Wood Stove Hot Water Danger..

Discussion in 'HVAC Heating & Cooling' started by molo, Oct 27, 2007.

  1. molo

    molo Member

    cold new york

    "A co-worker of mine put a heat exchanger in his furnace firebox. Without proper controls he quickly made high pressure steam and soon solder joints were failing and the system came apart in what sounded like several small explosions. His solution? Weld the damn thing together the next time. It held together, but it back-fed steam all the way to his artesian well and he had a nice geyser out back. True story."

    In this case the pressure had somewhere to go. Without a place to expand the resulting explosion could cause injury or death.

    I am very interested in heating hot water using wood as a fuel source. I want to do this safely and efficiently. What are some general considerations before I even begin to discuss specific ones? I have 2 good engineering schools neary to me, and both do work on alternative energy. I may be contacting them, but I wanted the opinion of you pros first.

    The biggest obstacle to adding DHW systems to wood stoves is that EPA emissions certification virtually eliminates the possibility of taking heat directly from the stove. Any DHW collector that robs heat from the combustion process would almost certainly create a dirty burn that would fail the stove. That is why manufacturers of EPA certified wood stoves don't offer DHW collectors as optional accessories. There may be one or two available, but we are aware of none at all.

    Last edited: Oct 31, 2007
  2. leejosepho

    leejosepho DIY scratch-pad engineer

    I would not want to use any heat source that does not have a high temperature limit to prevent boiling. A sensor can control a circulation pump, but something else first needs to control the burner.
  3. Cass

    Cass Plumber

    Sounds like he is trying to reinvent the wheel.

    People died long ago so safety systems could be designed into present day systems.

    Just buy a system already designed to do the job and and enjoy the results so no innocent people are hurt.

    Your friend ignored the warning he got when the solder failed to hold and made a dangerous change that was the wrong one to make by welding the pipe.

    Unless you live alone and you are the only one that can be killed, maimed, or injured should something go wrong I would say don't try and build your own system buy something that has already been designed and tested.

    Steam is nothing to fool with.
    Last edited: Oct 28, 2007
  4. molo

    molo Member

    cold new york
    I set 2 quarts of water on the stove, it heated to over 100 degrees in less than 10 minutes.

    If I'm home and the stove is running, would there be a problem with putting an open topped container on the stove that holds 10-20 gallons and then running it off into my hot water tank before I go to bed (I could direct pipe it, and I would have to determine how much weight the stove could hold, I'm looking for a plumbers perspective).?

  5. Wood burning stoves and insurance companies

    Another thing to think about......

    I had a very cheap wood burning stove in my old house in the basement, it could pretty much heat the whole place
    well ...but my home-owners insurance had a hissey fit
    when they learned of it......

    if he is still doing this or you are thinking of rigging up some sort of steam boiler---wood burning stove.....

    you will probably not be insured when the unit burns down
    your house or blows out a side wall or roof...

    they will worm out of covering the damages...

    becasue their is no UL or Antsi lable on that rigged up
    boiler you created....

    stuff like this was done a long time ago with the original manual water heaters called "side arms" in 1880'
    ...ocasionally someone would forget how to operate the unit they and it would cause a steam explosioin.....

    many people are trying to figure out a better mouse trap right now ........for home heating..and you can
    find their creations alll over the internet for sale...some good and some bad.....

    if I could talk my wife into a good wood burning stove in our downstairs laundry room we could easily heat our
    house very well with that alone.....
  6. leejosepho

    leejosepho DIY scratch-pad engineer

    No problem there other than the danger of scalding a passerby, and it would humidify the area ...

    That would require a pump and the evacuation of the water already in the tank.
  7. hj

    hj Moderator & Master Plumber Staff Member

    Cave Creek, Arizona

    The owner of the wood burning boiler in the initial posting was lucky. If there had not been multiple small explosions there could have been one big one, about he equivalent of 3 sticks, or more, of TNT.
  8. molo

    molo Member

    cold new york

    THIS IS AN OPEN SYSTEM, I'm talking about placing a metal pot on the stove with 5-10 gallons of water while I'm at home. IT WILL NOT EVEN HAVE A LID. Worst case scenario, it will evaporate out.
    Also, the first post isn't somebody I know, it is a quote from a site with extensive warnings about designing and building unsafe systems.

    Yes, I see that trying to add hot water to the tank would be tough when the tank is full. Does anyone have any suggestions for how this could be done?

  9. alternety

    alternety Like an engineer

    There are a couple of issues you need to overcome. Your water tank is pressurized and your pot is not. You don't want to blow up the tank.

