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Need info on how a blower on a wood furnace is supposed to work

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Cold in N.B., Jan 23, 2008.

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  1. Cold in N.B.

    Cold in N.B. New Member

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    Our century old farmhouse has a century old wood furnace that provides great heat. It is -35C outside and +28C inside. My problem is the blower on the furnace. When is it supposed to run? When the room temperature is down or when the heat in the furnace is high? We have a thermostat, of sorts, for the blower.

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  2. Eric Johnson

    Eric Johnson Mod Emeritus

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    Ideally, the fan that blows hot air into the house should kick on when temps in the living space drop to a set point, which would be the thermostat setting. It should also have a control that runs the burner (or if it's a wood-fired furnace, the draft blower) when there's a call for heat. So in a typical arrangement, when the room stat calls for heat, the blower should start and the burner should fire. In a somewhat more sophisticated setup, the burner would fire on a call for heat and the blower would kick on when the temp in the furnace got up to where it would actually provide heat to the house.

    But with a 100-year-old setup, who knows? It's a long ways from being efficient, that's pretty much guaranteed.
  3. Cold in N.B.

    Cold in N.B. New Member

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    So, if I understand correctly, set my thermostat at say 72 degrees and when the room temp drops to that, the blower should come on and send the heat up the ducts. I'm not sure if there is another control on the blower. My father (86 years old) has decided this year that the blower shouldn't kick in until the house reaches the high 70's but last year the blower was on an off and countless times in a day.
  4. ericjeeper

    ericjeeper New Member

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    The wall stat should tell a damper/forced draft blower to come on. That will bring up the firebox temps.. The blower should not come on unless the temp in the housing reach ,,, say 150 degrees.,
  5. Eric Johnson

    Eric Johnson Mod Emeritus

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    It probably also has some sort of overheat protection, which would blow hot air into the living space if the firebox got too hot, regardless of what the room stat is set at. At least that's the way I'd do it.
  6. Cold in N.B.

    Cold in N.B. New Member

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    We get excellent heat without the blower but use a lot of hardwood to do so. My fear is that we are overheating the furnace which is forcing the heat out and up. So I figured the blower sending the heat (once the furnace is hot) to the cool room is the safest.
  7. reaperman

    reaperman Member

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    The furnace blower is not directly connected to the wall thermostat whatsoever. Nor should it be. There should be a limit swith (circular dial) somewhere on the wood furnace telling the plenum blower to kick in, thus keeping the furnace from overheating, and blowing the hot air throughout the house. Normally this switch is set to run the fan at 150 degrees and shut if off somewhere 100 degrees. Or instead it may have a snap disc, that would sense the correct temperature to run the blower fan. But that too, would also be on the furnace.

    You mention you get excellet heat without the blower. Which tell me something isn't set right or something may be malfunctioning. The blower fan must run to cool the furnace, because most wood furnaces are insulated and cannot distribute heat. Therefore they can get very hot without a fan running to cool them. And yes, the blower fan will cycle on and off throughout the heating cycle. Especially when the fire is dying down. Good luck.
  8. begreen

    begreen Mooderator Staff Member

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    If you are running the furnace without a blower, what are the temps on top of the plenum on the furnace? What are the clearances to the nearest combustible? This has the potential to be very dangerous if safe clearances are not honored. Wood will pyrolyze if exposed to heat oover time and can combust at much lower temps than normal.

    For this reason theoretically, I'd like two switches. One connected to the thermostat that turns on the blower on demand and another hi-limit switch that turns on the blower under high heat to act as a heat dump. Or perhaps connected to an electric damper to snuff the fire in a high heat situation?

    I doubt there are any systems wired like this so it is important to keep the plenum generously away from combustibles for at least the first 6 feet from the furnace.
  9. Cold in N.B.

    Cold in N.B. New Member

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    My husband says there is a switch on the furnace but he doesn't think it is working.
  10. Cold in N.B.

    Cold in N.B. New Member

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    I will check the distances you mention and try to get back online again tonight.
  11. laynes69

    laynes69 Minister of Fire

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    I've never heard of a furnace with a heat dump on high. Maybe a relay that would open if the power would go out would be your best option. The thermostat shouldn't be needed all the time. Ours is set at 68 if the house gets to 68 it will kick in the forced draft fan. The can is on the limit control with it set at 175. So if the limit gets met, the forced draft fan will shut off. So we do have a thermostat on our wall, but the forced draft never runs, because the house heats easily. Now the woodfurnace blower to remove the heat should kick on and off depending on where its set. If its an older model, then it probably would have a limit control on the back of it. So if you are talking about a forced draft blower, then it should rarely run if your home heats well. Ours only runs when it goes close to 0 f or below. Really if not needed you should keep it low. You can easily double wood consumption with a forced draft running. If its the main blower itself, those limits can go bad. Look on ebay and just replace it. Pretty straight forward job. Also like said clearances are Very Important!! If your furnace blower isn't acting right, it could more likely be your limit. All that it takes is for the limit to stick, causing the blower to not come on, and a potential for a house fire. You should have at least 6" from combustibles for the first 6 to 10 feet of the ductwork. I love our furnace, but care should be taken.
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