Using Your Wood Stove to Heat Water

March 1, 2013 · 20 comments

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One of the most overlooked functions of a wood stove is heating water. A few decades ago many wood stoves (especially cook stoves) used what was a called a hot water front in conjunction with a tank (range boiler) beside/behind the wood stove to produce endless hot water.

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Wood stove heating waterThe hot water front (which is nothing more than a small tank with an inlet and an outlet) was placed in the fire-box of the stove and the tank was connected by copper, steel, stainless steel, brass or cast iron piping. None of these systems required pumps or fancy controls.

Instead of a circulating pump, the hot water would rise out of the water front into the top of the water tank. The colder water in the tank would fall out of the bottom of the tank and into the hot water front (small tank). In this system, the water circulates all by itself through the process known as thermosiphoning.  Years ago this type of water heater would have been the primary or only hot water heater in the home. Today these are more commonly used to preheat well or municipal water before it enters the main water heater although they can easily supply enough hot water for a typical family during the cold months your woodstove is required.

SAMPLE HOT WATER FRONTS

 

This hot water front is made from stainless plate and has 1 baffle to force water to travel more slowly and make surface contact with the hot surfaces.

This hot water front is made from stainless plate and has 1 baffle to force water to travel more slowly and make surface contact with the hot surfaces.

sample hot water front for heating water with your woodstove

This hot water front is made from stainless plate and has no baffles. Notice how the water travels from inlet to outlet making very little contact with the hot surfaces.

In the above illustrations is it obvious the more baffles you install in your hot water front the more hot water you can expect it to produce. However one is usually plenty. It gets very complicated to weld, the more baffles you install.

You MUST use pressure relief valves on the inlet and/or outlet tubing.

OTHER WOOD STOVE HOT WATER HEATING SYSTEMS

THERMA COIL (or homemade unit)

Heating water with your wood stove

There is a relatively new product available online called the “Therma Coil” which is a small u-shaped coil made to insert into your wood stove and attach to your range boiler. They can be installed at any time by drilling two holes in your wood stove and connecting the plumbing.

If you wish to make your own hot water coil only use rugged materials inside the firebox such as brass, steel, stainless steel with threaded fittings. Do not use copper pipe with soldered fittings or copper tubing with a 180 degree bend.

Copper is too soft and unforgiving if the pipe were empty of water with a fire going. Soldered joints will simply unsolder themselves immediately if overheated.

The same rules apply as when using a hot water front. You want the system to circulate without any pumps. Mount the U Shaped pipe horizantally as in the photo so there will be a top fitting and a bottom fitting.

The top fitting must be connected to brass, copper, steel , stainless steel or some other metal piping and go to the top fitting on your water tank (range boiler).

The bottom of the U-shaped pipe will connect with metal piping to the fitting closest to the bottom of your tank. The closer to the bottom of the tank the fitting is, the more storage of hot water you will have. If your lowest fitting is 8 inches from the bottom of the tank, the bottom 8 inches of your tank will store no hot water.

We have no experience with them but they look reasonably well made and the concept is proven.

You MUST use pressure relief valves on the inlet and/or outlet tubing.

BRASS FITTINGS PLACED ON TOP OF WOODSTOVE

Another slightly less efficient method of heating water with your wood stove is to make a maze of threaded brass or stainless steel pipe fittings and lay them on top of the wood stove if your wood stove has a flat top. You could also make a coil of soft copper tubing and set it on the top of the wood stove for the same effect.

Although this would not make as much hot water as a water front or loop of pipes inside the firebox, it will make a lot more than nothing and require no drilling or modification of your wood stove. This method of heating water would likely be one of the most insurance friendly systems as you are not modiying your wood stove and you could easily remove it if your insurance company decides to visit for a home inspection.

You MUST use pressure relief valves on the inlet and/or outlet tubing.

3. COPPER COIL INSIDE YOUR STOVE PIPE

Another method of capturing your wood stove’s heat is to make a coil of copper tubing that is just small enough to fit inside your stove-pipe. Usually you would use 1/2″ (13mm) soft copper tubing for this project and you would increase your stove-pipe diameter to make up for the space taken by the copper tubing. For example: If your wood stove’s manufacturer recommends 5″ stove-pipe. you would use 6″ as to keep the correct amount of draft for your wood stove.

