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Wood Furnace Overheating

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

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  1. garrymg7

    garrymg7 New Member

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    I have a Hotblast 1557M located in the basement of a ranch style home. The house is about 70 feet long and the furnace is in the middle of a long side and vented outside through stainless triple wall pipe. The furnace has dual blowers so I ran duct work about 15 feet out to each side to registers I put in for this setup. It is in the teens outside right now and we would burn up if it weren't for opening some windows. That's not my concern though. I am concerned about how hot the floor registers get and the duct work connected to the floor. There are many times, the registers are so hot, I can't even touch them. This condition happens even if I have the furnace air intake
    completely closed off. I may be overly concerned, but could this possibly lead to the floor or carpet catching on fire? Thanks for any help.

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  2. summit

    summit Minister of Fire

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    do you have a thermometer to stick on the register? usuall testing is 90 deg over ambient = safe
  3. JustWood

    JustWood Minister of Fire

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    Check to see what your high limit switch is set at. Mine is around 160 degrees.
  4. garrymg7

    garrymg7 New Member

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    Thanks for the quick responses. I looked for a thermometer the other day. The stores were all out or didn't carry them. Any suggestions for a particular brand? As far as the limit switch goes, The registers actually get hotter when the blowers aren't going, so I generally keep the blowers running. Thanks.
  5. EatenByLimestone

    EatenByLimestone Minister of Fire

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    I think any magnetic thermometer will do you fine. The air moving over the register grates cools them off like air moving through the radiator in your car does.

    Matt
  6. JustWood

    JustWood Minister of Fire

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    I would still check the high and low limit settings . Sounds like your blower is not running long enough to keep the unit cool. This could lead to overfiring of the stove and warping of the firebox.
    Check door gaskets also.
    Try closing your drafts completely. You may have a very strong draft.
    You should be able to hold your hand on the duct within a few feet of the furnace and not get burned.
  7. begreen

    begreen Mooderator Staff Member

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    This is definitely something to watch. You need to be especially careful to maintain a safe distance between combustibles and the supply ductwork leaving the furnace. The supply ducts should be spaced at least 6" or more off of the joists to avoid pyrolysis of nearby wood. This is where there is the greatest risk of fire. The primary furnace plenums are required to be at least 2" from the joists.

    For measuring at the registers, first, they should not be closed off. Leave them all open and let them vent. A duct temp measured at the nearest to the furnace register of about 120-130 °F is ok. If it is reading over 150, then that is hot, over 170 °F I'd be concerned about. Use a decent high temp, probe thermometer to measure. Often they are magnetic to hold onto the register.
    http://stores.ebay.com/DeltaTRAK-Instruments
    http://www.goodmart.com/products/1200704.htm

    As for the furnace, perhaps it is being run too hot? This is a wood/coal unit. For wood burning, except for starting, leave the lower door vent closed. The maximum btu output of the furnace may be too high for the house. If so, try burning less wood per load.

    Attached Files:

  8. LLigetfa

    LLigetfa Minister of Fire

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    Does that model have an anti-gravity damper on the plenum? If the fan shuts off due to high limit or power failure, the heat rising by convection alone can become greater than the central duct allows for clearance to combustibles. Without an anti-gravity damper, the duct needs to be a class B vent with greater CTC.
  9. garrymg7

    garrymg7 New Member

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    It does not have an anti-gravity damper on the plenum. I typically run the blowers all the time so that the duct work doesn't over heat, but lots of times the duct work gets very hot anyway. So hot that you can't touch the duct work. It gets this hot even though I have all intake air shut off. Have concerns about starting a fire. Thanks.
  10. JustWood

    JustWood Minister of Fire

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    Are you burning kiln dried wood or lumber scraps?
    Your door gaskets maybe leaking or gone.
    Is this an old unit? If so it may have a crack in the firebox.
    My furnace ductwork never gets so hot that I can't hold my hand on it. Something is definetly wrong.
  11. laynes69

    laynes69 Minister of Fire

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    Your running 2 registers? The furnace is made to run into the plenum of the central furnace. The furnace is rated at 120,000 btus, so 2 8" ducts will get hot. You can't get the btus off the furnace quick enough. Theres nothing you can really do about the heat with that setup. Like posted just make sure your ductwork has clearances. Tying it into the central furnace would ease the heat. My ductwork isn't hot either, but I'm running a large blower that takes the btus away from the furnace. Thats one of the things I didn't like about those 1500 series furnaces. If there was an outage, they overheat quickly because they don't have a plenum that will take heat away. 2 8" ducts don't cut it in an outage. Its wasn't fun waking up to the smell of burning paint!
  12. TomB

    TomB Member

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    Try running the furnace without the inducer blower running (air to the firebox), reduce the amount of air in the box little by little.

    I have a yukon, I run it like a stove not a furnace and have no problems with over heating. I never have to use the inducer blower.

