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  1. #1
    Join Date
    Jan 2009
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    Default Furnace too large?

    My furnace shut itself off today, and the HVAC guy tells me that the 120K btu unit is way oversized for the 1100 sf house. He says it's putting out more heat than the ductwork can evacuate, thus keeping the unit too hot and tripping the over-temperature switch. Unjit worked fine for several years I assume, but today (10 below zero) it decides to get too hot. Any ideas on how to keep it from shutting itself off?

  2. #2
    Join Date
    Jun 2007
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    Fayette County, Ohio
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    5,775

    Default Re: Furnace too large?

    The high temperature limiter cuts off the flame when the heat exchanger gets to hot then lets it turn back on when the blower cools the exchanger down to a safe level. With the furnace oversized it has probably been over heating and has been using the limiter as a switch turning the gas on and off on a regular bases. The limiter is not designed for constant use. One solution that may work is to connect the blower to run at a higher speed while heating.
    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  3. #3
    Join Date
    Aug 2007
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    The Great White North
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    Default Re: Furnace too large?

    No one can really comment over the internet whether the size of furnace is correct or not
    If this furnace has been in use with the same size ducting for a number of years I would agree .... something doesn't sound quite right.

    If your furnace is 20 years or newer it's likely to have electronic controls and sensors.
    There could be a number of things that could be causing the furnace to shut down.

    If it has something to do with over temp. it could even be the high limit fan control. Basically this control not only turns on / off the fan but also monitors the high temperature limit that has been set.

    If the fan/blower doesn't turn on this high limit will sense the excess temperature rise and shut down the furnace.
    Some furnaces will have a high temperature sensor near the burners as well.

    If any of those conditions or components were faulty would shut down the furnace.

    Without knowing the particulars it's difficult to exactly say what the problem is and at this point just guessing.

    However .... I don't believe the explanation of the furnace is overheating because it's suddenly too large.

    Sounds as though you better contact another HVAC technician to give a second opinion.

    Hope this helps.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  4. #4
    Join Date
    Aug 2007
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    1,131

    Default Re: Furnace too large?

    Quote Originally Posted by mossmanman View Post
    My furnace shut itself off today, and the HVAC guy tells me that the 120K btu unit is way oversized for the 1100 sf house. He says it's putting out more heat than the ductwork can evacuate, thus keeping the unit too hot and tripping the over-temperature switch. Unjit worked fine for several years I assume, but today (10 below zero) it decides to get too hot. Any ideas on how to keep it from shutting itself off?
    Is this a natural gas furnace, LP furnace or an oil-fired furnace? If oil-fired, you could likely install a smaller nozzle to reduce the output. (Installing a smaller nozzle would also require "re-tuning" the combustion air input, etc.)

    Also.....if an oil-fired furnace.......where is the oil-tank located? If outside......it might be that the fuel gelled in the line and that's what shut down the furnace. (This wouldn't be the likely cause though if the furnace fired up again shortly afterwards)

    While 120,000 BTU furnace does sound oversized for an 1100 sq. ft house, I doubt that is the real reason why the furnace shut down. An over-sized furnace causes furnace short-cycling and wasted fuel up the chimney, but really shouldn't cause high-limit switch tripping.


    How long since you changed the air-filter? If it's clogged with dirt...that will reduce the air flow over the heat exchanger which could/would cause tripping of the high-limit.

    On a similar note....have you covered over any cold air returns for any reason? See above.


    If this furnace has a belt-driven blower.......the belt could be slipping. This could/would also result in low flow over the HE which could then cause a high-limit trip.

    If this is a high-efficiency furnace with forced-exhaust.....it might have shut down because of some manner of "obstruction" in the flue pipe.

    Like canuk said.....there are too many possibilities to diagnose the real problem from here. I would also advise that you seek out someone different to come look things over.
    Last edited by goldhiller; 01-16-2009 at 11:51 PM.

  5. #5
    Join Date
    Jan 2009
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    2

    Default Re: Furnace too large?

    Hey thanks ****-man. It's me richardreau, now mossmanman, returning briefly to the boards for some hvac help. So what's new since I've been gone? Did lesliek finally fade away? Or did she morph into dwarfwitch? Was lesliek a she at all, and did he just enjoy aggravating so many people?
    Here in OH business was very slow in '08, so we bought a foreclosure and I spent the summer renovating it. It's now housing my son and 3 roomies as they are are freshman at OSU. Think animal house...Then came fall and the schedule got very busy with a large renovation for a client and some smaller stuff too. My knees are killing me!
    Thanks again, and best wishes...rr

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