Fan continues to run after burner shutoff

[aaronml]

Lennox G20 forced air gas fired system about 10 yrs old. After the burner shuts down the fan will continue to run forever. Only by cutting power at the furnace will it stop. Fan should shut off about 90 seconds after the burner shuts down, but is adjustable for a longer interval by a jumper. Does this indicate that what Lennox calls the blower control center (BCC) a circuit board, is faulty or is there sensor or bi-metallic control that might not be opening? Problem seems to be getting more frequent.

[Therm76]

I work for a lennox dealer, call for service and have that furnace heat exchanger inspected. The G20 exchanger failure rate is extremly high.

[aaronml]

Hi Therm76,

Thanks for the reply. I will have the heat exchanger inspected, but why would a faulty heat exchanger cause this problem? The time delay in shutting off the fan after the burner shuts off is apparently controlled by a timer. There is a jumper that can be changed to make the fan-off delay be either 90, 150, 210 or 270 seconds.

Thanks, Mark

[beenthere]

Have the heat exchanger inspected.
And hen you call to set up the appointment, tell them about your fan problem, and the tech can check it out also.


May have a safety going out.

Sorry, this is a NO DIY site.

[Therm76]

Is your air filter clean? Are you using a filter that is to restrictive? The high temperature safety switch may have something to do with your problem. The G20 heat exchanger failure rate is high and you should have the furnace inspected annually due to age.

[Focko]

If it was out on high limit, it wouldn't fire up.

[Therm76]

The high limit is a snap disc. Will cycle burners until temperature limit reached. Homeowner just call for service. therm76 out

[Focko]

Oh, that limit. Then there's the manual reset on the blower housing.

'tis surely true the heat exchangers in the G20's don't hold out as well as others. I just condemned one on Monday. On Friday I did maintenance on another G20 (a '93) and advised the customer to plan replacement before the furnace planned it for him. He said the HX had already been replaced once. I said "oh, ok then". He had an April Aire 2200 filter system so that HX should be good for a while. I adjusted the manifold gas pressure from 3.8" wc to 3.5.

[aaronml]

I have an annual maint scheduled in about 2 weeks...will see if I can get it moved sooner.

Problem is intermittent as it had occurred 3 times in a week until last night when it occurred 5 or 6 times. It occurred again this morning the first 2 cycles and has been OK the last several. Filter is changed regularly.


Thanks for all the input!

[coolwhip]

A well placed kick to the side of the furnace does wonders.

[aaronml]

I'll give it a thump next time it fails. If a thump stops the fan, what would that tell me?

[Senior Tech]

I'll give it a thump next time it fails. If a thump stops the fan, what would that tell me?

That it's suffering from "fonzarello" syndrome.

Dangerously close to Diy...sorry.