Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit

Lennox heat pump / electric heat thermostat troubles

I would like to thank you for taking a look at my application

I have a Lennox thermostat that recently I have to put into electric heat to bring house up to temperature .
When I put thermostat into electric heat the light on the front of the thermostat turns on . Thermostat heats the house .after a while I see the temperature dropping .
I have to put the thermostat switch from electric heat to normal , wait a few minutes , put switch back into emergency heat and the house starts to heat again.
Could the thermostat have a electric heat timeout feature that is stopping the heat after I put it into electric heat mode ?

My Lennox thermostat is # 21J72 ( 3AAT89B38E1) , heat pump is # HP27 , the furnace is # CB30U-31-1P ,S/N 5897G23448

Thank you for your help


  • GW
    GW Member Posts: 4,326
    That's 1997 stuff, not ancient, not modern.

    Seems like there's a glitch with the emergency heat as you indicated. I've never heard of a wait function on elec back up on normal heat pump thermostats. If the elec heat light is illuminated then you should be on your resistance heat. If you have some "duel fuel" thermostat (possible but unlikely) there may be an outdoor sensor disallowing the back up heat. Even so, emergency heat means emergency heat, no matter what.

    You could pull the stat and find the wire that handles the e heat, jump r to that wire, and see what happens.

    Or maybe the e heater relays are on the fritz. Could be several things. You may need to get someone that can diagnose with tools.

    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • Brp814
    Brp814 Member Posts: 22
    Is the heat pump coming on? If not, why not? Sounds like you need a tech to come out to find the problem. Emergency heat is not the way to heat the house, the heat pump is stage 1, and the strip heat would normally be stage 2. The HP27 is a good unit, but is only a single stage compressor. Unit could be off on high pressure, or something else. Let us know what they find. Check your filter too.
  • Davetfc
    Davetfc Member Posts: 2
    Gary ,Brp814

    Thank you for your replies

    I will check out the electric heat relays and high pressure switch
    I will also jump the r wire to the electric heat terminal to see what the result is.
    I will contact a professional if I can not find problem .

    I was hoping to find operating -troubleshooting information on the Lennox 21J72 thermostat on the internet but keep coming up empty.
    Any ideas whereI can locate info.The Lennox site does not show anything .
    Thanks again for your help -I will let you know my results.

  • John Mills_5
    John Mills_5 Member Posts: 935
    Might want to get a GOOD digital thermostat and put the old mercury bulb out to pasture. Not really recommending setback for heating but can for cooling. Might be a rebate for a WiFi stat though around here it is just the gas companies offering it not the electric utilities.
  • GW
    GW Member Posts: 4,326
    Ahh didn't realize it was mercury, yes time for a new stat!
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • Empire_2
    Empire_2 Member Posts: 2,343
    GW I was thinking along the lines of the differential. Depending on the stat, in emergency heat isn't the second stage bulb used? The diff would be 2 or 3* or so. My .02

    Mike T.