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Thread: Off the wall question about homemade heat exchanger

  1. #1

    Default Off the wall question about homemade heat exchanger

    Hi! Sorry this is a little long, but I guessed I needed to describe my situation.


    Ok, my winter "to-do" list requres that I make a homemade wood-burning-pool-heater by spring.
    What this does is to provide very warm water to the pool to extend the swimming season. As the wood burns it heats up pipes that are carrying pool water by way of the pool pump. Water pressure is about 25 PSI. Water going in would be anywhere from 55 to 75 F and leave about 78 or higher.

    As for my question, (I have no idea where else to ask this....)


    I have all the plans on paper for this woodburner, what is holding me up is the design of the heat exchanger.

    I want to use thin wall pipe as that would transfer the heat the quickest;
    I will be using about 100ft of pipe.
    I beleive copper is the best common thin wall pipe metal
    If I use 100 ft coil, it costs about $260.00.
    I'm cheap. I need to be cheap!
    So I am trying to figure a cheaper way than use of coil.
    If I do a vertical up and down along the heater walls, (which is cheaper cost pipe cause it is straight) that involves I imagine, soldering pipe to u-joints.

    A wood fire is about 2,000 degrees so I beleive it would melt the solder. I say "think "because someone said to me the "cool" pool water would prevent the solder from melting.
    Is this true?

    Is there another way of joining copper pipes and to have the joint "safe" from the heat? I dont need any wood leaks in a stove!!!

    I heard brazing or high temp solder, but I'm not sure, this is getting beyond me.
    Just wanted your opinions, ......in fact your opinions would be GREATLY apprecaited. Again, I have no idea who else to ask.

    If joining copper is impractical, then I will go with a coil pipe.
    Or if you have better ideas!

    Thanks

    Matt

  2. #2
    Homeowner geniescience's Avatar
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    Default

    a copper pipe cheminea is what this sounds like. Let me know how it all turns out. The water flow will certainly keep the copper pipe wall temperature down. Even if it reached a boil, which it won't if you keep the flow coming. A birchbark vessel doesn't burn over an open fire, if it is filled with water or soup or broth.

    See http://www.terrylove.com/forums/showthread.php?t=13761 for a funny look at how to get low cost copper that you don't need to solder.

    Getting the heated water to go downhill into the pool is the big task. Unless your cheminea is located downhill from your pool, and you set up a closed loop so there won't be any siphon effect.

    Sounds like you have an unlimited supply of free dry firewood. What about using solar energy? That will be free until they tax photon consumption.


    David

  3. #3

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    Thanks David,

    Do you thing the solder joint will hold up at that temperature with the water moving through the pipe.

    Should I go with a higher temp solder or braze?


    Imagine a metal box 3 ft x 3 ft x 3ft; that is a very ROUGH dimension of the stove. THe firebox will be about 18' w x 21" tall, 21" deep for example.
    In this firebox I want to run the pipe along the firebrick walls of the firebox.

    Water is gravity fet to a pool filter then to a pump then to stove then to pool.

    That is a geneal idea.

    I need to know how to go about piping.

    Thanks

    Matt

  4. #4

    Default

    I have tried scrap yards, e bay and craigs list for the copper pipe. No real luck so far...............

    Thanks for the suggestion though!!!!

    M

  5. #5
    DIY scratch-pad engineer leejosepho's Avatar
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    I could be wrong here, but I believe copper would be a poor choice of material for a heat exchanger directly exposed to flame. Then, there is the matter of how you would control and maintain a relatively low temperature for the water coming out of a high-temp, wood-burning water heater or boiler when such a unit is usually used to heat water to a temperature that is at least as high (120 to 140 or so) as a typical gas or electric residential water heater. I am also looking for an inexpensive way to warm my (small) pool on cooler days, and I am fairly sure a wood-burner would not be a good way to do that.

  6. #6
    In the Trades Bob NH's Avatar
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    I would shop around for an oil-fired boiler that someone is replacing. Then, modify the firebox or otherwise use the heat exchanger. You will probably have to modify the fire box to make it large enough to accept the wood and make provision for cleaning out the ashes.

    If the firebox is not usuable you could set the heat exchanger on top of your own firebox.

    Call a few people who do plumbing and heating and tell them what you want to do. They will probably be able to sell you something for small money and it may have a circulator, burner, and controls. You could use the blower of the burner to produce forced draft.

    EDIT on 10/14: Pool water will be very corrosive from the chlorine. It is probably not appropriate to use a steel heat exchanger with chlorinated water.
    Last edited by Bob NH; 10-14-2007 at 06:08 AM.

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