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



Woodstove Water Heating Copper Tubing

First, the coil of copper tubing is positioned inside the angle-iron frame.


PHOTO: MOTHER EARTH NEWS STAFF

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.

fiddlesticks
8/2/2017 7:11:07 PM

This is brilliant. Do you know if we were to do this in a narrowboat would you still require a preheated and main heater tank or would it be sufficient to connect it directly to a calorifier? And how do you calculate what size tank should be used?


livinghealthy
6/11/2014 10:46:58 AM

This is great! I have a wood burning stove and it's just perfect for those nights by the fire. A great heat source too! I'm always looking for ways to cut down on my heating bill, such as using http://www.underfloorheatingsystems.co.uk, solar panels and more. Energy bills are always on the rise so its important to keep on top and have a green footprint.


jeff arthaud
2/28/2014 11:30:27 PM

I'm thinking of adding another 500 gal.reservoir closer to my house from my primary reservoir which is from a spring It would sit on the hill above my house. Split that water to cold then to a cook stove I want to purchase, from there I send the hot water to each bedroom where I hook up an water heat exchanger (copper radiator). Then the hot water goes to my hot hot water tank, which I would turn off during the winter time, so it would just be a tank for the hot water. I would install a sink, in which it's primary job is to drip the hot water which keeps the water moving through the lines, it would empty in a tank below the sink so that it could be recycled back in to my secondary reservoir. I do realize that I would now need to move my water pump between the reservoirs and a small pump to send back the secondary reservoir. With the system moving, it would prevent pipes from freezing and also keep the system from building up steam. What do you all think, does it sound workable and or safe.


brady
1/23/2014 9:48:18 AM

is it possible to run the water through the pipes cast in plaster on the side of my stove and go directly into my baseboard heat system? Matbe increase pipe size to half inch? build two heat exchangers for the stove each would be 18" x 18"


solardave
12/10/2013 12:05:51 PM

Brilliant idea! I may have to do this myself for my "mancave". An alteration I am considering (there's always another way!){:-): Instead of coiled 1/4 soft copper, build a grid of the type used in solar thermal panels, with upper and lower headers (perhaps 3/4"), and multiple vertical risers (1/2"). Connections "kitty corner" on lower left/upper right. A little pricey for the fittings, but... Cast this into the gypsum. As long as the HW tank is raised suitably above the heat collection panel, you'll get thermosiphon flow w no circulator. The larger pipe diameters/lower headloss make thermosiphon flow easier. You'll need to have a stopcock (automatic, or just manual if you can be vigilant), or maybe a swing check valve, to disable reverse (cooling) flow when the fire is out.


kip worden
1/17/2012 1:41:29 AM

Could you put the water heating attachment on top (horizontal) of the stove and still get a thermosyphon? I don't have space on the side of back of the fireplace insert. My water storage tank will be about 2 feet above top of insert and about 3 feet away? Also, recently purchased "lehmans.com" hot water/wood stove booklet. Has some good info, especially on safety valves.


bob doyle
9/29/2011 2:16:04 PM

Doug, since the pipe run is not straight I would think you would definitely need a pump. I would look at Build it solar dot com and see some of the pump options they use there. My question/concern is that with all those spots for water to collect, the bottoms of each coil, does the possibility of steam build up pose a danger?A second question is that the back of my Jotul has a heat shield. Could the heat tubes be mounted to the back, so that heat output would not be lost from the sides?


doug h
8/15/2011 10:06:12 AM

in your home made woodstove water heater article,would you still need the pump,if the pre heat ta nk was elevated,by 8 feet?please let me know,thanks.


jerry_3
11/30/2008 7:52:27 PM

Is the copper tubing 1/4 OD or ID. Thanks, Jerry


michael sakowski
11/1/2008 10:56:02 AM

We are stuck with an oil hot water heater. Our electric is not up to speed so an electric hot water heater is not an option. Considered propane, but this is another crude oil derived product. Natural gas not available. Since our wood stove is not near our hot water heater, I decided to utilize some of the waste heat going up our chimney. I made an enclosure around the vent pipe on a thermostat - this kicks on and blows hot air over my temper tank, warming my cold water to around 70 degrees. I calculate this saves 30% and my other measures can save up to 20%. To see how I have rigged this up, go to http://www.savehouseholdenergy.com/save-hot-water.html I got this idea from some of the plans here on Mother Earth News that make use of wood stove heat.






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