Hi Everyone!

So, a strange behavior on our Lennox WhisperHeat heater (model: G20Q3/4E-100-5) for the past week or so. The heat starts off fine, the flame ignites, the fan blows fine but after 3 minutes the damper shuts off (flame is off) while the fan is still blowing. After about 2 minutes, the damper opens and the flame ignites again.
This cycle repeats itself until the temperature in the house reaches the one set on the thermostat. This takes longer than usual, obviously for the on/off flame cycle.
I noticed on colder days takes even longer.

I thought there are issues with the blocked vent (S62) and flame rollout (S47) switches, so I bypassed them with a jumper but the issue still persists. Also replaced the limit switch with a new one.

Any ideas what this could be?
I am attaching below the schematic.

Thanks!


Lennox WhisperHeat G20Q3/4E-100-5

 

I'm not a pro... but the obvious question I have is whether any of those sensors are doing more than a simple switch, in which case you can't jumper around them to fake the computer into thinking they're working right.

Is it possible you're running the blower either too fast or too slow so it is just detecting the temp rise as out of range and shutting down? Have any changes been made like a new thermostat, changes in the ductwork, opening and closing of dampers/vents in rooms?

I'm wondering if it is either your gas valve having a problem or your control board having a problem--neither of those parts are cheap enough to just swap out as a "throw parts at it guess" in my opinion so maybe you should get a pro to look at it.

Is this furnace > 20 years old? If so you should consider a new one anyway.

 

Welcome to the forums.

Yes.... that furnace is more than likely close to 25 years old.

What damper are you referring to ?
Sidewall flue damper ?
Draft inducer ?

When the furnace fails..... see if you still have 24vac from R to C.
Have you replaced the filter(s) ?

 

Hey folks!

PJMax - it's the damper in the front of the burners with the red silicone seal seen open in the photo below. - I'm guessing that's the draft inducer?

Thanks for the tips! I think I found it! It's the limit switch! That was the first I suspected so I ordered a replacement but even with that, the intermittent on/off flame would persist.

So, today I pulled the limit switch out and let it run like this and the flame was working all the time. (see photos)
I taped the limit switch hole and I tested the temp with an instant-read thermometer (the ones you use for meat, pretty well-calibrated) and when I pushed it in it was about 400 F after which it came down to 240 or so. Which explains why the limit switch was tripped because I believe it's rated to turn off at 250F.

Now the question remains, why do you think it overheats in there?

Thanks!!

 

Dirty blower wheel, plugged and or too restrictive filter, vents block, bad blower run capacitor, plugged a/c coil if applicable.

Do not continue to run the furnace with the limit bypassed, 400f is very high!

There is a high probability the heat exchanger has cracks and you need a furnace, the g20 is very well known for bad heat exchangers.

they crack along the curve internally at the back - you may need to remove damper to gain access, look up with a flashlight angled high into each burner inlet.

 

Thanks!

So the technician came yesterday and it was as I suspected - clogged A-coil and dirty blower wheel.
Works like a charm now, and the flow intensified remarkably well to the parts of the home where we barely were getting any airflow.

Thanks again for the help!!

 

Was the heat exchanger visually inspected?

Having a G20 pretty much means needing new furnace, just because it is working does not mean it is safe!