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  1. #1
    Join Date
    Oct 2005
    Location
    Omaha, NE
    Posts
    309
    I will ask my HVAC tech to do routine checkup soon.

    But from owning different houses and different furnaces and watching them go through a few complete cycles, I have made the following observation:

    1. Thermostat calls for heat
    2. Electronic pilot ignites gas, temp rises until it reaches "Low-Limit" (around 120F)
    3. Fan kicks in
    4. Burners continue to work, temp continues to rise until it reaches "High-Limit" (around 180F), then the gas is cut off, fan continues to run until it cools down to minimum (around 120F) then furnace shuts off.

    My questions are:

    a. Is it normal for furnace to reach High-Limit on every cycle?
    b. Is it a good idea if the HVAC tech adjusts the GAS VALVE down a bit so when burners run, they do not heat too much causing the HIGH-LIMIT switch to trip?
    Potentially saving gas and money?

    Thanks.

    cn
    1991 Tempstar 80% Furnace

  2. #2
    Join Date
    Sep 2002
    Posts
    3,708
    If your furnace is going out on limit then you have airflow problem.

    Or you are keeping the stat setting way above 80 degrees.

    Its could be overfired but, I doubt it.

  3. #3
    Join Date
    Oct 2005
    Location
    Omaha, NE
    Posts
    309

    Furnace well-maintained

    -Air Filter changed often (every 3 months)
    -Thermostat set at 68-69F.

  4. #4
    Join Date
    Sep 2002
    Posts
    3,708
    Originally posted by cn


    1. Thermostat calls for heat
    2. Electronic pilot ignites gas, temp rises until it reaches "Low-Limit" (around 120F)
    3. Fan kicks in
    4. Burners continue to work, temp continues to rise until it reaches "High-Limit" (around 180F), then the gas is cut off, fan continues to run until it cools down to minimum (around 120F) then furnace shuts off.

    [/B]
    What you have posted does not indicate that the furnace is goingout on limit.But actually it sounds like that stat is taking it out.

  5. #5
    Join Date
    Oct 2005
    Location
    Omaha, NE
    Posts
    309

    Actually I watched the cycle

    Originally posted by simpleman
    Originally posted by cn


    1. Thermostat calls for heat
    2. Electronic pilot ignites gas, temp rises until it reaches "Low-Limit" (around 120F)
    3. Fan kicks in
    4. Burners continue to work, temp continues to rise until it reaches "High-Limit" (around 180F), then the gas is cut off, fan continues to run until it cools down to minimum (around 120F) then furnace shuts off.
    What you have posted does not indicate that the furnace is goingout on limit.But actually it sounds like that stat is taking it out. [/B]
    and see the dial in the Limit Switch keeps rising until it hits the HIGH-LIMIT, then the flame goes out but the blower still runs.

  6. #6
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    If you see it going out on high limit,then I don't doubt it.

    Going out on high limt is likely due to lack of air flow,as stated above.

    Furnace has X btus of heat,if air flow is not sufficient ,the furnace gets too hot,and goes out on the limit.Not what you want to have happening.


    Might want to try it for aa few minutes without the filter,this will aloow a little more air flow,so it should run longer,or not trip the limit.

    What type filter do you use,if it's a 3M ,that could easily be the problem.

  7. #7
    Join Date
    Oct 2005
    Location
    Omaha, NE
    Posts
    309

    Yes using 3M (or similar Filtrete)

    Originally posted by dash
    If you see it going out on high limit,then I don't doubt it.

    Going out on high limt is likely due to lack of air flow,as stated above.

    Furnace has X btus of heat,if air flow is not sufficient ,the furnace gets too hot,and goes out on the limit.Not what you want to have happening.


    Might want to try it for aa few minutes without the filter,this will aloow a little more air flow,so it should run longer,or not trip the limit.

    What type filter do you use,if it's a 3M ,that could easily be the problem.
    I will try to run without the air filter and see what happens.

    On the other hand, I'd like to see furnace tuned so at peak operating temp, it should be around 160-170 F or so, but not 180F that trips the high-limit.

    Thanks

  8. #8
    Join Date
    Jul 2005
    Posts
    717
    Cn; If your furnace is reaching it's high limit setting (1800 then it is due to one or all of the following;
    obstructed/dirty filter.
    restricted return air
    fan speed set too low.
    Several supply air dampers /registers, or runs are not fully open and are causing too high a temperature rise at the plenum or near the heat exchanger, thus tripping high limit.
    Normally on a T stat call for heat the burner will fire up,then a few minutes or so later at approx 120-130 degrees in the plenum the blower will kick in. The burner will continue to run until the T stat is satisfied, then shut off while the blower continues to run until the supply plenum temp gets down to approx 90-95.

    Some questions for you;
    When did this "high limiting" start occuring?
    Did it just start acting up like this overnite.Have any adjustments been made to the system which would cause this problem?
    What size is this unit (btu output) and what size of home.?

  9. #9
    Join Date
    Sep 2002
    Posts
    3,708
    Originally posted by deejoe
    What size is this unit (btu output) and what size of home.?
    [/B]
    None of the above will cause a high limit problem.

  10. #10
    Join Date
    Oct 2005
    Location
    Omaha, NE
    Posts
    309

    I have noticed this to be the "normal"

    cycle during the last year that I own it.
    Serviced by HVAC (not sure they watch a complete cycle).

    I will ask if they can adjust the gas valve down a bit.

    All registers are open.
    Blower healthy blowing lots of air
    I will try w/o the air filter and see what happens.

    Maybe it was over-adjusted from day one (1991)!
    Probably no harm as the high-limit shuts the burners instead of the thermostat.

  11. #11
    Join Date
    Jun 2004
    Location
    Iowa
    Posts
    128
    i got a call about this happenin also, went to the house she had rugs covering the return air grills, plus all the registers were closed. took the rugs off and opened the registers problem fixed.

  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,031
    Doubtful adjusting the gas valve is the real solution.

    You more then likely have an air flow problem as others have said.

    Your dust work can still be undersized and the regs blow out alot of air, because there aren't enough of them for the furnace size.

    Your furnace is probaly over sized for your house and duct system.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  13. #13
    Join Date
    Jun 2003
    Location
    Florida
    Posts
    9,548
    Tripping out on high limit,wanting tech to adjust gas valve, you need a service call, where airflow/safeties are checked and valve/combustion can be dialed in.
    If everything was always done "by the book"....the book would never change.

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