficiency. But it's almost irrelevant as wood here is a waste product that almost everyone is trying to get rid of, so we have an endless supply of free firewood.

I do however refute the claims that tankless water heaters are more effecient; that depends on the size of the household. I have a family of (6), with 5 being women. That means we consume a lot of hot water in a day, so after reviewing my options, it was determined that I would save more money by having a tank type heater then tankless. That is because while a tankless does not have to keep a tank of water hot 24/7, it takes a lot of BTU's (fuel consumption) to make instant hot water. Effeciency also comes from how far the unit is from sinks, showers, appliances, etc. So it is not clear cut. It really depends on teh application.

We also like the old fashioned tank type water heaters. They have some distinct advantages over tankless. The most important one for us that it still heats water even when the power is out. We're in a canyon so it's like being at the end of the extension cord. Someone trips over it and we're the first ones with no electricity. Another advantage is if we lose water service, there's always 30 gallons of water safely stored for use in an emergency.

Graham, just putting the heated coil into water might generate steam initially, but eventually the water and coils temps would equalize. Heating the coil would draw the cooler water in from the bottom and out the top. Imagine your flue pipe with two shelves on either side one slightly higher than the other. A coil wrapped around the flue will draw from the lower vessel and into the higher vessel.

Let's see if I can remember how to upload a pic on Permies. You realy could make a cool looking coffee maker if you're into steampunk or just a simple basic functioning coffee maker.

I usually upload to imgur and link from there.

I guess the other issue relying on thermosiphons might be the speed.  If I had a container with a small tap at the top, I could slowly open the tap to let water drip down the pipe which I think would be faster.  The water at the top would prevent steam as the other side is open.  So, steam exits the bottom over the coffee.

Graham,
I'll let someone else take it from here. Steam in a pipe below a feed resevoir kinda scares me. I personally do not believe that is a good idea.

Yes, I agree that there is the possibility of the water exploding in my face.  But if the top container is open vented, and the water is trickling in from the top it's going to steam either as soon as it hits the copper, or shortly afterwards, and in the latter case, there's a plug of water now above the steam.  The steam has a free path down the bottom of the pipe and should exit there.  And it won't reach more than the flue temp which in my case is 250 deg C max.

Anyway, next chance I get to go to town and purchase some copper piping, and I'll go to town on this   I can gradually test it as the flue temperature rises.  Full protection of course.

At some point your resevoir plug of water above the steam coil is going to be overrun. That will depend on the length of coil and friction/weight associated with the volume of steam/water in the coil versus resevoir water weight.  Be careful Graham.  

Yeah, at some point the pipe will be open at the top end when the reservoir runs dry.  And then the steam will eject upwards.  So, I'll need a loose fitting lid if I were to do this inside.  But first runs will be done outside to assess all the risks.

Although copper will transfer the heat much faster from the flue to itself, I would imagine stainless steel or aluminium would heat up slower but get there in the time frames I'm working with.  So, those are possible choices as well.  So, I think it really depends on what is easier to bend.

I've got a mate who has a couple of metres of 1/2 inch copper tubing he says I can have but he thinks I won't be able to bend it around the flue.  I guess we'll see.

Here's a way to bend tight small radius copper coils without kinking the tubing.

If the copper is malleable, then it will bend.  Filling  it with water or sand are methods to keep it from kinking while bending.  For water, put a valve on one end or a threaded cap so you can leave little to no airspace, rather than trying to solder a second cap on.  With the fine sand, you can solder it full to the top.

If the copper a person has is sticks of hard pipe, they may be able to anneal it for easier bending with a blowtorch, or even a campfire’s  flame to make bending easier.

Now that i’ve shared the methods i’ve used for bending copper, i’ll watch that video above and see what his method of bending copper is.

When water turns to steam there is a 1700 to 1 change in volume and that is at 100c. There is a lot of horsepower in steam. So a milliliter of water will expand to 1.7 liters. so for a visual here you have talked 1/2 tube basically a roll of dimes so in a volume of 1 roll of dimes water to instant steam would change that roll of dimes to 1700 rolls of dimes, so from 3.5 inches roughly to 5950 inches of dimes in volume.  Now that wouldn't happen depending on ambient water temp volume introduced and actual temp of the tubing and ability to maintain heat.. Opening an overhead vessel with a valve to tubing heated to 250c will be initially pretty exciting there will be a cool down with the introduction of the water so depending on the tube diameter you might be making a horizontal aeolipile. Steam can be a volatile force.

