Heater not starting reliably, Payne 373L and Nest 3rd Gen Thermostat

thm1234

Hi there,

So the heat was running for a few days now as it should and then on Sunday around noon, it stopped heating. The current temperature on the thermostat again showed a higher temperature (74 and rising) than it was in the room (68 and falling) and the unit would not turn on. So I removed the display of the Nest and connected the red and white wire, which usually turned the unit on. It’s a Payne 373L Horizontal Heater/Cooling unit in the attic. I can hear a strong humming noise when the igniter gets hot and the gas should ignite (that noise is new). After a while, it stops and then after a second or so starts again, for I think 4 times. Then it is dead again and nothing happens.



Then I disconnected the power on the circuit breaker and reconnect the two wires. Now it also started up the fan for a short time, then stopped. Since the unit is in the attic I can not see if the gas ignites or not. Then the unit was just quiet and did not turn on. I left the wires connected. Finally, at about 11 PM (3-4 hours after the last try), the unit started to try to ignite again, without me doing anything but never started up completely.



This morning I again disconnected the two cables, waited a bit, also disconnected the yellow (cooling) cable from the thermostat, switched the circuit breaker on and off, then reconnected the red and with, and again the same thing. 4 times trying, a bit of fan, then nothing. 3 hours later the unit turned on by itself and is now heating.



Now I don’t have a clue about what the control board does. Could someone confirm that there is a delay built in to retry on its own after a few times trying? Why would it not ignite sometimes, then after a while work again? Can this be the igniter or the heat sensor, the gas valve, or the control board?



I called the Hvac guys again, but when its working when they get here, they won't figure out what's wrong.

Any help to diagnose further is much appreciated.

Thanks,
Thomas


Thomas

unclejohn

After 3 failed ignition there is a 3 hour lock out. Also the indoor blower will run for 90 seconds after power is turned on or off during a heat call. You need to see if the igniter glows red on call for heat if not replace that if it does but no flame may be gas valve.

thm1234

Ok, great, I will tell that to the hvac guy. Thanks. 

What piece can make a loud (electrical) humming noise? Like a brimming, almost like an old train track transformer?  

Jamie Hall

thm1234 said:

Ok, great, I will tell that to the hvac guy. Thanks. 

What piece can make a loud (electrical) humming noise? Like a brimming, almost like an old train track transformer?  

Anything with a coil in it can hum. Those things don't usually, because the coil is not free to vibrate, and built to no vibrate internally -- but that can change. Relays and transformers are the usual culprits.

thm1234

Thank you for all that information. Another HVAC tech was here again and said it's the coils in the condenser that need to be cleaned and it might be the flame sensor. I checked the flame sensor and it has close to 100 volts to the chassis of the furnace, so that should be good. I cleaned the flame sensor and put it back in ( https://photos.app.goo.gl/J4ncHatrTr5bA6B1A). They are writing up an offer for the cleaning.
I think there is more to it. After the guy left, the furnace again does not ignite. During the ignition process, when the igniter glows, the gas should come into the burner, but there is no gas flowing. I have a video here: https://photos.app.goo.gl/m5hv5DXHhwH2MBAq7. It lights up 4 times, then stops the process, for 3 hours. After several repeats, many hours later, it might suddenly ignite properly, or not.

What else could prevent the gas to start flowing into the burner? Are there any gas sensors, that often go bad or is there a heat sensor on the igniter that might be bad?

EdTheHeaterMan

I hear the click of the gas valve opening. That is when the gas should ignite. There are two possibilities.
#1. The gas valve is defective and does not actually open when you hear the click.
#2. The gas is there, but at the wrong place, wrong pressure, or the path of the gas is not properly lined up with the ignitor. If the gas does not pass close enough the the ignitor then it will not ignite.

What else could prevent the gas to start flowing into the burner? Are there any gas sensors, that often go bad or is there a heat sensor on the igniter that might be bad?

No heat sensor , no gas sensor, The valve opens and the gas will ignite if it get close enough the the heat of the HSI (Hot Surface Ignitor)

Try 2 things...
1. Tap lightly on the gas valve when the HSI is at the brightest glow and you hear the click.
2. Try moving the ignitor closer to the path of the gas.
The HSI is very delicate. You do not want to touch the part that glows. The oils from your shin may cause it to fail when powered up and glowing. In order to adjust the alignment, you need to loosen the mounting screw and move it by the white porcelain base then tighten the mounting screw. It may be only 1/32" away from proper alignment.