    You may have to make adjustments to your fan limit switch I run 110(off), 180 (on). try running yours at 150. If you pull the white knob on the fan limit switch the fan will run constantly, use it to check it you have enough air flow to drop the plenum temp.

    You may have the fan limit switch to high in the plenum. OR you may have a bad fan limit switch.

    My 2 cents... have patience, you have a great furnace. You'll get it tweeked.

    Stay warm. -7 this morning, house temp is 73.

    Tom
  13. LLigetfa

    LLigetfa Minister of Fire

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    You make a good point. Probably not enough air volume moved to dilute the temperature.
  14. laynes69

    laynes69 Minister of Fire

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    The forced draft is setup different than the yukon. It goes above the fire, and is worthless. If its a newer furnace it will have a snapdisc, no fan limit/control. Buying a fan limit control will cost around 40.00 if you shop around will be your best bet if you don't have one already. But even then if you have a good fire, no matter what you do it will still put out that heat at the registers.
  15. jeff_t

    jeff_t Minister of Fire

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    I have two 8" round pipes that run into the trunk on each side of my gas furnace. The trunk gets too hot to hold my hand on very long when I got it crankin, I couldn't imagine only having two registers. I just think you can't move enough air. The thing's made to heat your whole house, man. I think you need to make some changes.

    Be safe.
  16. begreen

    begreen Mooderator Staff Member

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    Moving this to the boiler room where there are more furnace owners and installers...

    Time to post a picture of this installation. We also need some measured temperatures. Measure the temp of the air coming out of the nearest register to the wood furnace trunk tie in and from the farthest register and let us know what you are getting.

    Regarding the wood furnace, is the blower intake of the furnace sucking return air from the basement, or is it tied to the central system return? Is there a barometric damper on the connector flue pipe?

    And last, what kind of fuel are you burning. Can you describe the size of the splits being used and how dry is the wood?
  17. smokinj

    smokinj Minister of Fire

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    quick answer would be a damper in the flue turn it down, but like other have said to set the hi limit switch to 160 degrees but to do this you need the honey well limit instead of the thermostat that comes stock in that unit.

    the honey well come in this kit not sure if it can be bought by its self
    http://www.usstove.com/proddetail.php?prod=11DIKL
  18. JustWood

    JustWood Minister of Fire

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    I don't think the # of ducts is the problem. Many furnaces come from the manufacturer with only 2 - 8" outlets and some only 1 - 10" or 12". My furnace only has 2 - 8" outlets and it never over heats. I had to adjust my limit switches from the recomended factory settings about 10 degrees higher to get mine to work better and cleaner.
    It sounds more like an over draft problem. Those blowers should be more than enough to keep the firebox cool when they run non stop.
  19. smokinj

    smokinj Minister of Fire

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    those blowers are "under power" at best and it has 2-8's on it, I installed a 750cfm inline blower to move more air. The pipes get to hot to the touch but just in the first few feet. Sure could be over fireing it but if the air intake is closed a flue damper would be the easy way to slow it down.
  20. jeff_t

    jeff_t Minister of Fire

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    Would be good to know about the chimney. I've noticed on cold windy days I just chuck more wood in it for more heat. I don't adjust intake air that much. The added pull up the chimney from the wind keeps it hot enough, since I don't have a barometric damper. But you did say it was a ranch, so I wouldn't think the chimney is overly tall.
  21. garrymg7

    garrymg7 New Member

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    I'm a novice wood burner, but I don't see where the limit switch comes into effect when I have the blowers going continuously. From what I have read so far, I think part of my problem is the size of duct work I am using. I go up from furnace with 8" and then elbow out from there a couple of feet with 8", but then I have it reduced with 4"
    rest of way to registers. I am guessing that the amount of heat my furnace produces doesn't have sufficient time and space to cool off that much. Does this sound logical?
  22. begreen

    begreen Mooderator Staff Member

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    Whoa dude. The 8" supplies off of the furnace are supposed to be directly connected to the supply plenum of the HVAC system. Not choked down to a measly 4" pipe. Yes it is overheating things and also causing the blower to race because of high static pressure. Read the manual again and follow it. This is not a toy, but a serious piece of equipment. If you are not qualified to connect it, get a pro in to set it up correctly.
  23. JustWood

    JustWood Minister of Fire

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    Reducing it to 4" is most of the problem.
    BG beat me 2 the punch!
  24. begreen

    begreen Mooderator Staff Member

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    I'm wondering if this is the only problem or if there aren't several other shortcuts taken here in a rush to hook this up cheaply or simply. Flue damper is required, but doesn't appear to be installed. Methinks this installation needs to be inspected carefully for proper installation, running and safety.

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  25. garrymg7

    garrymg7 New Member

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    I know I have the ductwork all wrong, but I was sure that the store that sold it to me and the instructions said it could be used as a stand alone furnace.
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