I am particularly fond of the idea of having a copper coil heat exchanger suspended in an open top water tank that is the heat limiter, and “ballast” mass, sitting atop a masonry walled bell, for additional mass, with insulation on the outside of the bell, and an “openable for inspection/service” insulated cover over the top of the tank.  The large volume of water in the tank becomes a significant portion of your mass, with the masonry storing additional heat.

 As long as you check to keep the open tank topped off before every fire you KNOW the water in the closed system copper coil in that water bath will never be exposed to more than 212F no matter how hot or high energy input your flue gas stream.  The open bath tank could be sized for the system’s output such that no more than a small percentage of the water in the tank could be boiled away, even through a full batch box burn, without any heat extraction from the coil.

 Would be good to have an exterior sight glass for monitoring “ballast water” level in the tank, with a manual fill valve on the exterior from your cold water supply, an overflow drain on the ballast tank, and a T&P safety valve located near the coil on the closed system in case anyone ever started the system with no ballast water in the tank.  

 An automatic fill valve could also keep the ballast topped off, if it can withstand the 212F in the tank, or be controlled by a float from outside the tank.

 Another idea I had for additional safety was for the system to have a float in the ballast tank close off the system air intake if water was below the heat exchanger top.

The other cool thing about this type of system is with it in a tank of water, you don’t have to worry about soldered joints failing/being destroyed, if you are a cheapskate scavenger like me, (ahem... environmentalist!) making your coil out of scrap, like you do if you start a fire in a “flue gas exposed” heat exchanger system with no water in the pipe.  So soldered joints can be reliable in the heat exchanger in this type of system.

One could also solder bare solid conductor wire on/around the coils connecting them and providing a mesh of additional heat exchange surface...

 Wrap around once at each coil and go to the next and continue, then repeat, and solder it all on, you will have a “comb” of wires all spaced a wire width apart, and greatly increased heat exchanger surface area.

 My rocket will be of this design, outside my house, like an exterior wood boiler, with water being pumped to “thermal mass tanks” in various parts of my house, and to hydronic floor heating when and where I am able to install it

I picked up a 1 m length of 15 mm copper pipe.  I'm going to heat it to 250 deg C in a fire and quench it in cold water to see what happens.

Matthew Goheen wrote:I am particularly fond of the idea of having a copper coil heat exchanger suspended in an open top water tank that is the heat limiter, and “ballast” mass, sitting atop a masonry walled bell, with insulation on the outside of the bell and an openable insulated cover over the top of the tank.  The large volume of water in the tank becomes a significant portion of your mass, with the masonry storing additional heat..

So this is a copper pipe closed at both ends that is exposed to the heat source, and just lying like a kettle stainless steel heating coil?  Like an electric hot water cylinder minus the insulation?

Greg Mamishian wrote:Here's a way to bend tight small radius copper coils without kinking the tubing.

Yep, that's the video that sent me on this path! lol

Graham Chiu wrote:
So this is a copper pipe closed at both ends that is exposed to the heat source, and just lying like a kettle stainless steel heating coil?  Like an electric hot water cylinder minus the insulation?

No... the coil is connected to the closed system, ie, one’s household plumbing circulated by a pump to your insulated hot water heater tank, or as in my case it could instead be connected to other tanks that are open topped insulated mass storage tanks or uninsulated “radiator” tanks around the house...  my house is spread out with at least 4 different areas to heat

For homeowners insurance reasons, my batch core will be outside the house and in an all masonry and metal enclosure, like a wood boiler system... with the insulated masonry bell

 It will have a group of 4 55 gallon drums insulated, with their own heat exchangers to preheat (hopefully fully pre-heat) domestic hot water going into my tankless hot water heater, and the water be directly circulated to the radiator tanks around the heating zones of the house as well...

Graham Chiu wrote:I picked up a 1 m length of 15 mm copper pipe.  I'm going to heat it to 250 deg C in a fire and quench it in cold water to see what happens.

I thought water or oil quenching hardened metals?  I believe it does for steel or iron?  I haven’t searched annealing copper to learn whether it’s possible, even, so if that’s the method for annealing hardened metals then I may have it backward, or steel and copper are different.