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.

scotzniovsky
3/13/2018 5:42:48 PM

Great Idea and a really nice writeup. I'm aware that heating the surrounding air is still the main purpose but your efficiency to heat the water is really low: one lbs of wood is about 6000BTUs of heat, at 60 lbs that would be 360000 BTUs. Heating 42lbs for water from 40 to 140 degrees is 42 * 8 * 100 = 33600 BTU. So your efficiency is < 10% Seems perfect for your application but definitively will not work for any hydronic heating application.


scotzniovsky
3/13/2018 5:42:31 PM

Great Idea and a really nice writeup. I'm aware that heating the surrounding air is still the main purpose but your efficiency to heat the water is really low: one lbs of wood is about 6000BTUs of heat, at 60 lbs that would be 360000 BTUs. Heating 42lbs for water from 40 to 140 degrees is 42 * 8 * 100 = 33600 BTU. So your efficiency is < 10% Seems perfect for your application but definitively will not work for any hydronic heating application.


scotzniovsky
3/13/2018 5:41:58 PM

Great Idea and a really nice writeup. I'm aware that heating the surrounding air is still the main purpose but your efficiency to heat the water is really low: one lbs of wood is about 6000BTUs of heat, at 60 lbs that would be 360000 BTUs. Heating 42lbs for water from 40 to 140 degrees is 42 * 8 * 100 = 33600 BTU. So your efficiency is < 10% Seems perfect for your application but definitively will not work for any hydronic heating application.






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