Hello out there. We have a gas heater/central ac. A digital thermostat. 2 days ago, the heater wouldn't come on. I thought to test the thermostat by simply adjusting the temp to below the ambient and switching to 'cool' - and system started. I have no experience w/HVAC and wondered if anyone could point me? Is it possible that if the pilot is out (if there is one) system wouldn't start?
THX

Model number and brand of furnace?
What is it exactly doing?
How long has the thermostat been installed and what brand and model?

It's an 'A' series induced draft,horizontal gas four-pass warm air furnace.P-HED16N0800A - electronic ignition - and the thermostat is a honeywell chromotherm III. The furnace is firing; so I thought the blower motor was bad; then all of a sudden it starts working. I'm wondering if this intermittent behaviour could be the result of a bad thermostat? Thermostat is most likely 15-18 yrs old. Heater maufacture 1990.

At this point it could be many things. If the blower delay is way to long it could be the fan and limit control if equipped.

There is a heat limit switch; which does break the circuit if it 'thinks' the unit is too hot. Any way to trouble shoot this>? I've included a pdf of relevant section of heater manual.
THX

I would start with the chromotherm 3. Is the screen starting to go blank and you can not see all the numbers on the screen? It's a old thermostat and time to up grade. Im not saying this was the problem but you would hate for it to quit on a hot summer weekend or cold winter night and cost you 3 times the amount of normal business hours to get it replaced. I have seen the same style thermostat act the same way before it went out and had to be replaced

((There is a heat limit switch; which does break the circuit if it 'thinks' the unit is too hot. Any way to trouble shoot this))

According to the pdf supplied by you if the limit was tripped the blower would run constantly until it reset. You did not mention that problem. Your blower is controlled by a electronic timer and that could be intermittent.

Since you have a honeywell chromotherm III it is possible that it is fading away losing control of some functions. T-TOP mentioned this in the above post and it would be a good idea to check that also.