Hot water from my cookstove

Discussion in 'Shop Talk' started by cradlerock home, Dec 17, 2004.

  1. cradlerock home

    cradlerock home New Member

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    Oct 17, 2004
    Location:
    Ontario, Canada
    I'm looking to hook up a water jacket in my cookstove but I'm not sure what size of hot water tank I should use. We are a family of 5, if that makes a difference. We currently use an electric hot water heater. I'm not sure if I should bypass the electric tank or use the water coming from the cookstove as a pre-heat. I can't get ride of the electric tank completely yet as I do not have a solar hotwater system set up. Any help or info would be a great help.
    Ray
     
  2. River

    River Well-Known Member

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    Ray,

    Can you locate your electric water heater such that it can be used as your water tank heated by the cookstove? If you can, that may be your cheapest and cleanest installation.

    River
     

  3. BobBoyce

    BobBoyce Well-Known Member

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    Ray, I've used electric water heaters as the heat storage tank in solar water heating systems just by tapping in at the top and bottom and circulate the water through the solar heat collector with a pump powered by a solar PV panel. I also used a water cutoff solenoid to prevent convection flow at nights. That way the water only circulated when the sun was heating it.

    A similar setup could be used with your cookstove. Set up a coil of copper tubing as your heat exchanger to take heat from the cookstove.

    Add a line from the bottom of your water heater (T it into the drain valve port, the coolest water in the tank is at the bottom), through a pump, then through your heat exchanger coil, and return the heated water to your water heater via a T fitting located at your hot water output port. This way, when you have your cookstove going, you can run the pump and add heat to your water. I would insulate all lines to limit heat loss.

    Bob
     
  4. John Hill

    John Hill Grand Master

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    I believe the best idea might be to install the water cylinder right adjacent to the cook stove with the bottom of the cylinder just a few inches below the level of the cook stove heat exchanger so as to allow convection currents to form when the stove is lit. The connections from the water cylinder to the cook stove would be as straight as possible and at least an inch internal diameter, bigger would be better. One connection from the lowest part of the cylinder to slope upwards slightly towards the cook stove, the second connection would be from the top of the heat exchanger and sloping upwards to a point higher up in the cylinder. I think this is a system that will give your reliable hot water with no reliance on electricity or anyone remembering to turn on a pump.

    An electric element could be fitted to the cylinder too, being thermostatically controlled the electric heater will not switch on if the cook stove is keeping the water hot. I have lived in houses with this combination of stove and electric heating.

    I have no experience of solar heating but from my experience the stove heat exchanger may be able to get the water really hot, right up to boiling and I wonder how the solar unit would like boiling water being fed up to it?


    I am just a little uneasy about what would happen if you are relying on a pump, rather than convection, to feed water to the cook stove. If electricity is not available and the system is not arranged for convection flow what happens when the stove is lit? Won't the heat exchanger overheat and possibly burn through? Not only would there be no hot water but no stove either as the water in the cylinder would flow back into the firebox, put out the fire and make an awful mess too!

    I am not sure how you could use the stove as a preheater, it seems that would require water to be just sitting in the stove getting hotter and hotter until some is drawn off the cylinder. It seems like a recipe for rumblings and grumblings, steam blockages and possibly overheated stove heat exchanger.

    As for the size of the tank, I think the bigger the better


    Don't forget to allow for pressure build up when the water heats! Especially important for stove heat exchangers! I guess there are blow off valves available but nothing is more reliable than a vent pipe up through the roof.
     
  5. BobBoyce

    BobBoyce Well-Known Member

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    Designing for convection flow is nice. Mine was set up for both, so it would fall back on convection if and when the pump fails. Convection alone will not be as efficient as adding a low powered pump. When my pump eventually failed, I just left it inline and let the system run by convection. On a system set up for convection however, when the water tank is hotter than the collection system, the convection will dump heat out of your water tank into your collection system. This is why I installed a solenoid operated shut off valve. When there was sun, the power from the solar PV panel opened the valve to allow flow.

    For a stove or other heat source, a temperature differential thermostat could be used to automate flow control. Either way, you're either going to have to power a control system, or manually operate a valve. I doubt there exists a backflow preventer that would work at convection flow rates, and reverse convection would waste your water heat.

    A solar heat collector can and will boil water if enough flow is not maintained. I've seen it happen on a system I set up to heat a swimming pool.

    Really the only problem we had besides the pump burning out, was the water would get so hot that it would trip the overtemp limit switch on the electric water heater.

    I set the thermostat real low so the electric element would only kick on if the solar wasn't able to keep up, like on rainy days ect. I don't think it ever kicked on because the overtemp protector was almost always tripped, and we never ran out of hot water.

    As far as pressure buildup, if you plumb into an existing water heater as your storage tank, they already have an overpressure protection valve.

    You heat collection can be as simple as copper tubing against the outside surface of the firebox or wrapped around the pipe to the chimney, to iron pipe plumbed inside the firebox itself.

    I worked for a while as a fabricator/welder building heat exchangers. I built some pretty darned big ones that weighed in at several tons, and some small ones that weighed just a few pounds. Small ones were typically copper tubing and brass sheeting, while the large ones were iron pipe and steel plate.

    Bob
     
  6. JustinThyme

    JustinThyme Active Member

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    If you use a heat exchanger idea with glycol be careful ,glycol burns at high temperatures ,I've seen the remenants of the shell of a gasfired lineheater that melted 1/4 inch steel when the pH of glycol went out of whack and over the course of several years ate through the firetube .

    I built a simple 'H' thing out of stainless tubing that fit over a barbecue burner ,seems to work pretty well .
     
  7. cradlerock home

    cradlerock home New Member

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    Oct 17, 2004
    Location:
    Ontario, Canada
    Thanks for all your help and advice. I think I understand enough now that I can hook it up. I think I play it safe and get it inspected before I try it.
    Ray
     
  8. agmantoo

    agmantoo agmantoo Supporter

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    Properly positioned the electric water heater tank can be used as a heat storage device from the wood stove using a thermal siphoning technique. This device will not require a pump, will use the electric water heater as a backup and will not backflow heated water through the heat extractor on the wood stove. No electricity required except for the water heater elements if you need the backup. No glycol required also.