[LennyLames]

Hi. My Carrier furnace, natural gas, installed 1990 (80+% efficiency) usually works. However, once or twice a season, we wake up to house temperatures below the set level, as if furnace didn't respond to thermostat setting.

When problem occurs, I fiddle with thermostat (off, then back to heat, and raise temperature setting), then the furnace eventually kicks in. It's a good feeling when I hear the 3 or so clicks (is that the igniter?), then hear the rush of gas flames, then eventually, the main blower kicks in.

I thought the problem was due to Maple Chase programmable thermostat (I also change batteries every year), so had it professionally replaced last December with Honeywell Pro 4000. Problem still occurred in January. When it did, I called the service company (this time, without fiddling with the new thermostat), hoping the service person would be able to diagnose fully.

However, the service person just walked in with a replacement part, apparently sure about what caused the problem. He described this little white part as something that would allow the furnace to continue trying to start, rather than giving up after a few tries. Does anyone know what this is? Is it a limit switch? CORRECTION: I've been doing some reading, and the term he used was lockout switch. After 3 or 4 tries without proper ignition, system locks out. That was the part that he replaced.

Should he have done more diagnostics, since I left the furnace in its "malfunctioned state?" I intentionally did not fiddle with the thermostat to maintain this state. After reading through several posts and responses here, I think he should have checked voltages in the red, white, black wires, etc.

Problem has happened in both program and manual temperature settings.

This same service person replaced the motor a few days after that January service call, since the 16 year old motor just gave up. The furnace malfunction happened again in early February, this time with the thermostat set to hold at a certain temperature.

Circuit board was replaced about 4 years ago, at a cost of almost $300.

Why could be causing intermittent problem? If furnace fails to start, doesn't thermostat keep sending signals to keep trying, as long as ambient temperature is already lower than the set temperature? Problem seems to happen during the coldest nights. What if there was a power failure in the middle of the night? Would that keep the furnace from starting?

It's a split unit, with an external Carrier A/C blower hooked up to it with hoses. The A/C unit has one of those remote control units installed by the power company so that they can shut it off during peak power demand days in summer. I do not know if that is even relevant, but I'm trying to look for all possibilities.

Thanks in advance, for any guidance.

[hvac1000]

A problem that shows up twice a season is almost impossible to find.
There was a service person there and he did not fix it.
How in the world would we know whet the problem is if the serviceman who was at your house could not figure it out.

I am not trying to be mean but trying to fix something that causes a problem 2X a year on a forum like this is impossible.

I recommend you call someone else who knows what they are doing and have them stop by to see exactly what your problem is.