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Trane furnace running too long - large heat differential

SnowDrift Member Posts: 3
Hi there. I'm new to this board.

We have a dual fuel propane furnace that turns over to the electric heat pump at about 30 deg. F and above. All components are Trane brand and it was installed as a new whole house system in 1998-1999. Trion Air Bear filter (MERV 8) is replaced every fall.

Last winter with the furnace running, only, the thermostat was set at about 72 (mercury switch analog) and the temperature would drop to about 70 before kicking on, which is pretty typical for our system. It would run, though, until the air temperature was up at about 75-76, before shutting off. The blue auxiliary light was also on when this happened (also unusual, since we rarely ever see this light). It only did this for about a week and then went back to normal operating.

It began doing this again yesterday. I suspected the thermostat, but the Trane dealer I called said he doubted it. I called another local tech, who is not a Trane only tech. and he suggested it's just because it's been really nasty cold outdoors. I have owned the system since it was new and it's never acted like this before, with exception of the week last winter that I mentioned.

Can I add anymore information to attempt a rough diagnosis of what is happening before having someone come out to look at this?

thanks in advance.


  • unclejohn
    unclejohn Member Posts: 1,749
    Is just the furnace running when this happens or is the heat pump on.
  • SnowDrift
    SnowDrift Member Posts: 3
    Just the furnace. The heat pump is locked out due to the outdoor temperature. I have determined that the blue light comes on because both mercury bubbles roll across when heat is demanded, due to the heat differential. Once the house heats up a bit, the light goes out when one bubble rolls back away.

    Is there a way to adjust the heat differential on a Trane Weathertron analog thermostat? If so, I'd try it. It's pretty inconvenient, as it is.
  • unclejohn
    unclejohn Member Posts: 1,749
    Sounds like the relay that switches from HP to GF is not working correctly. Hence the longer delayed on and then the over shoot. When it's cold enough for the furnace to run the first bulb should activate the furnace not the second. You may also have a bad out door air stat that's not telling the control to switch to furnace.
  • ProblemSolver
    ProblemSolver Member Posts: 190
    Sounds like a two stage stat, and the blue light is telling you that both stages (furnace and HP) are being activated, but the thermostat probly doesn't know about the HP lockout control so it believes it's activating both systems/stages. I am not a Trane specific tech, nor do i work on HP's here in Michigan, but those are my thoughts.
    JUGHNE Member Posts: 8,949
    Assuming you have an Emg Heat position on t-stat, what happens it you just set that switch there and let the system run at a comfortable setting.
    If I recall the functions of each bulb change when you do this; the first bulb will just run the gas furnace as if there is no heat pump involved.
  • SnowDrift
    SnowDrift Member Posts: 3
    No change when on Emergency Heat. Trane Tech. yesterday, said the Weathertron thermostat is bad. It actually started working last evening again. Should be getting replaced today.
  • unclejohn
    unclejohn Member Posts: 1,749
    I will be surprised if it's the tstat.
  • Spence
    Spence Member Posts: 316
    Here is how it should work. The HP and the furnace both use the 1st stage bulb (the top one), yet not together due to the ODT. Otherwise, the furnace would blow room temperature air until the 2nd stage bulb drops, which is about 1 1/2 degrees below the 1st bulb. The older Trane units energized a blue light when the 2nd stage was calling. The overshoot could indeed be the thermostat, or simply a maladjusted anticipator. I'd check that first because that vintage thermostat was practically bulletproof.