Trane XE78

[bmik2729]

I have the above mentioned furnace and it has been acting up recently. It is @ 16 years old. Every once in a while (more frequently each day) the furnace turns on but blows only cold air. I opened it up and cleaned up around the venturis, the ignitor and the flame sensor, etc.
Basically the unit cycles up normally, the inductor fan comes on, then the ignitor glows very brightly but the gas never ignites. Then the ignitor goes out but the air fan turns on and obviously blows only cold air.
Usually when this happens if I simply power off at the main switch, wait a minute and turn it back on it works okay for a few hours. I put a new Honeywell thermostat on last summer that tries to keep the temp within 1 degree. The furnace is turning on a lot...sometimes within a minute of shutting off. Is it just cycling too much or is something else to blame?

[HayZee518]

You set the heat anticipator according to the current draw marked on the gas valve. This adjusts the cycling of the furnace.

[bmik2729]

The cycling is a thermostat problem that did not exist with my original thermostat which had a 'swing' setting (or differential setting) set at 3 degrees. The Honeywell is permanently set at 1 degree...much too tight.
The real problem is the failure to ignite. It almost appears as though no fuel is being supplied.

[bmik2729]

I replaced he thermostat to one with a 2.25 degree swing setting and the seeming constant cycling of the unit has stopped. The temp must now drop more then 2 degrees before the heat will come on. I also discovered that the inducer motor was loose and was making a lot of noise and vibrating. I read somewhere that excess vibration could cause the pressure switch to not read properly. I tightened it back up and the unit is much quieter now and vibrates less. All worked flawlessly for almost 12 hours and then it failed to ignite again. All it needed to fire up was to shut the unit down at the main switch, wait 30 seconds (less) and power back on and it fired up as normal. It appears I am going to have to start replacing components. It is a Trane TUD080C936A1 with a WHite-Rogers 50A50-405 controller. Any suggestions are appreciated.

[bmik2729]

It ran for 24 hours before it failed to ignite again. This time though I was able to get the controller status: 2 flashes = System Lockout. I think I am going to jumper the air pressure switch closed to eliminate it as the culprit. The inducer blower motor is turning...is it possible that on occasion it is not creating enough airflow and it could be the problem?

[jnaas2]

Ive seen the impellor blades come off the inducer moto shaft and the motor would run but not the blades. And I found a single tree leaf that made it all the way to the furnance intake screen and wouldnt allow the inducer motor to pull enough of a vacume.