    Here is a thought or two about how. The plumbers can comment on the rationality of the approach. It is not necessarily real cheap; but not bad.
    For all the scenarios. You are probably not going to have good temperature control on the water in the tank - unless you do some simple control systems. On the other hand, the hotter the water in your tank, the more energy stored. Insulate the tank well. Install a tempering valve on the output side of the water tank. You can have dangerously hot water in that tank.

    Buy or build a flat plate heat exchanger. Pipe on one side goes to the open pot and has a circulator. Other side goes to the hot water tank with another circulator. Put an automatic fill valve on the open tank to replace evaporated water. Don't let the pipe to the pot touch the pot or stove. Only the water. This will limit the transferred heat to boiling temp.

    Same thing but just make a coil of coper pipe and attach the coil directly to the tank with a circulator. Put coil in pot water. Don't touch anything but water. Having the pipes contact only water limits max temp to boiling. Touch hot metal and there is no limit. Bad thing.

    Water heater needs a working T&P valve and properly sized expansion tank. The expansion tank should be a boiler capable tank to be on the safe side. The T&P you normally find on a water heater is set too low for potential max temp in the tank. One of the plumbers can comment on availability of something else. I would suggest no check valves back to the water source but that is just belt and suspenders.

    If you put a thermostat in the water tank you can then control the tank temp by controlling the circulator. If you go for high storage temps, make sure everything is rated for those temps.

    Good safety device if you store water above normal temp is the tempering valve. I would put that in even if you think you are controlling the temp. Just so a circulator sticking on does not surprise you with scalding water.

    There are details of piping to tank and stuff.
  10. Why would youwant to waste the time???

    Last edited: Nov 1, 2007
  11. molo

    molo Member

    cold new york
  12. alternety

    alternety Like an engineer

    My previous suggestion using a heat exchanger and an open loop to the pot keeps the water temp from exceeding boiling in the closed system. Thermostat control on the pump gets the water moved. No air gets into the system.

    If you just want to heat a pot and pour it in the tank you need to tie to the tank output and put the hot water there. Since your tank is pressurized hot water will come out of this when you open the valve.

    To make room you will need to remove water from the tank or pump your new water in at a pressure exceeding the supply pressure in the house. This would force water back into your cold water line. This requires that there be no check valves between your hot water and whatever your primary source is and/or an expansion tank with available capacity (at the time you do this) to accept the water.

    If you are on city water they would probably not be very happy with you pushing water back into their pipes. That is why many utilities put check valves on the water feed at the house.

    If you have a well you Will need to make sure there is a place for the water to go whenever you do this.

    You could open turn off the cold water supply for the tank and the output of the tank and open the drain the of the tank. And pump as above. You might be able to use the suction of exiting water to pull in your pot water. You are real likely to get air in the system.


    None of the ways above strikes me as being a good idea. They are a pain in the butt and could be dangerous. Also consider that some of the water you get rid of has already been heated to some extent. The tank stratifies but I suspect that even the water on the bottom has some absorbed energy. You have to get rid of five gallons to put in 5 gallons.

    You could just take the hot water and pour it in the bathtub or sink and use it there.
  13. Bob NH

    Bob NH In the Trades

    New Hampshire
    The problem with trying to heat water with a wood stove is an engineering problem. There must be a way to control the water temperature to somewhere around 180 degrees F in a system where the gas in the stove is 500 F. When there is no water heating demand the water temperature will rise to 500 F. It is not feasible to control the temperature in the stove to 180 F because the stove wouldn't work and you wouldn't be able to use it for the purpose of heating the house.

    Operating a boiler off a wood stove requires some kind of control system. The "open" boiler works because it can't exceed atmospheric pressure. I remember that we had a wood-burning kitchen range that had a copper tank on the side of it which was heated from the oven. You filled it up and when it got hot you carried buckets of hot water to the bathtub or the wringer-washer where you needed it.