BE SURE TO USE REFRIGERATION TYPE COPPER TUBING AS IT IS THICK AND SOFT (MAKING IT EASIER TO COIL)

The biggest issue we have found using this system is the creosite that forms on the copper tubing because of the large difference in temperature between the water and the super heated air going up your chimney. All of the systems mentioned in this article will form creosite to a certain extent but the copper tubing in the stove pipe seems to be the worst.

EVEN WITH SOFT TUBING IT IS  GOOD IDEA TO INVEST IN A TUBING BENDER OR FILL THE TUBING WITH SAND (LIKE SILICA SANDBLASTING SAND) BEFORE ATTEMPTING TO BEND TO AVOID KINKING THE TUBING!

Once a thick layer of creosite has formed it reduces the size of the stove pipe. The other issue is the system loses it’s effectiveness as the creosite acts like an insulator between the hot air and cold water.

You MUST use pressure relief valves on the inlet and/or outlet tubing.

4. COPPER COIL WRAPPED AROUND YOUR STOVE PIPE

This is an expample of a nice looking installation of a coil of copper tubing wrapped around a 5 inch stove pipe and covered with a 6 inch stove pipe. The cold water enters the bottom and the hot water exits the top. The hot water tank is on the second floor.

This is an example of a nice looking installation of copper tubing wrapped around a 5 inch stove pipe and covered with a 6 inch stove pipe. The cold water enters the bottom and the hot water exits the top. The hot water tank is on the second floor.

The last option for making hot water with your stove is to capture the heat from the chimney by wrapping soft copper tubing around it or some other type of heat exchanger. For cosmetic reasons you could add a larger stove pipe to hide the copper tubing.

BE SURE TO USE REFRIGERATION TYPE COPPER TUBING AS IT IS THICK AND SOFT (MAKING IT EASIER TO COIL)

For example: If your stove pipe is 5″, you could install a 6″ over it. This will also act as a double wall stove pipe.

If you decide to capture the heat from the stove pipe and the range boiler needs to be above the copper coil, you will either need to install it (range boiler) on the floor above or use a circulating pump.

EVEN WITH SOFT TUBING IT IS  GOOD IDEA TO INVEST IN A TUBING BENDER OR FILL THE TUBING WITH SAND (LIKE SILICA SANDBLASTING SAND) BEFORE ATTEMPTING TO BEND TO AVOID KINKING THE TUBING!

Like the brass fittings on top of your woodstove this stove pipe system would probably be fine for most insurance companies. You have not modified your stove in any way.

This system should not cause any creosite and should not ever get hot enough to melt the copper tubing if the system was to run dry (without water).

You MUST use pressure relief valves on the inlet and/or outlet tubing.

DESIGN CONSIDERATIONS FOR CIRCULATION

As we are not using any pumps to circulate the water we must be very careful to ensure the following:

  • The tank must be located higher than the wood stove
  • The tank must be located very close to the wood stove to avoid long pipe runs
  • The drain of the water heater must connect to the input of the water front or coil and be higher than it
  • The hot water piping coming out of the front, coil or U shape heater must go into the top of the range boiler
  • All piping must be sloped at least 12″ for every 24″ of pipe run (45°)
  • The larger the piping the better (use 3/4″ or 1″)
  • Avoid 90 degree elbows or other restrictive fittings as much as possible

SAFETY CONSIDERATIONS

  • ONLY USE METAL PIPING. NEVER USE PEX, ABS, PVC from the stove to the tank.
  • Insurance companies will never like any of these ideas
  • Always use at least one pressure relief valve (preferably two) between the woodstove and the tank
  • Use temperature/pressure relief valves to make system safer.
  • NEVER INSTALL VALVES BETWEEN STOVE AND HOT WATER TANK

The best arrangement would be to have the range boiler on the floor above the floor the wood stove is on. That would allow for excellent circulation.

