Wood Heat Hot Water: Part I

At a wedding a few weeks ago I met a guy (husband of one of J.’s friends) who is building a system  to heat their domestic hot water via the wood stove.  His set-up involves running a steel water pipe right through the stove (he has welding skills– similar off-the-shelf versions are available, though).  A pump then circulates the water from the hot water tank, through the stove, and back.  A controller senses temperature differential and only turns the pump on when it’s needed.  (In summer, they use the same controller to run a solar system.)  I’ll ask his permission to post a few pictures here– it’s pretty cool.

I love this idea.  I’d like to build a similar system.  And I think I can & will.  But, my ideas sometimes don’t work out in practice– so my M.O., now, is to require on-site proof-of-concept at every stage that involves either (a) irreversible damage to something expensive, or (b) buying parts.  So, before even thinking about drilling holes in the wood stove (which I doubt I can ever bring myself to do), I want to test a version that just uses the stove top for heat exchange.   Step 1 was to estimate how much heat could, reasonably, be extracted from the top of the stove.  Step 1a was to guess:  the Luckily, because it’s almost Thanksgiving, it was easy to procure a cheap aluminum pan that almost exactly fits the stovetop.

A quick test revealed that water can easily be heated in the pan at a rate of about 2,200 BTU/hr, with the stove at medium-burn.  At that rate, it would take about 5 hours to heat 20 gallons of water from 50F to 120F.  That’s encouraging.  We don’t typically burn the stove 5 hours a day, of course.  But neither do we need to produce all of our hot water from wood heat– the idea would be just to make a big dent in it.  And in fact, I don’t really know how much hot water we use– it’s probably less than 20 gal/day.

The next test was to put a 10-foot coil of  1/2″ copper tubing on top of the stove, and run water through it for a while to see whether this would be suitably efficient as a heat exchanger.  Unfortunately, it was not nearly so good, capturing only 940 BTU/hr.  At that rate, we’d only manage to heat perhaps 5 gallons per day.  But, there is much room for improvement.  I have given a lot of though to means of increasing the heat conduction.  For example, the coil could just be set into a water bath in the aluminum pan.  Some heat would be lost to evaporation, but since the cool water in the coil would keep the water bath from ever exceeding about 120F, this would be minimal.  Another idea is to put the coil in the pan, then fill the pan with steel shot (or even, and my guru on this matter suggested, pennies).  What I’d really like to have is a slab of copper with the coil cast in the middle of it– but that is not a DIY project!  More tests to come, stay tuned.

Total project expenditure thus far:

Copper coil  $22.  Turkey pan $2.

5 Responses to “Wood Heat Hot Water: Part I”

  1. Patrick Says:

    While I’m sure there’s a sizable online community of folks who are doing exactly what you’re talking about, if you run out of leads, check out the homebrewing forums for ideas – heat exchangers are a regular topic of discussion, from commercial to homemade to Frankenstein concoctions, and we’re a thrifty and creative bunch who repurposes all sorts of stuff whenever possible.

  2. BMc Says:

    Fun. Enjoyed the gadget experimentation. I have an easy (and cheap) something for folks out there w/ radiators to try. (I know CH does not have a furnace). I have yet to try it, but plan to do so this yr.

    Radiators are almost always situated along outer walls, and below windows. As radiators put out heat in all directions, much of the heat is wasted, as some goes behind the radiator into the outer wall, as well as to the outside via the wall and window.

    In order to capture some of this wasted heat, put something that reflects the heat back into the room behind the radiator (best to extend it somewhat in all directions, though the aesthectics migh bother some; but these are removable). Some possibilities: alumimum foil wrapped around heavy cardboard, some fire-resistive reflective contractor materials made for similar purposes (don’t have detatils handy), or buy a reflective car window shade and cut to size.

    Caution: Reportedly, the heavy cardboard wrapped w/ foil option (the most crude option, likely also the least effective) does not cause a fire hazard. BUT, I’ve only read that, so DIY-er beware. The other options out there sound better from that standpoint.

    • Cold House Journal Says:

      BMc: Serendipitous that you mention this– I’ve been composing a post on this very topic lately in my head. Perhaps I’ll get around to writing it soon.

      “Reportedly, the heavy cardboard… does not cause a fire hazard.”
      There’s no way you can start a fire with cardboard on a steam on hot water radiator– they can’t possibly get hotter than boiling water, far from hot enough to ignite cardboard. But I’d be wary of doing that with an electric baseboard…

  3. BMc Says:

    Just read your post on the topic — very interesting!

    As for the serendipitous timing, I’d say “cold minds think alike,” but, alas, my mind–and the rest of me–are enjoying the relative warmth lately (it’s been pretty nice weather in the Philly region, and I have been keeping my thermostat higher than I did last year when I came across the CH).

    Looking forward to more gadget stuff.

  4. Fred Says:

    I have been thinking a lot about this concept, but adding it as a alt zone to my oil burner to heat house instead of running fans all over. I have looked into to stainless pipes you can cut into your stove but not sure if I want to damage my stove and newer stoves have more parts in the way of running pipes. I was thinking of wrapping the stove with copper pipe on the sides but not sure of what size and if there would be good heat transfer there. Somewhere I read about putting a coil of copper on top of the stove but you ran into the same heat transfer issue. In that same article they encased the coil in plaster or concrete or something of that nature to assist with the heat exchange. What are you doing to prevent steam explosion and over heating of the water? That is the reason the EPA does not have water jackets anymore in the USA.

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