Heat Exchanger getting too hot.

[Lemmy]

I have a 25 year old Amana furnace and a Lux Geo smart thermostat. The thermostat is powered off of the red wire (24VAC) and blue wire (Common). The thermostat runs on a schedule. If the temperature setting on the schedule is 70 degrees and I decide it is too cold and want to override the schedule to 72 (it overrides until the next schedule change time), I noticed the heat would come on but then within 30 seconds or so turn off and the setting on the thermostat would be back to 70 degrees.

I used an Arduino board as a data logger and sampled the 24VAC line at the thermostat and noticed that when the heat would come on, after 30 seconds or so the voltage would go away for about 20 seconds. This would reset the thermostat so when the 24VAC came back on, it would be back to 70 degrees. I also noticed that even when I wasn't overriding the schedule, the heat would come on, then shut off within 30 seconds then immediately come on again and run normally.

I have looked at the schematic (it has no real control board, just a bunch of relays and sensors controlling everything) and found that the only thing that can kill the 24VAC to the thermostat is the primary limit switch. I hooked up volt meters to the 24VAC going to the limit switch, the wire coming back from the limit switch (which goes on to the thermostat) and also a meter set to measure resistance placed across the limit switch. When the heat would come on the primary limit switch was opening after ~30 seconds and shutting down power to the thermostat and the entire relay board.

I assumed the primary limit switch was bad and removed it to test it. I used a thermocouple reader and a hot air gun and it seemed to work just fine. I then decided to install it again and also put the thermcouple in right beside of it. I found that the switch is working perfectly. When the heat first comes on, the pilot lights, the inducer fan comes on, the main burner ignites and even though the blower comes on, the heat exchanger will rapidly climb to 220F and of course the primary limit switch opens up and shuts everything down. But if I immediately make the heat come on again, the heat exchanger slowly climbs to about 140F and stays there for the duration of the heat cycle.

I know some sort of blockage or blower motor issue can cause the heat exchanger to overheat. But I can't imagine a blockage would be there when the heat first comes on, but then suddenly go away for a 30 minute heat cycle that immediately follows the failed heat cycle. The filter is clean and the evap coils were cleaned last year. So it makes me think it is some issue with the blower, but I can't think what would cause it to not run properly then on a second try run perfectly fine. Any ideas?

 

[Lemmy]

Using the schematic and several meters I think I found the problem. First, it wasn't 30 seconds like I said, it was more like 60-90 seconds.

The Time Delay Relay has these connections:

24VAC
Neutral
24VAC from the other side of the primary limit switch
The "Gas Valve" connection.
120VAC
Normally Open output to the blower motor.

The line connected to the Gas Valve comes from the ignition module and pressure switch (which is verification the induction motor is on and running properly). I found all of these signals come on when they should. However the Time Delay Relay is supposed to close the contact to power the blower after a fixed time of 25 seconds. Then after a successful heat cycle, the blower is supposed to run for another 180 seconds (this is adjustable) before it turns off.

I found that the blower motor wasn't coming on in 25 seconds and would sometimes be up to 90-100 seconds and still not be on. At that point the heat exchanger had gotten too hot and the primary limit switch would open and shut everything down. The reason I thought the blower was coming on was that as soon as everything shuts down, the time delay relay closes the contact for the 180 second. The furnace is loud enough that it was hard for me to hear the blower running with the inducer fan running. But when it all shut down and I could hear the blower running, I incorrectly assume it had been running the entire time.

It seems that after a failed attempt, if the thermostat immediately calls for heat again, the time delay relay will turn the blower on before the exchanger gets too hot. So I expect the time delay relay is going bad. They don't seem to make my exact relay, but I have found one that is almost identical with the only difference being that the turn on delay is adjustable instead of fixed at 25 seconds.