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Old 03-06-2012, 10:21 AM   #1
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Lennox 80 series


Hope you guys/gals can help. What is the correct sequence for one heating cycle? We have an old 73% efficient Lennox furnace that needed some work recently (flame rollout sensor needed to be ?blown out?) My question is this: Before the service call, after the house heated up, the flame would go off, and the fan would stay on for minute or 2. Now, when the thermostat registers that the house has come up to the temp, the furnace just shuts down, no fan after anymore. Is this dangerous? shouldn't the fan continue like it did to cool down the furnace/pipes?
After re reading this, it is clear I will not be going into the HVAC industry...but if you need some welding...lol
thanks in advance guys.
D.

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Old 03-06-2012, 11:26 AM   #2
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Lennox 80 series


There are usually adjustable time controlled dip switches that determine how long the furnace fan will run after the burner shuts down.


Last edited by how; 03-06-2012 at 11:30 AM.
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Old 03-06-2012, 11:40 AM   #3
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?dip switch? If you mean the jumper that has #'s like 60. 90. 120. 180, it is on the 120..
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Old 03-06-2012, 11:53 AM   #4
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Yes. Different system - same function.
I would look at the furnace operation to determine if you have a problem or not. Watch for when the burners shut off in the furnace and then time how long the fan continues to run before shutting off.

If both the burner and the fan do shut off at the same time, then that's a loss of fuel efficiency.
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Old 03-06-2012, 05:36 PM   #5
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Lennox 80 series


that is exactly what is happening. It didn't used to be that way, and i am wondering what the tech might have done to change this. The flame and all fan/blower shut off at the same time, and i want the fan to run a bit after the flame shuts off.
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Old 03-06-2012, 09:59 PM   #6
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Lennox 80 series


I can only think that you have a faulty board where you no longer have time delay for the fan relay.
I've never seen a board fault this way.

I have seen a number of older lennoxs just set up with the lowest motor speed running continuously. There are pro's & conns to doing this. This is usually for folks with heat distribution issues or those wanting continuous air filtering. Sometimes it's just a matter of pressing the summer fan switch in or using the fan "on" thermostat control.
If setting this up means a change of fan speed then a temp rise should be done on the furnace.
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Old 03-06-2012, 10:15 PM   #7
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Thanks fir the help, but I think I am over my head on this one..diy woodwork, welding yes.. Gas furnaces no.. Thanks anyway.

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