Water Pre-Heater

Discussion in 'Homesteading Questions' started by whodunit, Oct 6, 2005.

  1. whodunit

    whodunit Well-Known Member Supporter

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    I eventually want to use our wood stove to pre-heat water before it goes to the water heater.

    My FIL had a set-up where pipes ran from the tank, through or on top of the stove and then back to the tank. The water circulated itself through "convection" (is that the right term?).

    My wood stove has a heat shield on the back and it looks like it would be simple enough to run some copper tubing between it and the back of the stove. Would this have to be attached somehow or could it just "float" in this space between the back of the stove and the heatshield?

    My thought is to use a closet space behind the stove to house the tank. The water heater, which is in the basement, would then draw its water from the tank.

    I would also like to have some type of valve to send the water directly from the tank to our faucets, thus bypassing the water heater, in case we have a power outtage. We would then still have some hot water in the winter. Is this possible?

    Any thoughts or advice would be appreciated. A website with some type of plans would also be helpful.

    Thanks in advance.
     
  2. rambler

    rambler Well-Known Member Supporter

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    I won't comment on the rest, but - that seems un-needed. If your water heater is properly insulated, it will store hot water for several days. Just continue to use water (are you on city water - your own pump will not work if you have your own well?) and all should be fine. The wood will heat the water, & the water heater will store it.

    I thought most just ran insulated pipe to the water heater, not involve another storage tank - which you need to insulate well - but perhaps that works too.

    --->Paul
     

  3. Drizler

    Drizler Well-Known Member

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    I was looking for the same thing not long ago. If you dig around in google pictures doing searches you will find a line drawing of an old installation used decades ago. The thing you need to do with that arrangement is locate the holding tank or header tank up above the stove. That sort of killed it for me as my stove is upstairs and theres no place to put the tank so that it remains inside.
    Thats about the only issue. The tank has to be above slightly for the thermosiphon effect to take place. Thats about all there is to it beyond the plumbing connections. Even a simple tank in the basement ahead of your hot water heater will help some by bringing the water up to basement temp from what groundwater is. Check out Mother Earth News they may have something in there.
     
  4. whodunit

    whodunit Well-Known Member Supporter

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    Thanks for the responses so far.

    Positioning the pre-heater tank above the stove shouldn't be a problem. I would just have to build a deck for it to sit on.

    I have an old water heater that I can use for a storage tank.

    I did read some plans that talks about a coil of copper inside a metal frame and insulated with plaster of paris. I wonder if that is really necessary. Couldn't I just coil some copper in the space between the heat shield and the back of the stove?
     
  5. raymilosh

    raymilosh Well-Known Member

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    good questions.
    Most folks call it thermosiphoning. Hot goes up, cold goes down. the tubing inside the stove is called a waterjacket.

    If your stove has a heat shield, it may not get hot enough behind it to heat the water enough to be worth the effort. The heat from the fire should radiate onto the tubing to the tank directly. If you soldered the tubing to the back side of the heat shield, that'd prolly do the trick.
    It is important to protect the tubing from being hit by the wood as you load the stove. It is also important that the tubing maintain a mostly upward slope with the water going in the bottom and going out the top. Anyplace in the system that can trap air will completely stop the thermosiphoning effect and may lead to dangerous buildup of heat.
    Using the closet near the woodstove sounds like a great idea. There are general rules about the elevation and distance of the tank from the woodstove. Use a dead electric water heater for the tank. They are PERFECTLY suited to your needs. Plumb the hot water from the water jacket to the top of the tank where the sacrificial anode was screwed in and plumb the cold out from the tank to the water jacket out in addition to the spigot at the bottom of the tank. Tanks are available free from plumbers or from the scrap yard or the metal recycling dumpste or from the curbs. They're plentiful.

    There are some VERY serious saftey measures that MUST be installed in these systems to prevent the possibility of DEADLY steam explosions. One such device is a temperature and pressure relief valve.

    Lehman's founder wrote an instructional 40 page (or so) manual entitled "How to get hot water from your woodstove." I bought a copy. I am glad I did. I am a pretty handy guy, but the amount of safety explanations and practical advice and the large number of possibe ways of doing it made me feel very confident that I was doing my project the right way.
    Here's a link to the book:
    http://lehmans.com/shopping/product...RODUCT&RS=1&keyword=hot+water+from+your+woods

    I have been plumbing a water jacket from a waterford Stanley wood cookstove to a solar waterheater on the roof. We'll see how it works when it starts getting cold out.
    ray
     
  6. Explorer

    Explorer Well-Known Member Supporter

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    I don't think you will get much heat transferred with this method. The coils probably need to be inside the firebox. Years ago these types were sold, but I don't know about today.

    Measure the temperature where you would like to place the coils. I think you will be lucky to get 30-40% heat transfer. In other words, if your incoming water is 50 degrees, the temp you measure is 150 degrees, then the water would heat 30-40 degrees. Is that enough to make all this work worthwhile?
     
  7. paperboy-7

    paperboy-7 Well-Known Member

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    hi i have set-up preheat on every hot water system for 40 years. neverrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrput coils inside firebox. you will have a boiler and may never need hotwater again,your dead. the book on this subject is a must, talked about before. a small pump is needed for trouble free service plus plumbed in w/thermo correct lines there can still be a back-up or stoping of the thermo flow. please dont get hurt later preston
     
  8. whodunit

    whodunit Well-Known Member Supporter

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    Raymilosh, thanks for the book link!

