Oil furnace cycle problem

Discussion in 'HVAC Heating & Cooling' started by taiwantim, Nov 4, 2008.

  1. taiwantim

    taiwantim New Member

    Messages:
    7
    My oil fired furnace will cut in and heat the house to the thermostat's requested temperature, then turn off. But, a few minutes later, the burner will come back on and heat the house again even when the requested has been met. ANd again a few minutes later.

    What could be the cause of this?

    Tim
  2. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    it could be a faulty T-stat,(seldom) but there are other things to check out first.
    First of all, are you sure the actuall requested temperarure setting is being reached?or is it shutting down on high limit? When did it start doing this? is the air filter clean?
    Take a probe thermometer and take a reading at the main duct near the SUPPLY plenum. Then take a reading at the RETURN AIR entry to the blower. The temp difference is the "heat rise". Compare it to the suggested 'heat rise' specified on the furnace label name plate.
    Example; supply air reading=135
    return air reading = 69
    heat rise is 66
    A heat rise more than what the furnace name plate is the reason for the high limit shutting it down.
    Post back with some info and I or others will try and help.
  3. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    You have a bad thermostat or the heat anticipator on the thermostat does not match the amp rating of the primary control. There can be no other cause. Ductwork, fan speed, air movement and fan limit switches can be the cause of blower cycling but not the oil burner. If the burner is cycling after it has reached it's set point then it's the thermostat. If it cycles before it's set point it can still be the thermostat but it may also be caused by an auxillary hi limit control that is shutting down too soon. Very common in mobile home and any counter-flow furnaces.
  4. Hube

    Hube New Member

    Messages:
    156
    Location:
    Ontario
    Nhmaster; fyi, an oil burner WILL SHUT DOWN if the hi limit control exceeds its setting. Also the blower speed ,if too slow,will make the hi limit TRIP,thus shutting OFF the burner.:)
    Last edited: Nov 6, 2008
  5. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    Yes it will if the thermostat continues to call and in fact it is normal for that to happen, but he said the thermostat has been satisfied and then it cycles.
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