Is it Normal for furnace to reach high-limit switch often? - Page 4
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  1. #40
    Join Date
    Oct 2005
    Location
    Omaha, NE
    Posts
    309

    Please explain...............

    Originally posted by simpleman
    Originally posted by cn


    Adjusting OUTPUT temp down to 125F (from 145F) should save some gas (I will give a follow-up post later in the winter).
    [Edited by cn on 10-20-2006 at 03:13 PM]
    I doubt it.By reducing the radiant heat and the convective
    transfer to the hx to me would increase the gas bill.

    This why I ask what were the readings on the flame side.

    No worry..If you're happy then so be it.
    When I cook spaghetti, when water is boiling, I keep the gas burner low, any more heat will create flame that "licks" outside of the pot and is lost as heat.

    Of course there is the "recommended" temp rise of 45-75F in the mfg manual, and my HVAC tech adjusted to 60F temp rise (65 Home temp + 60 rise= OUTPUT temp of 125F).

    What do the pro's in this forum adjust their customer's gas furnace OUTPUT temp to? Just curious!

  2. #41
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,631
    Shoot for the middle.

    But check the temp the high limit is sensing by inseting temp probe in hole where limit is mounted.

    I use a tech 60 for combustion testing.

    You said he adjusted the gas down alittle. But you didn't say what the manifold pressure is now.
    He should have done a combustion test on it.
    The inducer is pulling x amount of combustion air into the burners, and if the gas volume is too low, it will cause poor combustion and lower efficency.

    Not to bust on the tech, but he may have cured the symtom, and not the ailment.


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  3. #42
    Join Date
    Feb 2004
    Location
    Ontario
    Posts
    700
    One further thing to add. Check that the insulation in the blower compartment is still firmly in place. On that type furnace it's not uncommon for the piece of insulation that should be glued to the side panel in the blower area to come loose. Then, when the blower comes on, it's drawn against the blower, blocking off much of the air flow. May not be your problem, but worth checking.
    Question authority!

  4. #43
    Join Date
    Oct 2005
    Location
    Omaha, NE
    Posts
    309

    Thanks, HVAC tech already checked that

    Originally posted by jetstream
    One further thing to add. Check that the insulation in the blower compartment is still firmly in place. On that type furnace it's not uncommon for the piece of insulation that should be glued to the side panel in the blower area to come loose. Then, when the blower comes on, it's drawn against the blower, blocking off much of the air flow. May not be your problem, but worth checking.
    and the blower is fine. No obstruction.
    Thanks.

    cn

  5. #44
    Join Date
    Oct 2006
    Posts
    28
    If the furnace has a fan/limit switch, then the burners would light once the limit side has cooled down (limit minus differential). You stated that the fan switch cools down and then shuts off. Sounds like the thermostat is satisfied.

    You did not mention the type of thermostat (mechanical or digital). If the thermostat is mechanical, it would hava a heat anticipator, which would have to be set per manufacturers recommendations.

    If the furnace is cycling on limit, a static pressure test should be performed to determine if it is within the maximum external static pressure as stated on the furnace nameplate. If the static pressure is above, corrections should be made to bring it down. The proper airflow is based on the maximum eternal static pressure.

    Adjusting the gas pressure can result in incomplete combustion. A combustion efficiency test should be performed to maxize efficiency. However, airflow must be coorect.

  6. #45
    Join Date
    Nov 2006
    Posts
    4
    To whom it may concern,

    I have had some of the same problems as the previous writer and have not been able to completely fix.

    The high limit switch will open from time to time and will need to be reset. The unit is a Trane XE80 that came with the house.

    I am currently using one of the cheaper 3/$10 buck filters, so I do not know if this "adding" to the problem, however I noted this issue shortly after starting to use a filter. Previously the unit had used a plastic/weave factory type filter which did not filter too much at all. As an aside, I did note a service sticker, on the side of the unit, that reported this trouble shortly following the unit replacement, and the tech noted the the limit switch was "cleaned and reinstalled".

