Furnace cycles while heating the home

Discussion in 'HVAC Heating & Cooling' started by nestman, Dec 4, 2009.

  1. nestman

    nestman New Member

    Messages:
    1
    Location:
    Utah
    I keep the heat at 65 degrees when I goto bed. When I wake up in the morning, I set it to 70 degrees. The furnace will come on, but it takes lots of breaks as it makes its way up to the newly set temperature. Because it takes so many breaks, it take forever for it to eventually reach 70 degrees. Here is some info I found on the controls that may be helpful to you smart guys:

    Lennox Elite Series
    Johnson Controls (G776RGD-14)
    .44 orifice
    Intermittent Pilot Ignition Control
    85 second trial
    15 second prepurge
    60 minute retry

    When the unit is taking a break, the fan is not blowing, but I hear the furnace still "on", whatever that means. On the Johnson Controls, I see a LED light that blinks 1 second on, 5 seconds off. According to the printed label on the controls:

    "in retry mode"

    The weird thing is, a "professional" said there was nothing wrong that he could see. Can someone help me figure out why the heater doesn't run continuously until the desired temperature is reached?

    Thanks!
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,133
    Location:
    New England
    It sounds to me like the flame detector is shot. When a flame generated heating device turns the main burner on, it normally has a detector that determines the burner is actually burning. If it isn't, or it thinks it isn't, it will shut the gas off, then wait awhile for the unburned gas to 'purge' out of the chamber, then try again. Your system is not getting the signal that it actually lit. Could be dirty, could be out of position, could be bad, or the main board that signal goes to could have a bad receiver, and not see the good signal. Most common is a dirty sensor. There's often a way to test them, but you'd need the spec sheet or diagnostics and the proper tool (often a multimeter).

    Note, some burners have a maximum burn time, and will shut off, even if the set point isn't reached, for a few minutes before firing again. This is much longer than a few minutes, though, and wouldn't give an error code.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,815
    Location:
    Cave Creek, Arizona
    furnace

    You have to find out what is happening at that time. Normally, even if the burner goes out, the fan will continue to operate until the furnace is cooled down. So you have to know if the fan is off because the burner has not been operating, or what ever else the reason is. The burner will shut off if the thermostat is satisfied, the high limit switch is activated, or the burner flame has failed. Again, you have to determine the cause before any action can be taken.
  4. john152

    john152 New Member

    Messages:
    5
    Location:
    Texas
    Did you find your answer? I'm having the same issue. Changed/cleaned several things but nothing worked. Tech couldnt figure it out either.
  5. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    If you checked everything over and cleaned/changed the flame rod and nothing is seeming to help you probably need a new control board.
  6. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    It is normal. when the burner comes on it needs to heat up the exchanger to what ever the fan on speed is set at, then the fan comes on and blows for awhile and stops because it's bringing back cold air and cools off the exchanger, the burner either stays on or it may go off on high limit before the blower comes on again. It's real common when you set back the thermostat. You could change the high limit switch which is most likely the cause of the cycling but a lot of furnaces do that.
  7. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    It's not normal for a furnace's fan motor to cycle, after the furnace has fully started up, while the burner is still going. It should be the other way around with the blower going and the burner off.

    Nestman > I just read the manual for this furnance online and it's says it's a 2 stage furnace. Is it possible that maybe the furnace is cycling from hi to low fire and you're just not hearing the fan motor running while it's on low fire?

    What you need to do is run the furnace with the cover panel(s) off. There is probably a door switch on the fan compartment you'll need to tape closed. Ofcourse do ALL of this with the power OFF to the furnace. The fan motor is a dangerous thing. Once you are ready to watch the furnace run through a cycle turn the power back on to the furnace and watch it run through a cycle.

    This way you'll know for sure if the fan motor completely stops running or not.

    If the motor does stop completely while the burner is still going once the furnace is up and running (after initial startup) I would again say you need a new control board. These new furnaces are run by the control board and it tells the furnace to do everything it does basically. So if the fan is cycling on and off it's because the control board is telling it to cycle on and off. These newer furnaces rarely have heat switches like the older furnaces to bring the fan on and off. It's all controlled with timers and such built into the control board.

    I would suggest you phone technical support and talk to them too and see what they have to say about! They'll want you in front of the furnace with the model & serial #'s ready!
  8. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    I'd say the duty cycle should be 100% until it reaches the setpoint temp. Then the duty cycle varies to maintain that temp.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,133
    Location:
    New England
    Most should run until the thermostat is satisfied. Every once in awhile, you'll find one that has a max run time timer and it will automatically turn off if it runs that long, but the on time is quite long, not a short cycle.

    It would be unusual to have the return air be cold enough to drop the heat exchanger temp low enough to have the fan turn off IMHO. If it did, the setting probably isn't optimum.
  10. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    This isn't even possible on a new furnace with a control board because like I said above they don't have heat switches to turn the fan on and off. It's all controlled through the board electronically.
  11. MG

    MG New Member

    Messages:
    160
    Location:
    Illinois - Near St. Louis
    I just replaced one of these ignition modules tonight. However, the flame sensor that they provide with it does not match the one I have. I cleaned up the one in there and it fired right up but its doing the same thing - cycling a lot. The new sensor is shorter than the old one and doesn't have the threaded fitting that the old one has. For now it's working though.
  12. MG

    MG New Member

    Messages:
    160
    Location:
    Illinois - Near St. Louis
    Followup - so far it's working fine and it appears that its working a lot because its doggone cold here this week.
  13. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    Great. Glad it's up and running!
  14. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,815
    Location:
    Cave Creek, Arizona
    cycling

    If the burner turns on and off, it is because the unit is heating faster than the fan can disperse the heat so it goes into high limit thermal shutdown. If the fan cycles on and off, it is because the burner cannot keep up with the demand, and the fan's LOW limit turns it off so you do not circulate "cool" air. It then starts up again when the temperature reaches the fan's turn on point.
  15. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    On an older furnace yes, on a newer furnace no.

    On *most* newer furnaces the fans are not brought on and off based on temperature. They are usually brought on a certain amount of time after the burner has fired and should run until a certain amount of time after the burner has shut off. Again all of this is controlled through the control board.

    So if on a newer furnace you have a fan that is cycling on and off it's usually because the control board is shot. Notice I said usually :p
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