(Please, no one tell me to get a new furnace. I've heard that before. When I can, I will.)
I woke up early and the 2nd floor temp was 60 instead of 68.
Question 1: When the thermostat is set to auto, as it always is, the air circulation fan goes on about a minute after the furnace starts. In a furnace of this age, is that because of the Fan and Limit Control???? As opposed to something on the small control board, which has only about 15 resistors, capacitors, transistors, including 2 relays.
Question 2: Does the Fan and Limit Control keep the furnace running once it reaches fan termperature (and until the thermostat says for it to stop.)? I would think the answer is No, and that the control board alone starts the furnace and keeps it running, until the thermostat says no.
To try to get the furnace going, I pressed the reset button, but it didn't need to be reset. I checked the circuit breaker; measured the voltage on the secondary of the power transformer, and removed the control box cover, just in case anything cried out, "I'm broken".
Besides the relay with the reset button, there is another relay next to it, about 3/4" cubed. I pressed the armature down and the furnace started. Let go after 3 seconds and it stopped.
Thought about checking out the thermostat, but didn't know what color wires were used for heat. Thought about replacing the whole control circuit board, but figured I'd better test the thermostat first.
To get some heat until I could learn wire colors and maybe replace the thermostat, thought I'd jam a piece of wood between the relay armature and something above it. Didn't find the right length of wood, but even a shorter piece was heavy enough to hold down the armature, just leaning against the cabinet.
That started the furnace and immediately I thought, "Will the house fan start? Can't run the furnace for long without the fan" Ran for about 3 minutes but didn't hear the fan start, so I removed the piece of wood. Furnace is still running!!!. Does the Fan and Limit Control on a 34 yo Carrier keep the furnace running once it gets hot enough but the thermostat hasn't said Stop yet?? I thought not, but the furnace is still running. If the answer really is NO, something has changed since the furnace didn't run all night.
I had to leave the furnace room to find an air vent, and indeed, the fan is blowing.
I'm 99% sure it will turn off when the house is warm, but not at all sure it will restart when it should.
Any suggestions, other than getting a new furnace?