    I will continue to try a less restrictive filter but would also like to replace the existing limit switch in case it is slightly cooked. I have been unable to find a source for this part and most of my local dealers do not carry Trane parts. Is there a generic replacement? On a note adjacent to the switch, the part is listed as a Bayvent 800B0.

    As this unit replaced an older oil fired burner and has reused the existing duct, I do not know if the new unit has been sized correctly or if I am fighting any other issues that are related to this. All ducts are clear, supply and returns are open and unblocked, and balancing vanes are fully opened.

    I hate to give in but I may need to throw in the towel on this one. Any advise would be appreciated. Thank you.


  7. #46
    Join Date
    Jul 2006
    Location
    Connecticut
    Posts
    271

    Cool

    An experienced Technician should clock the meter, and check the manifold gas pressure. If the input is too high & the Pressure is OK, reduce the orifice size(s). Could also raise the limit to 200*f.
    Tom D. - Long-time Journeyman from CT.

    On account of being a democracy and run by the people, we are the only nation in the world that has to keep a government four years, no matter what it does.

  8. #47
    Join Date
    Jun 2004
    Posts
    13

    thermal

    if it worked for yrs ok and now you have the problem try removing the front cover on your cooling coil and if its an a frame coil carefully remove the shield and ill bet you find a matt the looks like dirty dryer lint which has cut your airflow down resulting in higher discharge temps

  9. #48
    Join Date
    Nov 2006
    Location
    Rhode Island
    Posts
    8
    is the blower motor direct drive or belted. If it is belted make sure it is not slipping. Also if it is belted make sure the motor pulley is set for the maximum rpm. IE not riding way down in the pulley opening. If all above is correct then it would definitly be a air flow problem. A course check would be to remove the blower compartment door and hold it a a 45 to 60 degree angle and let it go. If the blower pulls the door shut and slams it, you have an air flow problem. If it does not then check the above.

  10. #49
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914

    Re: Simpleman, Thanks for your input

    Originally posted by cn
    He kind of laughed and mentioned that furnace is like a gas stove. If all you need is to boil the pot of water then turn your gas burner just to get enough gas output to boil the pot, any excessive gas creates extra flames that "lick" the outside of the pot and lost to the air.
    Your so called "tech" is dangerously wrong and is not qualified to service fuel burning heating equipment.
    If more government is the answer, then it's a really stupid question.

  11. #50
    Join Date
    Dec 2005
    Location
    Spokane Wa
    Posts
    169
    To your knowlege has the furnace ever operated without tripping on high limit? If it has'nt is the furnace sized properly? If it is sized properly and airflow checks out, clock the meter. You may want to find out haw many ton blower you have and use the sensible heat formula for determing your CFM.

  12. #51
    Join Date
    Nov 2006
    Posts
    4

    Trane XE 80 - High Limit Switch Cutout

    Thanks for the replies.

    I cleaned the evaporator coil...a little dust etc. but not enough to think this might be the problem.

    I also changed filters to a reusable filter that was in the unit when I bought the house. I really doesn't filter anything but large particles, so I do not think it is offering any significant restriction. The same symptom was noted after the change.

    The unit has always tripped the high limit occasionally but it is worse now. I just finished sealing all open seams, taping joints etc....the unit was a disaster after some one cut in a humidifier and replaced the furnace. The old plenum with the evaporator and duct work was NOT sealed and was leaking everywhere. One of the ducts on top was also partially disconnected. I am sure I added quite a bit more static pressure to the fan with the above.

    Is there a Trane reference to compare factory gas consumption to my existing unit so that I may clock the meter and check my existing pressure.

    In addition, can I diagnose the existing limit switch and can some one advise on the replacement part temperature rating? I am assuming one of the universal limit switches can be used, however is there a 'best' fit replacement part. Before I call in an expert, I wouldn't mind changing a $40 part to see if this item is defective.

    Thanks for your patience.

  13. #52
    Join Date
    Nov 2006
    Posts
    4

    Trane XE 80 - High Limit Switch Cutout

    Oh forgot to add....unit fan is direct drive. No belt. Fan is also set up at factory speed.

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