Pulse 21 runs for 5 minutes then shuts down

[sscallon]

I have a Lennox Pulse 21 that is 17 years old, runs well for 5 minutes then shuts down, and the LED on the ignition module goes out. If I reset by shutting off the electrical for a while (more than 5 minutes) it will do it again. Do I have an intake or exhaust * I switched out the ignition module, and a service technician cleaned the intake flapper, no help.

[Freon]

Possibly a high limit safety. Accurately measure the time from ignition to shutdown. Now at the thermostat set the FAN to ON. Now call for heat and see if the time to when the furnace shuts off is longer. If it runs longer then check all return and supply registers for adequate air flow. Make sure all filters are new.

[sscallon]

It ran 5 1/2 minutes on auto, and only 4 1/2 minutes on "fan on". After a technician cleaned the intake filter inside the heavy sealed box, it ran for a long time (hours), but only for that one day. I have checked the drip leg and it is not gunky.

[ericka89]

I agree with you. sscallon -
> It ran 5 1/2 minutes on auto, and only 4 1/2 minutes on "fan on".
> After a technician cleaned the intake filter inside the heavy sealed box,
> it ran for a long time (hours), but only for that one day. I have checked
> the drip leg and it is not gunky.

[Freon]

"After a technician cleaned the intake filter inside the heavy sealed box, it ran for a long time (hours), but only for that one day. "

It seems there is either inadequate air flow or the high fan limit safety id not working properly. Look on the front of the furnace for a 3"x5" metal box very close to where the heat exchanger is (above the burners in a vertical flow system). See if the cover to that box comes off easily if you squeeze the sides. If it does and there's a dial inside, let us know.