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Wood Furnace Overheating

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by garrymg7, Nov 1, 2010.

  1. garrymg7

    garrymg7 New Member

    Joined:
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    Loc:
    central Ohio
    I have a United States Stoves Hotblast furnace with dual blowers, two 8 inch air ducts, and 6 inch exhaust piping. It has a lower air feed control on dust pan door, and another air feed control on main opening door. I have round 8 inch ductwork going to two separate places up to the floor above. By the way, it is located centrally in the basement. I am really having some problems controlling how hot the stove burns. Last night I loaded up the stove at about 11 pm and at about 1 am I got up to a stove which had pegged my thermometer on the face of the stove to well over 900 F. The air controls were only opened slightly and the seals are good on the doors. This really concerns me, because of possibly igniting a fire. Any suggestions? Thanks.

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  2. bigburner

    bigburner Feeling the Heat

    Joined:
    Aug 28, 2010
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    438
    If I read this correct. you have a jacket wood furnace. air flow is every thing. fans running, wheels loaded up, short circuit air flow some how ?? get us some pictures.
  3. garrymg7

    garrymg7 New Member

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    Sep 26, 2008
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    Loc:
    central Ohio
    Yes, it is a jacketed furnace. The blowers seem to be putting out a lot of air. Do you think the blowers are undersized even though they throw out a lot of air? I have no way to measure the CFM. At least, none that I know of.
  4. vvvv

    vvvv New Member

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  5. flyingcow

    flyingcow Minister of Fire

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    is the furnace fan forced draft? if so is the high limit on boiler jacket not set right?i.e. 180/195ish?

    You said 900 degrees on the face of stove? Stack temp was 900? Water jacket wouldn't be that high? Stack temp would be, maybe. What surface is the thermometer attached to?
  6. FixedGearFlyer

    FixedGearFlyer Member

    Joined:
    Oct 8, 2010
    Messages:
    87
    Loc:
    Michigan's Upper Peninsula
    For those of you asking about the water jacket temp, he's got a forced air wood furnace, not a boiler. That Hotblast model is just like our Vogelzang Norseman 2500, just under a different branding.

    When we moved into this house last spring, we also had trouble with the furnace overfiring. Not only did it run really hot, but it chewed through wood like a beaver with an agenda. As a previous poster mentioned, a barometric damper made a big difference for us. The draft was too strong without it and the stove would draw too much air, even when it was mostly closed up.

    A couple of thoughts come to mind. First, if you have the optional draft induction blower installed, you might want to turn it off or disconnect it. It's really for coal burning. If it's not installed, check the threaded plug on the back side of the furnace to make sure it's installed properly. If it's loose - or missing - you'll have a huge air leak that would contribute to this issue.

    Second, make sure that the bi-metal thermostat on the loading door is doing its job. It should open and close a flapper draft door that you can see through the bottom of the metal box on the loading door by raising and lowering a chain. If the thermostat or draft door is mis-adjusted or stuck open, that would also cause too much air. If it is correctly adjusted, try turning it down, if you haven't already.

    Finally, I recently posted another thread about lining the bottom of my furnace with firebrick. It just went right over the top of the grate and made a huge difference in how the furnace functions. Granted, it made it burn hotter and longer, but you might find that blocking the rocker grate with firebrick helps you control your draft, too.

    When our furnace is really burning well (after the firebrick install), we get good secondary combustion in the main fire box below the baffle, even without a secondary air supply. The stove pipe temp hits about 550 right after the heavy burn starts, then settles down to about 450. I don't have a stove front thermometer, so I can't tell you what that measures.
  7. vvvv

    vvvv New Member

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  8. jeff_t

    jeff_t Minister of Fire

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    The Hotblast doesn't have a bi-metal thermostat, just a sliding damper under the cover on the door. Mine was really sticky, and there is no good way too see how much it is open. I took the sheet metal cover off the door and drilled a couple holes and reattached the handle, leaving the outside cover off. I freed the slider up so it moves smoothly, and with the cover off I can see exactly how far it is open. I also took the spinner off the ashpan door and ground it flat so I can close it all the way. I usually only use the damper on the door to control airflow, and only open the ashpan spinner on cold starts and reloads.

    I also rarely fill the firebox all the way up. It's tempting because it's so big, but with good dry wood it is just too much, especially to get burning cleanly. My air controls almost always get set about the same, and I control heat output with how much wood I put in it. It will take a 27" split, and I've got wood cut 2'+, but I save that for really cold nights and give it a little more air. This time of year I burn 16-18" logs shoved all the way back under the baffle. Not the best for burn times, but otherwise it will be 80 in here.

    I don't have a thermometer so I don't know how hot it runs, but I don't think I've ever had it at 900. That would make me pretty nervous with a 7 gauge firebox. I would get a manometer and check your draft.
  9. FixedGearFlyer

    FixedGearFlyer Member

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    Loc:
    Michigan's Upper Peninsula
    Jeff_T - thanks for the Hotblast specific info. I thought it was the same as my furnace, but I was obviously wrong!
  10. jeff_t

    jeff_t Minister of Fire

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    The thermostat is the only noticable difference. I looked at the Norseman at Menard's, then got the manager at the local TSC to match the price on the Hotblast. Well, it also helped that I bought the Huskee splitter at the same time. Got a 'package deal'.

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