we have a Heil DC90 ultra high efficiency furnace which would turn on and off OK for one time, the next time it would turn on OK, but then it will keep running for a while, then the heat will stop, but the fan will keep running. If we turn off the thermostat, the fan still won't stop, and I'll have to turn off the electricity switch to turn the fan off. So first question is, is it safe to keep doing this - turning off electricity to turn the fan off?

Then the heating won't come back up again, unless I push the auxillary switch on the gas chamber once. After pushing the auxillary switch manually, everything will come up fine when the thermostat is turned on. So second question is - is this method of pushing auxillary switch safe?

What could be causing this on and off problem?Any insight would be appreciated! Thanks.