Replaced Ignition Control Unit, Then Blower Fan Ran Non-Stop at Low Speed


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Old 02-17-24, 04:15 PM
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Replaced Ignition Control Unit, Then Blower Fan Ran Non-Stop at Low Speed

Hi, first time messing with my Lennox gas furnace. Has a SP735L Intermittent Ignition Module and wasn't getting a pilot flame most of time. Voltage between P and C terminals on the gas valve would either not get any 24V or would get it for around 2 seconds only. Replaced ignition control with an SP845-780 and problem solved.

HOWEVER, when I turn power on to the furnace, after about 45 seconds, the blower fan will run at a relatively low speed, and it does not stop. If I turn temperature up, burner fires up and the blower runs at higher speed and all is great. But when the burners stop, the blower continues to run, although at a slower speed, nonstop. I checked the thermostat (Nest) and there is no setting to run the fan continuously. I start the fan from the Nest and the blower in response will run at high speed. I stop the fan from the Nest and the blower continues to run, at low speed. This wasn't the case before the ignition control module replacement.

I unplugged the Nest face module, same result. I measured the voltages across the thermostat terminals on the fan control board, and all is good. When I start the fan from Nest the terminals get the right voltage, otherwise nothing. So it doesn't look like Nest is the culprit here? Anything I might have missed?


I googled around and some old posts say it could be the relay or some capacitor on the BCC. I think mine is the original BCC2. But otherwise it's been working fine though, running with the furnace and the A/C.

Any suggestions?
 
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Old 02-17-24, 04:19 PM
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Replaced Ignition Control Unit, Then Blower Fan Ran Non-Stop at Low Speed

Hi, I replaced the SP735 ignition control unit with an SP845-780 and fixed the no pilot flame issue. But at the same time, it seems to have introduced a new problem. That is, the blower fan now runs non stop at low speed. It still runs at high speed when the furnace burners have fired up, but it remains running, at low speed, afterwards, nonstop until I turn off power to furnace. When I turn power back on, in about 45 seconds, blower starts running again. I've pretty much ruled out the Nest thermostat and/or thermostat wiring. Any other suggestions? This is in a Lennox furnace. fan control board seems original, BCC2 perhaps. Thank you.
 
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Old 02-17-24, 05:04 PM
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Welcome to the forums.
Normally we delete duplicate threads but these two are not the same.
Combined threads.

We'll trust you got the right ignition module. The suppliers have the most up to date lists.
The ignition module has no effect on the blower so somehow a wire got misplaced.
The ignition module is a stand alone device. It creates spark and controls the gas valves.

Sounds like the blower has a 45 second delayed turn on.


Robertshaw modules - pdf
 
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Old 02-17-24, 06:53 PM
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Sorry I wrote up the first message in length and got an error when I posted it. I didn't realize it actually had gone through, and wrote a shorter version of the same problem. I think you can delete one of them.

One update: the fan continuously runs at a low speed. The fact that it would run at high speed when the burners fire up turned out to be the work of my Nest Thermostat. There is a setting to "activate the fan" when it turns on heat. When I disable this (do not activate the fan) on the Nest, the blower fan will just continue to run, at low speed, even when the burners are running.

The new ignition module is the manufacturer "direct replacement" for the old module.
 
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Old 02-17-24, 07:00 PM
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The fan timing starts when power is applied to W. This would be the slow/heat speed.
Power applied to G would be the A/C - high speed blower setting.

So if the fan is still turning on in 45 seconds... try removing W wire and wait for timed blower shutdown.
 
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Old 02-17-24, 07:00 PM
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I don't think the blower itself has a 45 seconds delay to turn on. If I use my Nest thermostat to "run the fan," the fan runs immediately, at high speed. This 45 seconds delay that I have observed happens only when I restore power to the furnace. It will then auto start, but at a lower speed. Without the Nest activating the fan, now this blower stays at a low speed even when the heater is running.
 
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Old 02-17-24, 07:23 PM
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now this blower stays at a low speed even when the heater is running.
Yes.... that is correct.
 
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Old 02-17-24, 07:41 PM
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Right. The issue I guess is that it stays running (at a low speed) nonstop, even when the heater is NOT running. I checked and the "continuous fan" terminal on the boiler control board is NOT connected.
 
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Old 02-17-24, 08:26 PM
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I'm guessing you have an old G20 furnace ?
It would be 25+ years old.
Make sure you get the heat exchanger checked for rust thru.

Lots of good stuff in the manual. Diagnostics too.
Lennox G20 furnace manual - pdf
 
workflow voted this post useful.
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Old 02-17-24, 08:40 PM
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Yes. Thank you for the manual.
 
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Old 02-19-24, 03:17 AM
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In the Lennox manual you shared, page 17 wiring diagram, there is a reference to S68-1,2 switch-continuous blower. Maybe that is the reason? Where is this switch on though? Nowhere else in the same manual is this switch mentioned again. I've found all the other switches referenced on page 17.




 
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Old 02-19-24, 12:28 PM
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The first thing that comes to my mind when I hear of this problem on an old furnace is someone bumped the button on a fan limit. That would make the fan run continuously in low speed unless the stat calls for fan.


 
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Old 02-19-24, 12:35 PM
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Nah.... it's a G20.... blower is timer controlled.
 
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Old 02-19-24, 06:23 PM
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What post has the model?
 
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Old 02-20-24, 09:41 PM
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Hi, just wanted to update here that the problem is solved after replacing the BCC board (with an ICM289).

Yes it is a G20. G20 3/4E-100 in fact. The blower has no visible switch.

I did one more test before deciding to replace the BCC. I disconnected the ignition control from the BCC. This time blower started, low speed, right away after power was restored, w/o a 45sec delay. So I concluded I could ignore the ignition control. It was most likely a stuck relay but I couldn't tell if any of the 4 relays were stuck. I did try "tapping" each of them a few times, with no luck. Then I replaced the BCC board and all works as expected. When the gas valve is sensed, a 45sec delay starts before the fan starts in low mode. After gas is stopped, the fan keeps running for another 90sec. I can continue to run the fan manually from the Nest.

Thank you all for your help. The manual also helped a lot.
 
 

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