[jrschreppel]

I have a model Ultra SX 80 Lennox furnace and the combustion blower is energizing with out a call for heat from the thermostat...

[hvac1000]

Lennox has its share of problems. Disconnect the wiring from the thermostat and it should stop. If it continues to energise the blower you have a wiring short in your low voltage wiring or a bad circuit board. If this does not hekp with the situation I recommend you contact a Lennor repair person.

[labman]

By combustion blower, do you mean the little one that supplies combustion air? If the only problem was in the thermostat, Once the blower came on, the furnace would advance to ignition.

Does it show any error codes? If not, the relay that controls it could be stuck. A thwack with a screwdriver handle might unstick it. Unfortunately once they stick, they become more and more likely to stick. Plus, replacing the relay means replacing an expensive circuit board.

[jrschreppel]

By combustion blower, do you mean the little one that supplies combustion air? If the only problem was in the thermostat, Once the blower came on, the furnace would advance to ignition.

Does it show any error codes? If not, the relay that controls it could be stuck. A thwack with a screwdriver handle might unstick it. Unfortunately once they stick, they become more and more likely to stick. Plus, replacing the relay means replacing an expensive circuit board.

Yes the small blower. The unit is about 10 years old and there are no visible code lights.

Once it energises it will run for about 10 seconds then shut off for 10 to 15 seconds and then it keeps repeating this process over and over unless the thermostat is turned up then it will advance to ignition. After cool down it is usually a half hour to an hour before it starts repeating the process...

[Felrio]

Have you done what hvac1000 recommends?. The unit should not start doing anything w/o a call. If the symptom stops, then you got the culprit, if not, keep posting with details.

It is strange that it stops for an hour or half after the call for heat, so it sounds unlikely that the relay might be getting stucked. If you happen to have the diagram and post it, we can trace the options of what is causing this...

I hope this helps..