CIRCULATION PUMPS

High efficiency circulating pumpIf your wood stove and water heater are a long distance apart or you are unable to make the system circulate by gravity, you can always use a circulating pump move the hot water back and forth. If you go the circulating pump route, resist the temptation to purchase a standard “off the shelf” pump. As you have to generate the electricity required for the pump you will need a super high efficiency pump.

thermal-snap-switchIf you use a pump you WILL need a thermostatic control to turn the pump on and off. You have 3 options:

  1.   Use a thermal  (low limit) snap switch – A thermal snap switch is very primitive yet reliable way to operate your hot water circulating pump. They are super cheap so buy a few spares.They uses a bimetallic strip much like your thermostat on your electric baseboard heaters to make contact when your room becomes cool except it operates in reverse. As the wood stove (or your water jacket/loop) heats up, the switch turns the pump on and when temperature lowers the contracts open, turning of the pump. I would recommend a turn on temperature of about 100 F if mounted to wood stove as the wood stove will take longer to heat up than in internal loop or hot water front.

Here are some more examples of thermal snap switches.

samples of thermal snap switches

2.   Use a boiler aquastat (water jacket or external) – The most reliable ( and more expensive) method of turning the circulating pump on and off is the aquastat that has been used in hot water boilers for many decades. As the water heats up in a typical gas, wood or oil hot water boiler, the aquastat (a thermostat for water…hence the AQUA) closes the circuit to start the circulating pump. As the boiler cools down, the pump shuts off. These are robust and usually outlast a hot water boiler. The second bonus is they are completely adjustable for on and off temperature. Something to remember…most aquastats are built to turn an oil or gas powered burner which operate exactly the opposite of what we need. You are looking for a REVERSE ACTING (OR LOW LIMIT) AQUASTAT  unless you know how to convert a “normal” one to reverse acting. Honeywell is a common name in aquastats in North America.

They come in external models that come be mounted to your wood stove or external piping using magnets in some cases or hose clamps in older models.

external low limit aquastats for heating water with a wood stove

 

They also come in water jacket models or internal models where the temperature probe is actually in contact with the water of your water jacket if applicable. If you can find an old boiler at a scrap yard you will be able to find a suitable aquastat for little to no money. They almost last forever if kept dry.

aquastats usede for hot water wood stove heating

There are

3.   Use a solar hot water temperature controller – These are awesome while less reliable and bullet proof that the above mentioned items. Google “solar hot water temperature controllers” for more info. There are many.

Do not buy any circulating pump before reading this.

WARNING! PLEASE READ THIS FIRST!!!

When water is heated, it expands. The cold water that enters the system is heated to 120-180 degrees F and expands slightly. This expansion is usually handled well by the same expansion tank that keeps the pressure constant in your water system. This is how a “normal” hot water heater doesn’t always build pressure or blow the pressure valve.

However if that hot water turns into steam it will expand 1600 times. That will not be handled by the pressure tank. Instead, the range boiler, water front or piping will explode. Many people have been killed by this during the past century. Always have a pressure relief valve between the range boiler and the water front and preferably two of them. Steam is very dangerous.

Do not install valves on either pipe between the hot water front and the range boiler. You are just asking for trouble by doing  this. Someone (maybe a child) will close the valves turning your water heater into a bomb!

If your fire is lit and your hear bubbling in the system, it is likely the water is starting to boil. The correction is simple. Just open a hot water tap (or a few of them) and dump that excess hot water. If you don’t (or aren’t home), the pressure relief valve will do it for you. Our forefathers did not have pressure relief valves so they had to be very aware of steam production.

Try this method for making cheap hot water and let us know how it works for you!

Leave a Comment

{ 20 comments… read them below or add one }

Paul July 1, 2016 at 12:22 am

Can I buy something like the stove that was shown in #4 ? thank you

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Jody Graham July 5, 2016 at 5:59 pm

Hi Paul,
Thank you for your question. There is nothing off the shelf you can buy that is like the copper tubing wrapped around your stove pipe to heat water. What you need is a soft roll of copper tubing (usually 3/8″-5/8″ inside diameter) and wrap it as neatly as possible around your single wall stove pipe.

You must use single wall stove pipe and NOT double wall as required by some insurance companies. Then it is a good idea to cover it with a stove pipe large enough to account for the extra diameter created by the soft copper tubing.

