Lennox Furnace stays on


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Old 11-05-22, 02:19 PM
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Lennox Furnace stays on

ML193UH070XP36B-3, heat stays running after thermostats satisfied. Flame not just blower. Both thermostats changed. Stats running even if thermostats removed from back plate. If unit is shut down and the restarted does not ignite until thermostat calls but then stays running. Have checked wiring and everything goes where it is supposed to on thermostats and control board on furnace. Looking for troubleshooting tips. Would limit switch cause this ? hoping not control board

Thanks

 
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Old 11-05-22, 05:01 PM
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Take a look at the diagnostic LED's on the control board. Under normal conditions with no call for heat, they should both be flashing slowly. When furnace is calling for heat, they should both be flashing faster. If they are both flashing fast when you think the call for heat has been satisfied, the furnace thinks there is still a call for heat.
That could be a thermostat wiring issue or a control board issue.
If they return to flashing slower when the call for heat is satisfied but the burner and blower continue running, that is likely a control board problem.

I couldn't find a schematic for that board, but it looks like there are several relays on it. A few light taps on the relays when it is "stuck" may show up a sticking relay.

Remember that the blower will continue to run for a while after the burner shuts down.
 
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Old 11-05-22, 05:35 PM
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Sounds like a bad control board. A limit will not do that.

Does the flame turn off when you remove the wire from the furnace W terminal?


 
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Old 11-05-22, 06:25 PM
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Thanks for both replies, will remove W from board and report back. May not be until Monday

Thanks !
 
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Old 11-06-22, 05:20 PM
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Both thermostats changed
A furnace only has one thermostat.

If you have two then you may have a zoning board. That board will control the burner.
Houston's W test still needs to be done.
 
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Old 11-07-22, 04:34 PM
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Yes, 2 thermostats, thiu 2 zones. Zoning control Honeywell HZ311. This is at 2nd home so not at it till weekend.

Houston could you LMK what would determine bad board? ie remove W wire at board and flame stays on means ? or remove W wire at board and flame goes out means ?

I have purchased board and would like to diagnose and install if necessary this weekend. Not great wifi to check in here, when there so trying to get as much info upfront. really appreciate all the help !
 
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Old 11-07-22, 06:38 PM
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If you remove the wire from the furnace W terminal and the fire stays on you need a new furnace control board.
 
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Old 11-08-22, 10:16 AM
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Either thermostat will activate the HZ311 and the HEAT LED will come on.
You'll want to make sure the HEAT light is out.
If it stays lit.... the HZ311 could be the problem.
 
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