Here's the background- It's an older Payne furnace / AC unit from the late 80s maybe. No, digital controls with error codes. The issue is that the blower motor never starts when heat is called for by the thermostat. This is the sequence of events that I observe...
1 switch thermostat to heat and set temperature
2 induction / draft blower starts running
3 I can hear the "tick" and see the tiny arc from the ignitor striking the small hood for the gas
4 within 30-45 seconds of turning on the thermostat, the flame ignites and everything seems good so far
5 the draft blower is still running and things are heating up
6 after about 1-2 minutes the flames shut down and the entire unit is locked from what I assume is a heat-activated switch
At this point I cannot go to the thermostat and switch the blower from "auto" to "on"; it's completely unresponsive. I've figured out that if I let it cool off for a couple of hours it "unlocks" and I can run the blower from the thermostat. Also, I have been getting heat into my house by switching to heat and because the unit is on the roof I can hear the draft blower start up and eventually the flames ignite. I count about 45 seconds then switch the fan to "on" and run it like this until the temperature reaches my thermostat setting and the furnace shuts off on its own, though, the blower will still continue to run if I didn't notice that the heat shut off and switch it back to "auto".
I have done the following on my own so - removed the ignitor / hood assembly and scrubbed the surface with steel wool. Removed the tube that leads from the gas valve to the hood and blew both pieces out with an air compressor. I figured this wasn't the problem but, I had it open and now notice the flames ignite sooner than they used to after doing this. I tested the limit switch with an Ohm meter and while it would open and close it felt sticky and didn't always want to return to its rest position easily. So, I replaced it; it's a Cam-Stat 3" adjustable. Basically it didn't help. I tried it in the middle range, and then high and low. On the lower range I think it opens too soon and I can't even start the blower manually by switching the thermostat to on. I haven't looked at that pressure switch with a diaphragm built into it yet and I can't find any other heat activated switches on this thing. There is a diamond shaped device that has a nickel sized round face which is situated above the flames where they shoot into the heat exchanger. I disconnected that and put the Ohm meter on it. There's a loose button on the back of it and I can put pressure on that buttom and watch the resistance values change. I believe it to be a "blow-back" sensor.
In summary, my furnace ignites but after burning for over a minute, maybe close to two, the blower doesn't start to run and the flames shut down. The blower motor DOES work because before I switch the thermostat to heat I can turn the fan to "on" and it blows air through the entire house.
Please advise as to what I can look into next. Thank you.