The cold water inlet will be on the bottom (which connects to the bottom fitting on your tank) and the hot water outlet will be at the top (which connects to the top of the water tank). Hope you are able to do this or find someone locally to make it for you…Jody

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Del Williams July 8, 2016 at 10:42 am

Good tip. Having worked with soft copper pipe a few times, I quickly learned that it very easy to kink it when attempting to bend it. To prevent This, fill it with sand, cap the ends, then bend it.

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Jody Graham July 9, 2016 at 7:43 pm

Hi Del,

Thank you for the helpful tip. You are right. Soft copper tubing as used to wrap around a stove pipe can be kinked easily. Using a tubing bender or filling the pipe with sand (and capping it so the sand cannot get out) will prevent the tight bends from making a kink in the tubing. Once tubing has been kinked it is very difficult to fix the kink or at least fix the kink and make it look pretty. I will add that to the article when I get time and again I thank you for your helpful advice…Jody

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Donavin May 3, 2016 at 10:52 pm

What an awesome and insightful site.
What I’m trying to do is heat water to keep it from freezing outside during the winter for my cow.
I haven’t purchased anything yet but have an old tub to use as a trough. I also have an abundance of firewood.
Any ideas or experiences will be greatly appreciated.

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Jody Graham May 5, 2016 at 11:09 am

Hi Donavin,
Thank you for your kind words and comment.
What I would do in your case would be to setup a wood stove (preferably one you are already using to heat your home. Install a hot water front or coil (which is described in the article) and connect that to a large well insulated water tank. Then add a coil of copper tubing into the water of your hot water tank and add a coil of copper pipe (or iron pipe) somewhere in your water trough being sure to protect it from the cow. Somehow cows tend to destroy things like this.

Connect the two coils of copper with PEX pipe and add a high efficiency pump (high efficiency pumps article 1 and article 2) and an aquastat (water thermostat). When the water in the tub gets close to freezing, the pump will turn on and circulate the water from the hot water coil to the cow’s water tub. I know it may sound complicated but if you draw it out it really isn’t. You could also add another copper coil in your hot water storage tank and use that to feed your standard water heater or radiant heating system. Hope this helps…Jody

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Del February 27, 2016 at 5:37 pm

I’ve installed two systems like this over the years. Both involved a range boiler near the wood stove to preheat the water going into the conventional hot water heater.

The first system I installed used a water coil that bolted to the outside of the wood stove. This was never very efficient. At best, it created ‘warm’ water in the range boiler despite running the wood stove to heat the house 24 hrs/day. We then moved to another community and bought another house. The previous owner had installed a water coil and range boiler, but it was dangerous! The range boiler was not positioned above the wood stove so constantly got vapour locks and we could hear the water boiling. I dismantled the system and re-positioned the range boiler on a brick pedestal slightly higher than the wood stove. As well, I made a new water coil out of 3/4 inch soft copper pipe. This was installed inside the wood stove firebox . The rebuilt system kept our family of 5 supplied with hot water all winter with no problems! We have since sold that house, but the new owners are still enjoying the system.

Here is a hint for bending soft copper pipe: Fill it with sand, and solder end caps on to keep the sand in. Now you can bend the pipe around a jig without it flattening or pinching shut as you bend it. Remove the end caps and draine the sand out when you have it bent the way you want it.

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Jody Graham May 5, 2016 at 11:18 am

Hi Del,
Awesome. You have done everything right and fixed all the previous owner’s mistakes. I don’t think people realize the massive amount of hot water these systems can make. If you are running a wood stove anyway you might as well enjoy the hot water too.

Thanks for telling our readers about the trick for bending copper tubing. We have used that technique for years but only after making crappy and kinked bends many times. Thanks so much…Jody

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Bill January 27, 2016 at 3:47 pm

I am curious if anyone has attempted to use a system such as this to heat a cast iron radiator heater in a more remote area of the home that doesn’t receive as much of the wood stove radiant heat such as an upstairs room. In other words, as a means to transfer the heat elsewhere. Any experiences or thoughts on this would be appreciated.

