Coal Stove - Water/Air Heat Exchanger Setup.

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by Siphen, Oct 6, 2008.

  1. Siphen

    Siphen New Member

    Messages:
    5
    Hello,

    New poster here, with some questions, hopefully someone with a little plumbing know how can advise me if this looks safe. Basically my plumbing know how is extremely limited.

    I currently have a forced hot air oil furnace system. Due to the high cost of oil I have no desire to continue to drain my bank accounts for the oil companies this winter.

    I am installing a coal stove in my house that has water coils in it; the water coils feed out of the unit and connect to 1†copper pipe. I also bought a water to air heat exchanger sized to fit in the duct work of my existing forced air system also with 1†copper. I have enough space between my air conditioning coils and the extended duct work to install this exchanger. There will be about 5-7’ of copper pipe between the coal stove and the water/air heat exchanger. I plan to install a automatic air valve, pressure gauge, thermometer and pressure release valve between the stove and exchanger. On the return from the exchanger I am going to install a pump.

    My main concern is not allowing the water to boil in the pipes. Once I get the house hot enough I will need to continue to circulate this water and was thinking about adding a secondary zone to push the water to. This zone would bypass the water to air heat exchanger and route the water to potentially baseboard heating I would put in my basement This would hopefully cool the water down enough before it gets back to the coal stove. Does this make sense? Can anyone see potential flaws? Am I missing any components what will make this system safe? Last thing I want is an explosion in my coal stove.
  2. Siphen

    Siphen New Member

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    5
    Here is my horrible diagram.... lol

    Attached Files:

  3. Southern Man

    Southern Man DIY Hillbilly

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    Location:
    North Carolina
    There was a good episode of Myth Busters over last weekend where they wanted to test the "hot water heater turns into a rocket" myth. They disconnected the electric over temperature controller and the T&P valve on an electric hot water tank. They started with a 6 gallon unit that created a small steam explosion, probably not big enough to kill you unless you were in that room. Then they went to a 30 gallon unit that shot up into the air so high it took 40 seconds to come back to earth. Then the built a small house to California building codes and put in a 50 gallon unit. When that took off it destroyed the house instantly, shot through the roof like it wasn't there and took about 45 seconds to return to earth.

    In other words, confining water and heating it up can kill you quicker than just about anything. One gallon of water makes 1000 gallons of steam. You are basically making a coal fired boiler so you have to design it and build it as such.

    Coal boilers can be safe but you need redundant controls to keep the pressure in the system below 15psi. Make sure that the system that you are buying conforms with local codes, hire a a professional to install it, and get a permit from your local CEO before you do anything.
  4. hj

    hj Moderator & Master Plumber Staff Member

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    heater

    They must not have tried hard enough with that 6 gallon heater. With a little bit of effort, I or you, can find the link to the Wisconsin school which had a 6 gallon heater explode and destroyed the cafeteria and kitchen. In any case, for this problem. What are you going to do with the heat when the furnace heated area is hot enough and also the baseboard? Will you create a third, then a fourth, etc. until you can finally use up all the heat?
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    You would also need something like an automatic damper control to turn the burn rate down. Note, this could produce a lot more CO. The local inspector might just revoke the occupancy permit. You'd be okay if you had an air-to-air heat exchanger and ran the heated air. I wouldn't be comfortable rolling my own heat exchanger - too risky with CO as well.

    With a boiler, they've got usually at least three safety devices: under pressure, over pressure, and high limit (of the water). You've only got one: overpressure.
  6. leejosepho

    leejosepho DIY scratch-pad engineer

    Take a look at this thread:
    http://www.terrylove.com/forums/showthread.php?t=16248&highlight=wood heat exchanger

    Steam locomotives have large whistles for releasing excessive pressure from coal-fired boilers, but I doubt your neighbors would tolerate one at the peak of your roof.
  7. Siphen

    Siphen New Member

    Messages:
    5
    I am not interested in blowing anything up. I bought the unit from a guy who was using it to heat his domestic hot water. With the air release valve, pressure release valve and pressure gauge, I figured that should prevent it from potentially exploding, but I am no plumber. The Stove would be on the first floor my air handler is in the basement.

    The stove I am using is a Harman 150, these are sold with an option for hot water coils. http://www.harmanstoves.com/doc/sf-150-250m.pdf Page 17 is there recommended method for setting up a thermo-siphon.

    I am not interested in creating steam at all. I have heard that a type of antifreeze can be used in hot water plumbing that raises the boiling point to about 380 degrees fahrenheit. This may give me a little more overhead as I would not need to pull off as much heat before the water heads back to the stove.

    For a secondary zone, I figured I had 2 options, dig a trench below the frost line and run a loop underground to bleed of heat outside, or run a loop around my basement and have it bleed off down there. I was leaning toward the basement as it is currently unfinished and I could always crack a window down there to wick away some of the heat. I realize I will potentially be wasting heat here, but it would just be going up the flue anyhow.

    I would really like to make this work, if I can.
    Last edited: Oct 6, 2008
  8. Bill Arden

    Bill Arden Computer Programmer

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    Location:
    MN, USA
    1. You need an expansion tank near the pump inlet.

    2. keep the pump and fan running when ever the boiler has a temp over 90F and keep the water temperature and pressure low as possible.

    3. You do want to pull off as much heat as possible to increase efficiency. The lower the water temperature, the more efficient the system will be.

