A Woodstove Water-heating Attachment

Enlarge Image Image Gallery FAR LEFT: First, the coil of copper tubing is positioned inside the angle- iron frame. LEFT: Then plaster of paris is worked in around the tubing. LOWER LEFT: Water warmed in the coils is piped to a storage tank that feeds into the main heater. BELOW: The assembly simply bolts to the side of the woodstove.

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This sensible approach to heating your water could drastically reduce your monthly utility bill!

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, heating domestic water with a woodstove 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 circulaton have been developed.

YOU CAN HAVE
YOUR HOT WATER . . .

The majority of the modern woodstove 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.

. . AND (ALAS:)
 YOUR CREOSOTE, TOO

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.

SAFE AND SANE

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.

A 10-gallon-per-minute pump installed on the heater's drain circulates water through the coil and back to a "T" just below the pressure-relief valve at the tank's top (the valve was retained as a safety precaution). Cold water enters the vessel through the normal inlet, and the wood-warmed water moves on to a conventional electric heater through the standard hot outlet. All of the lines are well insulated with 1"-thick high-density foam.

Of course, if the water were circulated constantly, heat could be lost to the stove when no fire was burning. To prevent this from happening, researcher Dennis Burkholder made an automatic on/off control from a line-voltage air-conditioner thermostat wired into the pump's power supply line. (You could also use a more commonly available combination heating/air-conditioning control, set on the cooling mode.) The thermostat is attached to the wall three feet away from the heater and about a foot above its top. When the air temperature reaches 80 °F, the 120-volt control turns the pump on, and water starts warming up. The built-in differential switch shuts the circulator back off again when the temperature drops to 76 °F.

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