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  1. #1

    oil furnace tripping on high limit.

    Went on a "no heat" call to find system tripping on high limit. I had a 70 deg. RA and a 238 deg. SA!!! The blower motor is working at 120v on high speed. I found no obstruction in the RA duct, filters, registers open, etc. I then opened up SA plenum and also found no obstruction. Nothing touching limit switch. Shined flashlight into heat xchanger and couldn't see any light penetration indicating a crack. Pump pressure rated at 140 psi and that is what I got. I bumped it down 15 psi but didn't help. Nozzle clean and clear but changed to factory nozzle. Found a little crud between electrodes and cleaned off. No flue blockage either. System has a history of bad control boards and this current one is also malfunctioning. I don't feel the board is related to the high delta. This was my first time at this residence and noticed last tech added a time delay relay between blower heat tap on board and blower. Why would this have been done? Also, the system (according to the customer) had been working fine until now. Any ideas?

  2. #2
    Join Date
    Feb 2010
    Location
    In a boiler room
    Posts
    7,050
    Low airflow.

  3. #3
    Join Date
    Jun 2002
    Location
    Upstate Central NY
    Posts
    580
    Blower motor going the correct direction?
    Too big for the house/ductwork?

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,750
    Moved to tech to Tech forum.
    Contractor locator map

    How-to-apply-for-Professional

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

  5. #5
    Join Date
    Jul 2009
    Location
    Columbia, MD
    Posts
    3,788
    airflow x 2

  6. #6
    Join Date
    Sep 2002
    Location
    Virginia
    Posts
    3,082
    Blower wheel clean ?

    Is there a capacitor on that 120 volt fan motor ? if so change it

  7. #7
    Join Date
    Jan 2010
    Location
    Toronto,Ontario Canada
    Posts
    355
    Where are you measuring the temperature for the supply air. 238 seems to be very high.....what is the high limit set for? and when does the blower motor turn on? does it turn on?

    1.Make sure the blower wheel is clean. [If you have to take out the wheel and inspect the squirel cage spin it and see if its going in the right direction.]

    2.inspect the size of the furnace and the duct work.

    If the furnace is too big for the house and the temperature difference is still excessive, tag the furnace. This can cause many problems!!
    Get er Done!

    Do what has to be done
    when it has to be done
    as well as it has to be done
    And doing it all the time.

  8. #8
    Join Date
    Nov 2006
    Location
    Wisconsin
    Posts
    280
    What size nozzle is in it?

  9. #9
    Join Date
    Dec 2007
    Posts
    4,670
    most oil furnaces will have a max high limit of 200, so 238 seems odd.

    adjusting pump pressure will not do a thing for this situation, and should be at mfr specs for the unit or higher if you understand the correlation.

    why the time delay, maybe it WAS coming on too soon and blowing cold air.

    what changed from last service to now?
    air filter
    blocked return(s)
    closed supply registers
    repairs by less than knowledgeable individual
    status of components-as mentioned blower wheel, capacitor, blower motor operation, etc.




    is the correct nozzle in it?
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  10. #10
    Join Date
    Jan 2002
    Location
    tip of the mitt
    Posts
    1,974
    is the blower wheel slipping on the motor shaft? let us know what you find.
    I have my own little world. But it's OK...they know me here.

  11. #11
    Join Date
    Feb 2010
    Location
    In a boiler room
    Posts
    7,050
    I don't think he's coming back.....

    He did state that he installed the factory spec nozzle and pressure is correct.

  12. #12
    Join Date
    Aug 2012
    Location
    NE wisconsin
    Posts
    402
    Is there ac, is the coil plugged with cat hair?

  13. #13
    I don't recall the blower having a run cap., but will check. The squirrel cage was a little dusty but nothing major. When the fan did kick on, it did seem to run pretty slow for high speed. Obviously something has changed to cause the high limit, they haven't had a service tech out for a couple years.

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