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Jody Graham January 27, 2016 at 4:17 pm

Hi Bill,

Using a radiator to move the heat from your wood stove to an unheated room is an excellent idea and has been done many times throughout the ages. We have done some testing with this and have a few suggestions.

1. This system is a closed system and can become a bomb if the water in the system is allowed to become steam. You MUST add at least one high pressure/high temperature pressure relief valve. Using the pressure relief valve will dump the steam in the event the circulating pump or aquastat fails.

2. You will need a circulating pump and aquastat (thermostat) to circulate the hot water from the wood stove to the radiator unless the radiator is located above the wood stove. If the radiator is above the wood stove, you can design your system to thermosiphon (circulate automatically as the hot water will rise from the wood stove to the radiator and the cold water fall from the radiator back to the wood stove).

3. You can also use a fan, thermostat and duct work to move hot air from your wood stove to your colder room. This is a much simpler and safe system.

4. If you decide to use water as your medium to transfer heat, then be sure to use a high efficiency pump such as one made by Ivan Labs or ecocirc.

If others have anything to add please do so. If there is enough interest in this topic, we will write an article regarding safe methods of moving heat from your wood stove to another room.

Good Luck and keep thinking outside the box…Jody

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Bill January 28, 2016 at 1:59 pm

Hi Jody- Thanks for the response/info. A couple issues I have with the duct/fan/thermostat concept: 1) Depending on how it’s done etc, I wonder if this could not actually be more dangerous (ie. if you have a chimney or other fire, this set-up could allow the fire to spread faster and to other areas of the house), 2) it doesn’t seem terribly practical in most settings because if you are considering doing this in the first place I would imagine the room you are trying to heat remotely is quite a distance from and has many walls and levels separating the stove. Generally speaking, I would think it’d be easier to run some pipes than running ductwork.

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Jody Graham January 28, 2016 at 5:57 pm

Hi Bill,
You have some good points. It is possible the duct work method could spread a fire more quickly but I am not sure how much faster if at all. Moving air is less complex than moving water in this case as you will only need a fan and a thermostat. If the fan or thermostat fails, nothing will happen. If your thermostat or water pump fails you will be dealing with a house full of steam if you used a pressure relief valve. If you did not use a pressure relief valve, you will likely arrive home to a large hole in your home as if a bomb detonated.

The other issue with a closed loop of water is expansion. As water heats up, it expands. You will need an expansion tank just like the type you would see on a typical boiler heating system or you will continually have steam/water leaking from your pressure/temperature relief valve.

The last issue with a closed loop water system is air. Any air in your system will cause the pump to fill with air and stop pumping. To avoid this, mount the pump low in the system (as air rises it will go through the pump and go to the top of the loop) and add an air bleeder valve to your closed loop. Some bleeders will dump the collected air automatically and some will require you to manually open the valve or remove a threaded cover every so often to remove any air from your system.

You are correct…running small pipes is much easier than running large ductwork.

Usually a high efficiency pump such as one made by (Ivan labs or ecocirc) will consume less power(and be much quieter) than a fan unless you use a 12 or 24 volt (DC) direct current SNAP-FAN which are very efficient…Jody

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The Dude January 25, 2016 at 4:16 am

Now as “Paul Harvey” God rest his Soul would say… The Rest of the Story, Mine… I have a “Root Cellar”
built into the hill side IE: terrace immediately behind the back of my utility room. 19′ long 7’6′ wide and 7′ high.
All of concrete block with a poured floor. I acquired a 10 x 10 outside Beer Cooler with 4″ walls that I am using as all walls , floor, and top with dimensions being 58″ Deep 80″ wide and 90″ long retained by dirt on outside
walls and 8″ cross rebarred walls lined floor, sides, ends, and top. 417,600 cu” 1807 Gals. 15000lbs. of water.
My Boiler is a 55 gal “Recovery Barrel” 4″ taller and 4″ wider than the standard 55 gal. I have my local Steel
shop bending up 22″ loops as close to 3″ spread which will line my Fire Bricked barrel from front to rear with
continuous loops, Inside ! out of Schedule 40 Iron Pipe. The pump from Northern tool is 21 G per Min.
Will have to let you how it all turns out,