    Paperboy, thanks for the warning. Safety is a definite priority since I have small children. I'd never jump into something before I know all the ins and outs.

    As far as the amount of heat, I'm mainly interested in cutting down on electrical useage rather than relying solely on this system for all our hot water. We probably take two showers and a bath a day, plus do several clothes washer and dishwasher loads a day. I'm just looking for alittle something to cut costs.

    I'm also interested in a low-tech solar pre-heater for the summer months.
     
  9. Explorer

    Explorer Well-Known Member Supporter

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  10. bill not in oh

    bill not in oh Well-Known Member

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    I must be missing something here... So I'm gonna put on my devil's advocate hat for a minute. We're going to spend a few hundred dollars and probably 40-80 hours modifying a woodstove and running pipe through walls and/or floors to a waterheater - for what? How much could one expect to heat water from [50 degrees] with this type of system, and how much would that translate to in terms of $aving electricity and dollars? I'm thinking there might be other ways to spend that type of money and effort with a faster and higher return - more bang for the buck so to speak.

    As to plumbing it to be a direct supply of hot water in case of power outages... again, how hot can you get this water and how many days in the last five years has this address been without electricity?

    So, since this community is cram-packed with smart, experienced and thoughtful folks, I suggest we re-route this thread (with the originator's permission, of course) to be alternate ways to accomplish the objectives of:
    1) saving some money through more efficient water heating,
    2) utilizing the woodstove beyond heating the air around it

    What do you say whodunit and have I correctly interpreted your objectives?

    Of course I could be completely wrong about the efficiency of a system such as this...
     
  11. SolarGary

    SolarGary Well-Known Member

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    Hi,

    The amount of energy required to heat 45 gallons (typ for 3 people) a day from 50F to 120F is about: (45gal) (8.2lb/gal)(120F-50F)(1 BTU/lb-F)(1 KWH/3412BTU) = 8 KWH per day, or about 80 cents per day if you are paying 10 cents a KWH. About $300 a year.
    So, you would get some part of the $300 depending on how much preheat you get from the wood stove.

    Being a big fan of solar, I would suggest taking a look at a solar water heater. The batch style solar heaters are inexpensive to build and work well. They are good build it yourself projects. In mild climates people report getting all of their hot water from the batch heaters in the summer, and 50% to 70% in the winter -- a pretty good saving.
    In cold climates it would be less because the batch heaters have no freeze protection and need to be drained in the mid-winter months.


    The closed loop and drainback style solar water heaters can be used year round in cold climates, but are more complex to build. Still a good project for serious DIY people.

    Free plans and info her:

    http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm

    Gary
    www.BuildItSolar.com
     
  12. whodunit

    whodunit Well-Known Member Supporter

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    Thanks Bill and Gary for your imput.

    'm tired since I've been hunting all day. I got a small doe which will butcher out to about 40-50 lbs. of meat. Sorry if the below thoughts sound rambling...

    Being we are a family of five, we could be using about 75 gallons of hot water a day.

    Using Gary's figures, this works out to about $400 a year to heat water. We pay 8.5 cents KWH, plus a $12.50 fee.

    That works out to $33, a month, which is about a third of our electric bill.

    If I could figure out a way to eliminate the majority or even all of that portion of the bill, I would be very happy.

    I'm not partial to any particular system, it's just that all I know is the one using the wood stove because I've seen one in action.

    We are in north central Idaho where we typically use heat 8-9 months out of the year and we do get freezing temps, so a batch water heater would not work during the coldest months, right?

    One of our big considerations is we want to be always working toward a more self-sufficient lifestyle. I think it would be a comfort to know that this part of our lives (hot water) would continue basically uninterrupted if the power went off and stayed off.
     
  13. Auric

    Auric Registered Doofus

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    When I was a kid, my dad came across a water heater at the township dump. He hauled it home and removed the tank from the jacket and insulation. Then he suspended it over our woodstove, horizontally, at ceiling rafters' level. He ran the cold water into the suspended tank before flowing into the gas water heater. I have no idea how much of a savings this was for us, but I know he was pleased with his work. A tour of the house for new visitors always included the furnace/woodstove room!
     
  14. SolarGary

    SolarGary Well-Known Member

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    Hi,

    Your numbers on the energy saving look right to me -- you are getting a good electricity rate, but I would not count on it staying that way

    You would have to bypass and drain the batch heater during the coldest months. But, they will take some freezing weather if the pipe runs are well insulated. The batch tank itself has so much thermal mass that it won't freeze easily. One simple improvement you can make to it for winter climates is to have an insulated cover that you open on sunny mornings and close at night -- I have even seen ones where this could be done from inside via a rope. The plans for the Maine Solar Association use this scheme -- here:

    http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm
    (its about half way down the page)

    The lid can be reflectorized (e.g. alum foil or aluminized mylar) -- this adds more collector area when the lid is up without adding heat loss area.

    If you can mount the batch heater right against the south wall of the house, then the pipe freezing can be less of an issue. I think that the Maine Solar one could be used year round even in a cold climate. With five people using it, you would want to make it on the large side.

    Maybe some clever person could figure out a way to automate the raising and lowering of the insulated/reflective lid???

    It sounds like a combination of a solar and wood stove one might work well together, since the solar one would be best in the summer when you aren't using the wood stove?

    If you want a solar design that can be used all year round, and that you can build fairly cheaply, take a look at the first two in the "Closed Lop and Drain Back" section at the same link listed above. I think the one that uses the poly barrel as both the heat storage tank and the drain back tank is pretty clever.

    Gary