    The water heater never blew up.
  14. solarsteve

    solarsteve New Member

    how to get the hot water into your tank

    Well first of all, i see no problem with putting pots on top of your wood stove. I have been doing this, and as mentioned its an open system, so the highest temperature that can be reached is boiling. I live in a dry area, so i always had a pot, to increase the humidity. Then i started putting the hot water into the bath to preheat the tub, and a little water [3 gallons]. Of course thats not enough water, so i now have 3 pots on the wood stove equating to around 7 gallons. What the heck throw the teapot on there and make tea for free. Of course the problem with this is that yes it is work, and of course you can burn yourself transporting the hot water. But i live at 6000 ft, and water boils here at only 190f. Reach in boiling water and pull out an egg...
    I don't agree that 5 or ten gallons of hot water is nothing, but then again i am one person, so it would get logistically hard for a family of 4 to heat 30 gallons or so on top of the wood stove. Using this system i have already had 2 baths today. You just might need to change your habits a bit. Take the hot water and pour it into your washing machine, or put it next to your sink to wash dishes, or fill the pot to cook your pasta with hot water. I ran a pipe up next to the wood stove, with a valve, and then a flexible line, so i can fill the pots easier.
    So yes this takes time, and i guess thats work.
    But yes with increasing fuel prices, why not reduce your usage which reduces your costs. Also reduces our dependence on foreign oil, and reduces profits for the oil companies who have been making record profits.
    So anyway back to your question. I don't see an easy way to put the actual water into your hot water heater. All sorts of problems can arise such a burning out the element because the elec is on and there is no water in the tank.
    A question: Is your hot water tank higher or lower than your wood stove?

    What I am thinking of doing is installing an open system tank above the wood stove. Then running coils around the flue pipe, and possible some kind of heat exchanger on the side or back of the stove. As long as the pipes are always heading upwards, the water will naturally convect up to the tank. This line should go into the top of the tank, not the very top, but say 1/4 down. Then the tank needs an overflow also. But of course this is a separate system. But i used a system like this with a self built solar collector, and it heater 20 gallons, and didn't need to turn on the hot water heater all summer.

    Ok, so i was thinking for your scenario...maybe buy a rheem 80 or 120 gallon solar tank. The loop collecting heat from the wood stove will travel to the copper coils in the bottom of the tank transferring the heat to the pressurized water in your house system. These tanks have an electric element at the top, so if needed it will automatically add heat to keep your water coming out of the tap at a constant temperature. Of course you would need a pump, and an expansion tank, temperature sensor, other stuff, and of course for any type of pressurized system, and probably a good idea for any heat exchanger with a wood stove I think an overpressure relief valve that was plumbed outside the house "wood" be mandatory. [REDUCE THE EXPLOSION FACTOR]
    Another idea is maybe some type of drain back system like they use for solar also...no heat at the stove, then the pump is off and the water drains back down to the tank...of course the exchanger has to be designed for the fluid to flow uninterrupted...I am just learning this, also maybe a vacuum release valve...
    cool thread
  15. shluffer

    shluffer New Member

    I did see a system at one point that amounts to a wood burning boiler. It has a damper to shut the fire down (At least t a smolder) when the water is not circulating, or gets up to temp. IT was an expecive system and at the oil prices back then, didn't justify the price. We went with a wood burning stove in our fireplace which we are very happy with.

    There are a couple systems for outdoor wood boilers that will heat you house with wood. They also have hot water attachments. They will run you around 8K in CT if you install it yourself. They are hard to get this time of year (most of the dealers are sold out).
  16. hj

    hj Moderator & Master Plumber Staff Member

    Cave Creek, Arizona

    The ONLY way to move the water from your pan into a sealed water system is with an injector pump. And the hot water system could not be a closed system because when you inject ANY water into it, that same amount has to be eliminated somehow, usually by allowing it to backfeed into the incoming piping.
  17. nhmaster

    nhmaster Master Plumber

    S. Maine
    The truth is that trying to heat water with wood always ends up being a compromise. There have been many systems that have made the attempt over the years. Most if not all never meet expectations.
  18. Steven62

    Steven62 New Member

    I agree!

    Absolutely the safest best way if just have to do it.
    I also have had experience like SolarSteve. I used to live in a remote location where I had to haul water from a well, heat it on my woodstove for a bath.

    It brought back good memories!
  19. Bill Arden

    Bill Arden Computer Programmer

    MN, USA
    The problem with this idea is that once the circulator turns off, the water in the fire box coil will turn to steam. Worse yet you could get a water hammer when the pump turns back on.

    If You really want to go this route, I would suggest using some other method to control the heat flow.

    One zany idea would be to Y out the flue and use two flue dampers to control which flew the gasses go out. and then have a coil in one of the flue's.

    This would be a waste of money since it would cost more to set up and run.

    A simpler option is a water pre-heater. Run the water that goes into the hot water heater threw a coil mounted on the wall near the wood stove. The idea is to heat the water to 80F or so. This reduces the amount of energy needed to heat the water.

    Some math (I am using my numbers for reference)
    60F incoming water temperature
    120F hot water heater temperature
    80F water after pre-heater

    Usage Savings = ((120 - 80) / (120-60)) = .66 or %33% lower
    StandBy savings = 0

    Ok, that would be a wast of time as well.

    Lets try this again
    1. Copper coil $50
    2. bronze Recirculating pump $200 + (80 watts * 24/7) = 60Kwh per month = ~$6 per month
    3. Something... Something... Something...?
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