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The Dude January 25, 2016 at 3:24 am

Just read your other commentators. Let me share some thoughts and friends experiences. My friend has taken
a broken A/C window I had along with a 55 gal. barrel he has installed a 25′ loop of type “L” cooper spread out
egg shaped with apx. 2′ out the rear top suspended by 2 pieces of rebar close to the top out the barrel. Now for
A/C, He has removed the compressor and soldered the “big” suction line to 1/2 copper tubing (easy} and then
the 3 small “capillary” tubes (all from the front IE: the Evaporator Core) by inserting into the 1/2 and then smashing the 1/2 copper then soldering. This is now the “Air Handler’ mounted in the rear of his 25′ 5th wheel and using insulated PEX tubing from Boiler to insulated tank the “holding tank” a 55 gal poly barrel encased in “Great Stuff” foam from a can.
With a “Magnetic driven pump” @ 6 G per a min. (remotely mounted in a 5 gal. bucket) Bottom Line:
It WORKS with thermostats that doesn’t turn on the A/C’s fan till 12o’ is present and has achieved 130′ deg
output. Now the BAD, have to refuel barrel every 6 to 8 hours not leaving much time to sleep.

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The Dude January 25, 2016 at 2:19 am

Very well written and informative. I am sharing your web site around,
Thank You
Mark

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Jody Graham January 25, 2016 at 2:06 pm

Hi Mark,

Thank you. We are just trying to help everyone in their quest to become energy independent and want to ensure folks don’t repeat our mistakes. It is all a learning process. We have so much to learn and continue to try things that sometimes work and sometimes don’t. I will try to answer your other comments/questions as soon as I can. Keep up the good work and open mindedness…Jody

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Roy Underwood July 31, 2015 at 1:20 pm

Hi! This is my 1st comment here so I just wanted to give a quick
shout out and say I really enjoy reading through your articles.

Can you suggest any other blogs/websites/forums that
deal with wood stove water heating? Thank you so much!

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cam mullally May 20, 2015 at 4:31 pm

I have heated water for free and I mean FREE for years at my off grid cabin. Water is gathered as rain into gutters and down into cisterns. April through October its solar and November through March it’s the wood stove (copper coil in the flue). No pump, just thermo siphoned. Free and works beautifully. Have had to learn some lessons about plumbing along the way, some of them hard. Pex or cpvc upstream from the woodstove? Disaster! Pressure Relief Valve in system, but not in the heating loops. Maybe should put one in. My biggest challenge is freeze, far and away. Many vacuum relief valves and drain valves all through the cabin.

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John Messick October 7, 2013 at 12:55 am

I have converted my old AC unit to a wood stove by taking out the coils – compressor and placing two welded drums together for the stove. Inside insulated box house.
The existing duct works and blower will be used off the thermostat. When heat is called for the fan and small furnace blower = 2″= will come on.
I am going to use your system to heat hot water. Using my existing refrigerant heat tubing. “Heat Exchanger”.
Thank You, John Messick

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Loren Amelang May 30, 2013 at 12:19 am

I started my off-grid adventure with a water heating insert in my (Elmira Oval) cookstove. I suspect if you live in a climate with serious winter, where you keep the stove going constantly for months at a time, the concept works. In California it is just wrong – it works too well! When you build a nightly fire for a bit of warmth, the water heating load sucks all the heat and you literally can’t fry an egg on the stovetop until you’ve heated all the water. You can’t just turn off the flow, because as this article warns, you’ll get serious steam.

The next morning when the sun comes out and the solar panels take over, the storage tank is still hot and the free heat is wasted. Totally incompatible with solar. The insert is still stored in the attic, like new…

The other problem was having the storage tank above the woodstove, as required for convection. Putting 600 pounds on the second story is a bad idea for many reasons, not least of which is that when it eventually ruptures it makes an incredible mess. The tank was too far from the places the hot water was used, and hot water really hates to flow downhill, so not only did it take forever for the taps to get warm, but the hot water flow was always weak despite 3/4″ pipe everywhere.

My solar backup is now a serious outdoor water heating stove, that can take 30″ logs and only runs when there is no hope of solar heat. The cookstove is dedicated to cooking.

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