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.
By the MOTHER EARTH NEWS editors
January/February 1984
Add to My MSN

Then plaster of paris is worked in around the tubing.
MOTHER EARTH NEWS STAFF
Slideshow


Content Tools

Related Content

Firewood Splitting Tips

Steve Maxwell, Canada’s Handiest Man, provides tips on splitting firewood for home heating via woods...

Go Ahead … Use My Home Urinal

Home urinals are catching on. First found in high-end homes, many people are installing them when bu...

Share Your Thoughts on Woodstoves

Do you have a woodstove or pellet stove at home? We’d like to hear more about your experiences with ...

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.

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. 

Construction Tips 

The components of the heat exchanger system are shown in the accompanying illustration, but of course each installation will require some alteration of the basic dimensions. For example, if your stove is larger than ours, you might be able to enlarge the panel enough to get a full 60-foot coil of the 1/4" soft-copper tubing inside the up-sized exchanger framework. Those of you who have smaller heaters, however, will have to use a smaller amount of the line.

In any event, it's easiest to work with the tubing as it's been coiled for shipping. We just laid the curled line into the frame and gently bent the tubing to fill the rectangular shape. The flexible material can be arced down to about a 1-1/2" radius without kinking, so it'd not difficult to force it into any potential "hot spots". We worked from the outer edges inward, wiring the coils to the backing plate as we went. (Without the wire to hold the outer circles of tubing in place, the whole thing wanted to spring out of the frame.)

Once you get the copper tubing evenly distributed inside the frame, stir up a thin batch of plaster of paris and pour the mixture into the frame. Level the surface by running a straightedge across the angle iron, and allow the material to dry for a couple of days. Then the panel can be attached to the side of your stove, and the 1/4" lines can be plumbed to the preheater tank's 1/2" tubes.

Safety and Performance

We ran extended tests to determine the most effective configuration for the exchanger and to satisfy ourselves that the device would perform safely. For example, to see what would happen if a power failure shut down our pump, we sealed up the tubes exiting the preheater tank and installed a pressure gauge on the relief valve. The highest pressure we were able to develop in the system was 3 PSI . . . and that was after allowing flow to stagnate for eight hours at the highest possible burn rate for our Atlanta Stove Works Catalytic!

Also, to determine if conductive heat exchange through the wall of the stove was being encouraged to an unhealthy extent, we checked the inside of the woodburner's firebox each day for increased creosote accumulation. We found no difference in the appearance or depth of the deposits on any of the four walls, which suggested that the exchanger was receiving mostly radiant energy from the exterior stove wall. (The ceramic may have exerted some insulating effect, counterbalancing increased conductivity.)

How much hot water will the exchanger produce? Well, during a typical seven-hour cycle we would load 55 to 60 pounds of wood into the Atlanta Catalytic, which would boost the 42-gallon tank's contents to just short of 140° F. This eight-pound-per-hour burn rate is probably somewhat higher than most folks use, so the volume of hot water that you might get from a similar unit could be slightly less. Of course, if you maintain a strong burn all day long, the total over a 24-hour period should still be more than 100 gallons per day of plenty-hot water. And even if you often operate your stove in a "closed down" condition, the system will significantly reduce your utility bills.

Depending on the size of your family and the amount of water each person uses, the system could eliminate your wintertime hot-water bill. Consequently, if you're able to get your wood at a price that's substantially lower than that of an equivalent amount of electricity or gas, the energy you put into warming water from your woodstove (which, of course, will be subtracted from the space heat the appliance would have delivered) will be well worth the investment. Besides, you'll have the satisfaction of knowing that you've taken one more step toward replacing nonrenewable sources of energy. 

Materials List

pump (Richdel R798)

thermostat (Dayton DE158)

(50') 1/4" Type L copper tubing

16-gauge 2' X 3' steel

(8') 1/8" X 1" angle iron

pressure-relief valve

(14) 1/4"-20 X 3/4" bolts with nuts

(3 qt.) plaster of paris

(6') baling wire

miscellaneous plumbing


Previous | 1 | 2 | 3 | Next






Post a comment below.

 

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.








Subscribe Today - Pay Now & Save 66% Off the Cover Price

First Name: *
Last Name: *
Address: *
City: *
State/Province: *
Zip/Postal Code:*
Country:
Email:*
(* indicates a required item)
Canadian subs: 1 year, (includes postage & GST). Foreign subs: 1 year, . U.S. funds.
Canadian Subscribers - Click Here
Non US and Canadian Subscribers - Click Here

Lighten the Strain on the Earth and Your Budget

MOTHER EARTH NEWS is the guide to living — as one reader stated — “with little money and abundant happiness.” Every issue is an invaluable guide to leading a more sustainable life, covering ideas from fighting rising energy costs and protecting the environment to avoiding unnecessary spending on processed food. You’ll find tips for slashing heating bills; growing fresh, natural produce at home; and more. MOTHER EARTH NEWS helps you cut costs without sacrificing modern luxuries.

At MOTHER EARTH NEWS, we are dedicated to conserving our planet’s natural resources while helping you conserve your financial resources. That’s why we want you to save money and trees by subscribing through our earth-friendly automatic renewal savings plan. By paying with a credit card, you save an additional $5 and get 6 issues of MOTHER EARTH NEWS for only $12.00 (USA only).

You may also use the Bill Me option and pay $17.00 for 6 issues.