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  1. #14
    Join Date
    Jul 2006
    Location
    Seattle, WA
    Posts
    103
    Course on a 15 yr old tempstar, it could be the combo fan/limit switch has a weak spring, or the heat exchanger has a partial plug....

  2. #15
    Join Date
    Jun 2004
    Location
    Iowa
    Posts
    128
    ive seen where the homeowner has tons of cats and the A coil is plugged with tons of cat hair, blockin the air flow.

  3. #16
    Join Date
    Jul 2002
    Location
    St Paul, Minnesota
    Posts
    3,468

    Re: I have noticed this to be the

    Originally posted by cn
    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.
    I see a lot wrong with your thinking. Some has been picked up on.

    160-170 is the temp you shoot for? Who told you that?

    Unit has been service by a company? Not sure if they ran a complete cycle? I would hope they checked a few things:

    Gas intake and manifold pressure: They are rated by manufacturer and posted on name plate.

    Temp rise: Also posted on manuf. label. Best to follow their advice.

    Clean blower wheel: A dirty one just spins in the air, it can't grap and throw air.

    Not sure if constant overheating is harmful?
    What planet are you from? Go out and start your cold car in the morning and once it starts just floor the acelerator for about 10 seconds. Won't hurt the car a bit, right? After all you floor it on the freeway all the time, right?

    I'm thinking you better rethink your thinking.

    /Anyone thinking of a defective heat exchanger/seal allowing hot combustion air to influence the limit?

    [Edited by MikeJ on 10-17-2006 at 12:07 AM]

  4. #17
    Join Date
    Jul 2005
    Posts
    717
    [QUOTE]Originally posted by simpleman

    None of the above will cause a high limit problem.
    ************************************************** **
    oh yes,lol, the size of the unit and the size of the home has a lot to do with it.
    but thanks anyways for that JOKE statement.(lol)

  5. #18
    Join Date
    Oct 2005
    Location
    Omaha, NE
    Posts
    309

    Gas Furnace High-Limit...what do people set them at?

    Originally posted by MikeJ
    Originally posted by cn
    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.
    I see a lot wrong with your thinking. Some has been picked up on.

    160-170 is the temp you shoot for? Who told you that?

    Unit has been service by a company? Not sure if they ran a complete cycle? I would hope they checked a few things:

    Gas intake and manifold pressure: They are rated by manufacturer and posted on name plate.

    Temp rise: Also posted on manuf. label. Best to follow their advice.

    Clean blower wheel: A dirty one just spins in the air, it can't grap and throw air.

    Not sure if constant overheating is harmful?
    What planet are you from? Go out and start your cold car in the morning and once it starts just floor the acelerator for about 10 seconds. Won't hurt the car a bit, right? After all you floor it on the freeway all the time, right?

    I'm thinking you better rethink your thinking.

    /Anyone thinking of a defective heat exchanger/seal allowing hot combustion air to influence the limit?

    [Edited by MikeJ on 10-17-2006 at 12:07 AM]
    Now I am rethinking my thinking....

    My point is that: if the "high-limit" is set at 180F (is this industry standard), then if the furnace is adjusted in such a way the temp hovers around 160-170F, a bit below the High-Limit then it seems common sense to me.
    BTW: Is 180F industry standard?

    (Like a car with temp gauge below the HOT zone, rather than running into the HOT zone).

    The rest of the heating system is checked:
    - all air ducts (return and supply unobstructed)
    - A-coil cleaned last year. House has no pets, no carpet ( a few rugs and that is it)
    - No smokers
    - Air filter brand new but it is 3M type
    - blower cleaned with vacuum machine. No dusts.

    - Blue flame in the burners about 2 inches high or so (per tech).


  6. #19
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    It is almost certain that you have an air flow problem.

    Ducts could have been undersized since day one,or the coil was only surfaced cleaned and is impacted(has dirt and stuff in the middle),or a duct could have been crushed by the cable guy,or etc.,etc..

    Furnaces are not designed to cycle on the limit,call a Pro ,to get the problem solved.

  7. #20
    Join Date
    Oct 2005
    Location
    Omaha, NE
    Posts
    309

    Thanks will call the HVAC tech back but

    1. How to clean A-coil properly? I hear some people use a spray???

    2. If ductwork undersized then I doubt there is anything I can do about it.
    But back to my original question: what is wrong with asking the tech to adjust the gas valve a tad so it does not trip the high-limit?
    House is nice and warm and air flow seems fine.

