Build a Woodstove Water-Heating Attachment

Follow these tips to construct a woodstove water-heating attachment and save on utility bills. Includes a materials list and diagram.

| January/February 1984

One of the advantages of heating with wood is the variety of needs that just one stove can meet. Besides keeping us warm, a woodburner can cook dinner, dry clothes, and toast chilly toes. But wouldn't it be just dandy if that black box would draw a nice hot bath, too?

Actually, domestic woodstove water heating is nothing new . . . many cookstoves had water-tank attachments more than a century ago. The advent of the "airtight" woodburner and pressurized water systems has left most of those old batch-heating techniques by the wayside, though, and new methods based on closed circulation have been developed. 

Modern Woodstove Water Heating 

The majority of water-warming attachments employ heat exchangers that are fitted inside the firebox or the chimney of the appliance. The best commercial examples of this approach work very well indeed. If the stove is run most of the day, they can supply a whole family's hot water. For safety's sake, however, these devices are usually made from stainless steel (an expensive commodity) and must be pressure-tested to insure that they are able to withstand the very high temperatures they may encounter inside the heating system. As a consequence, quality internal heat exchangers carry pretty hefty price tags. Homemade internal devices, on the other hand, have developed a nasty reputation for scalding steam explosions.

Furthermore, extracting heat from either the firebox or the chimney of a woodstove can have unfortunate side effects: Pulling Btu directly from the fire (with a firebox exchanger) can reduce combustion efficiency . . . and if the products of incomplete combustion are cooled below the temperature at which they condense (by either a firebox or a chimney heat exchanger), heavy creosote accumulation may occur. There is doubtless no need to mention that the combination of a chimney fire and an internal, water-filled heat exchanger can spell disaster. 

Sensible Design

Recognizing the fact that there is no uncompensated noon repast, we adopted a conservative approach to designing our own water-heating attachment for a woodstove. Rather than chance placing an exchanger inside the heater or stack, we attached one to the outside of the firebox. By taking this tack, we avoided making any major modifications to the heater, which maintains Underwriters' Laboratory certification. What's more, a couple of safety criteria that we've already mentioned are met: The temperatures encountered outside the heater's skin won't boil water (as long as that liquid's kept circulating), and the heat used to warm the water is that which would have been radiated by the heater anyway, so no extra heat is being removed from the firebox.

Our water-heating attachment consists simply of about 50 feet of 1/4" copper tubing coiled into a plaster of paris-filled panel. The gypsum-based material helps distribute heat evenly to the coils and allows the exchanger to be in direct contact with the stove body without chancing overheating. (We'd like to thank Ed Walkinstik for this suggestion.) The assembly bolts to the side of the heater and is plumbed into a salvaged 42-gallon water heater (we used one with a burned-out element but a sound tank) in much the same fashion as would be a solar preheater.

2/2/2018 12:42:31 AM

Hi, Lets see if I can explain this properly. I built this system and used it while living in a remote cabin in Oregon some time ago. Heat was provided by a 55 gallon drum style wood stove but I had no running water when I moved in although a spring was on the property. Things I added a 50 foot line of flexible 1/2 inch cooper tubing which I wrapped around the exhaust chimney on the vertical part this is very important as you do not under any circumstances want any part of this system to boil water out of it. Next I placed a 50 gal water heater I picked up for free a safe distance away from the wood stove yet close enough to connect 3-4 feet. Then bought the needed fittings sorry can't remember what was needed, but this is important I went from the bottom drain to the bottom of the coil of copper on the chimney then the top of this line was feed into the hot water exit pipe of the hot water heater which had a tee in it to allow hot water to be used from the system cold water entered through the cold water side. Think about this for a minute. the colder water is pulled from the bottom of the tank out the drain and heated in the copper coil then naturally rises and enters the top of the tank endlessly repeating. In reality the heating coil could have been much shorter, as the water gave very steamy showers. The over pressure valve was still in use on the side of the tank and never blew as the hot water was allowed to continually circulate. Other than the water heater tank copper tube misc fittings I did build a small water tower only a 55 gal plastic barrel which filled off the spring the gave me water pressure. Not a hard to build system to build and if kept so everything has to naturally keep rising it just works actually water races so fast it sounds amazing. I wanted to place a hydraulic motor inline and see what kind of energy I was producing but that's another story. I hope this can help someone.

1/9/2018 9:15:36 PM

I was hoping to find a simple, straightforward way to heat water that wouldn't take up space on my stove. For years, we have heated our water in two five-gallon stainless steel pots that stay on the stove pretty much all the time (unless we need the space temporarily for something else, like a big dinner or dipping candles....). I would very much like to free up the top of the stove, have a larger quantity of water available, and have a way to dispense the water without having to dip it out of the pots. I don't have (or want) electricity, so pumps and thermostats are not an option. The tank will be right beside the wood stove, so will hopefully be something that is not an eyesore. The system I need would be more like the old water reservoirs that won't blow up. Maybe there is another article about a simpler system somewhere.... (?) I'll keep looking...

9/21/2017 2:28:35 PM

"Uncompensated noon repast" is one of the premier commencement scribes ever. I wish to use this system to heat my cement, hydronic piped, floor is is ok to use "1/2 inch rolled copper instead of 1/4 inch? Fred

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