Lennox Pulse 21 fan/blower turns on too late in AUTO mode

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Old 11-11-17, 02:56 PM
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Lennox Pulse 21 fan/blower turns on too late in AUTO mode

Our Pulse 21 (G21Q3-80-1 installed in 1995) started doing this infrequently (intermittently) last Spring. The furnace lights off, runs for about 90 seconds, then shuts off. The fan will usually start about another 60-90seconds later. Sometimes it doesn't start at all though. It will run the blower for a couple minutes, shut off, and the whole cycle starts again. Sometimes the cycle will re-start when the fan is still running, in which case it will continue as it should and run a normal cycle. Sometimes it starts and runs a normal cycle on the first try. If I switch the thermostat fan control from Auto to ON, it runs the burner normally every time.

We had a tech out last Spring, who admittedly was not a Pulse guru, and he couldn't figure it out (in his defense, it wasn't very repeatable for him, he only saw it a couple times), but it was late Spring, it got warm, life took over.

Now that it's getting to be furnace season again it's happening again, more repeatably though. I sat down with the circuit diagram and my DMM and figured out relay K36 ("RELAY-HEAT BLOWER") wasn't doing it's job. Turns out the diagram is bit lacking in details and K36 is a time-delay relay, I found the adjustment pots and found the one responsible for the fan start delay after the gas valve is opened and turned it all the way down. This took care of the issue for several cycles while I had everything apart testing it, but after putting it all back together it still started late on the first cycle. I suspect that either there is an intermittent connection somewhere, K36 is going bad, or the time delay pot is oxidized/gummed up. I have some contact cleaner somewhere which I was going to try on the pot next, but thought I'd post here and see if there are any other recommendations.

Thanks in advance,
-Brad
 
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Old 11-12-17, 12:15 PM
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Not to beat a dead horse but make sure your furnace gets inspected routinely for heat exchanger issues. These furnaces are pretty much out of service now due to the heat exchanger problems. There was a major recall on the 21's but that is long over.


Fan Control
Fan--On Delay ---- Factory set at 45 seconds and adjust-able from 30 to 60 seconds.
Fan--Off Delay ---- Factory set at 180 seconds and adjust-able from 120 to 240 seconds.


Once the relay get the ON signal.... it should turn on in under 60 seconds.
 
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Old 11-18-17, 07:50 PM
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Thanks for the info. I was under the impression the heat exchanger issue was primarily on units older than ours, and IIRC the tech we had out mentioned that too, but we do have it serviced regularly and the HE gets checked/inspected each time. We also have CO monitors on every level of our house.

I used some cleaner on the pots and it worked as expected the first 3 cycles, which I watched and timed. The times were not exact and a little inconsistent, but it always started the fan before the burner shutdown. Then several hours later in the evening I noticed it started failing again, with the fan coming on 1-3 minutes after the burner shutdown, with the burner having run for 90-120 seconds. I suspect I need a new relay timer board.
 
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Old 11-18-17, 08:52 PM
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I would agree on replacing the fan timer board.
 
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