    3. You will want to make sure the small blower fan has a flap to stop airflow when it's off. Not having a flap is the easiest way for the system to overheat.

    Remember you want to regulate the fire using the thermostat, not the water temperature.

    Edit: Remember that the best way to increase safety is to ensure that the system can dissipate more heat than the fire will create. You might want to look into adding a few PEX floor heating zones.

    FYI: They also make Water "A" coils for furnaces.
    Last edited: Oct 6, 2008
  9. leejosepho

    leejosepho DIY scratch-pad engineer

    Yes, and like with the whistle in relation to steam pressure, that must be a guiding principle. When your system can *always* keep the water temperature below about 140* even when the fire in the box is at its greatest, no explosion should be possible. However, and here is where I would shy away:

    Being dependent upon electricity to keep the water temperature below about 140* would keep me awake at night. If the power fails or a fuse blows or a relay burns out or whatever else might happen, the water temperature could, and it probably would, soon disrupt everything within a quarter-mile.

    Also, I grew up in a house with a coal furnace, and I remember my dad having to get out of bed and stoke the fire in the middle of the night and early in the morning ... and I remember the overall coal odor (rising from the bin in the basement) that permeated the house. But, we did have a nice, black driveway after several years of hauling out and spreading the firebox leftovers.
    Last edited: Oct 7, 2008
  10. nhmaster

    nhmaster Master Plumber

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    3,189
    Location:
    S. Maine
    This sort of installation usually requires a heat dump zone. Also installing the hydro coil in the ductwork is going to make a major change in air velocity over the AC coil and may have some very adverse effects. The best way to handle the situation is with a separate air handler for the hydro coil and a couple of motorized zone dampers.
  11. Siphen

    Siphen New Member

    Messages:
    5
    Thanks for all the comments.

    My main problem as you all can see, is stability. This coal stove will be running from the day I turn it on until the end of the winter, unless global warming kicks things up a notch this year. The water coils in the unit are within 1 foot of the burning coal bed, one turn of the coil would be 6" or closer depending on the depth of coal bed at the time. With a 2/3 or full burn these coils will be seeing temperatures between 750-950 degrees fahrenheit it seems inevitable the water or whatever liquid I put in there would vaporize at some point no matter how much I circulated it.

    Perhaps I can go a dry route, just pushing air around the system. The water/air heat exchanger would become an air/air heat exchanger. With appropriate release valves to keep the air pressure in the system low.

    I still have some time to flesh out ideas until I get my hearth and chimney done. Once I actually get the stove up on the hearth and fired I should have a better idea of the temperatures that I will be dealing with.
  12. hj

    hj Moderator & Master Plumber Staff Member

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    Location:
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    locomotive

    A locomotive has a fireman to control the input and a relief valve to control overpressure. I doubt that the engineer would want to run for miles blowing his whistle. My wife's uncle used to work in a brewery where everything was run by steam. When they would get a new power plant operator, the guys would all run their lines at maximum capacity and the engineer would have to fire the boiler like crazy to keep up, and then on a signal everyone would shut down and the boiler would blow its relief valve. It made a sound that could be heard for a mile and everyone would know what happened and the engineer would be embarassed.
  13. Southern Man

    Southern Man DIY Hillbilly

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    530
    Location:
    North Carolina
    If you're going to go the DIY route I'd suggest sticking to air only heat exchangers. Do the math on this, and if you can afford the up-front costs, you can probably heat the house, the floors, the domestic hot water, and melt snow on your driveway with the coal stove and a hot water coil and all the bells, whistles and safety devices that this calls for. You won't get the efficiency with air to air, but you will get something out of it for a lot less money.
  14. Southern Man

    Southern Man DIY Hillbilly

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    530
    Location:
    North Carolina
    Yeah I think that little 6 gallon on Myth Busters had a seam defect that gave way prematurely. Or maybe it was designed to fail that way. The larger tanks failed at the bottom end cap, instantaneously ripping apart the welds all along the circumference. That itself was amazing, as I thought that one weld would rip, followed by a sequential failure of the adjacent welds, emitting steam from one side and the "rocket" would lean over and scoot across the ground.

    The whole setup had me a bit worried, as their viewing bunker was a shipping container with the open end towards the tank, and bullet proof glass at the front. I thought there was a significant chance that the tanks would fail as I described, smash through the glass and create a bit of excitement inside the container.
  15. alternety

    alternety Like an engineer

    Messages:
    650
    Location:
    Washington
    Just out of curiosity, how does one find a coal dealer for a residential application. I thought about using coal, but yellow pages and Google did not find anything. It would appear they are kind of sparse.

    I still have a space to put a coal boiler, and I would use a big tank of water for a dump. Batteries to keep things circulating. I have no idea if coal is less costly than wood. A feeder would make coal a bit lower operation man-hours.
    Last edited: Oct 7, 2008
  16. Siphen

    Siphen New Member

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    5
    Depends on where you live. I live in PA so coal is cheap.
  17. Southern Man

    Southern Man DIY Hillbilly

    Messages:
    530
    Location:
    North Carolina
    In a lot of areas its almost a specialty item. In Upstate NY my neighbor had a "pellet" stove and his dealer had a pickup with a dump body and delivered the stuff. This same guy also delivered corn (yeah that burns too) and "anthracite pea coal", which was a similar size as, well, peas, and was apparently washed, as it was relatively dust free.
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