    The other way to phrase this question is: is there any harm to adjust the gas valve down a tad?
    Isn't this what HVAC techs do when installing the brand-new furnaces anyway?
    I suspect the furnace was not adjusted properly even from day one.

    The reason I say this is because from seeing the HVAC setup and ductwork in this house, I know it was done by inexperienced people (I am the second owner). The register layout is another story!!!!

  8. #21
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Duct systems can often be improved to and acceptable static,by adding turning vanes in accessable 90° elbows ,often those near the furnace.

    Often the return drop9main return duct along side the furnace),is to small or doesn't allow turning vanes to be added,but it can be replaced,with a wider one that allows vanes.

    Post pictures of the furnace and duct,sizes and model number to get suggestions.

  9. #22
    Join Date
    Sep 2002
    Posts
    3,708
    [QUOTE]Originally posted by deejoe
    Originally posted by simpleman

    None of the above will cause a high limit problem.
    ************************************************** **
    oh yes,lol, the size of the unit and the size of the home has a lot to do with it.
    but thanks anyways for that JOKE statement.(lol)
    Why you're laughin? If you assume that a oversize furnace
    and a small home would have the furnace going out on limit then I should be the one that laughin.

    Oversize furnace and a small home will cause short cycling.
    LOL!

  10. #23
    Join Date
    Jul 2002
    Location
    St Paul, Minnesota
    Posts
    3,468

    Time to learn about temperature rise.


    It is what each manufacturer deems appropriate for that model of furnace. Temp rise can vary but should fall within manufacturers specs.

    Let's say the label inside the furnace says that a temp rise of 35 - 70 degrees is on the label inside the unit.
    \
    I have 68 degrees coming into the furnace, usually taken at the filter and a temp reading a few feet down the trunk line away from turbulance caused by the a/c coil or plenum of 103 degrees. That is at the very bottom of the specs:35 degree difference.

    Now, let's say the temp down the trunk line was 138 degrees. That would be a 70 degree difference, that is, at the top end of the specs. If I let the unit hover around 165 degrees down the trunk line, I would have almost a hundred degree temp rise, far over specs. My warranty on the heat exchanger would be invalid if the company knew this, or, my heat exchanger would be stessed out and probably fail prematurely. Constant overheating kills athletes as well as furnace heat exchangers.

    Too low of a temp rise causes condensation to buildup in the heat exch., venting, inducer housings, and chimneys. Not good either.

    Hope this helps.

    BTW, changing the manifold pressure rather than finding the real cause is also ill-advised. Tweaking down to 3.25 probably won't make a diffence anyway.

    If your return temp is 78 degrees and you had 148 in the trunk line you still would have a 70 degree temp rise.

    3M makes several filters. Which one are you using? What is the temp without a filter?

  11. #24
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,907
    If your using the 3m filtrete, its very restrictive.

    Adjusting down the gas valve could cause condensation in the flue or chimney. And cost you more to repair then fixing the real problem.
    Contractor locator map

    How-to-apply-for-Professional

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

  12. #25
    Join Date
    May 2004
    Location
    Rapid City, SD
    Posts
    7,415
    [QUOTE]Originally posted by simpleman
    Originally posted by deejoe
    Originally posted by simpleman

    None of the above will cause a high limit problem.
    ************************************************** **
    oh yes,lol, the size of the unit and the size of the home has a lot to do with it.
    but thanks anyways for that JOKE statement.(lol)
    Why you're laughin? If you assume that a oversize furnace
    and a small home would have the furnace going out on limit then I should be the one that laughin.

    Oversize furnace and a small home will cause short cycling.
    LOL!
    Well... both of you are correct. "normally" an oversized furnace would just short cycle. But lets say the 1st system was correctly sized, then Bubba's heating came in, they always oversize a unit by one size... now the duct is too small, and this problem comes up.

    The beauty of this trade is all of the variables that pro's consider when they do their job properly.
    "If you call that hard work, a koala’s life would look heroic."

  13. #26
    Join Date
    Sep 2001
    Location
    Vancouver Canada
    Posts
    996
    For starters, fan switch should be coming on before the unit reaches anywhere near 165-170 degrees unless you have the house temp at 100 degrees. That is way too high a temp rise on any furnace on the market. I can see if the fan is coming on at 170 degrees it could easily hit 180 before it has a chance to get rid of all that built up heat. Bottom line, as you have been told quit dicking around with this thing and get someone in there who has a clue...to fix this unit. You talk about the system being H.O. installed and here we now another H.O. diagnosing and repairing it. Two wrongs don't make a right.
    "Go big or Go Home"

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