1989 Westinghouse Hot Air Furnace starts but doesn't restart

[JerrB]

Hi,
I hope you guys can help me out, I'm very familiar with hydronic systems but not with hot air systems.
I have a model NUGI100AH01 hot air furnace. When the thermostat calls for heat, the furnace starts up. I see the pilot light come on then the gas valve turns on with full flame. After it heats up a bit the circulating fan comes on and the heat works fine. After about 3-5 minutes, the gas valve shuts off but the circulating fan still runs. I assume this is normal as the furnace gets to its high temperature point then shuts off while the air keeps recycling. However when it tries to restart the flame I hear it but the gas valve doesn't turn on. After about a minute or so the whole thing shuts down. After everything cools down it will restart the cycle. I removed the air filter, so I don't think it's an air flow problem? I can change the flame detector module and some sensors, but they are expensive and I'd rather isolate what the cause is rather replace a bunch of items. if someone can point me in the right direction to troubleshoot it that would be great. Diagram attached
Tx

 

[Fitter30]

Have a volt meter? Burner module has two 24 volt terminals. Need to check power on those terminals. Power drops out units going out on the hi limit. Need to inspect the blower wheel the cups on the wheel might be full of dirt. Would need to be.pulled and cleaned. If unit has ac the coil could also need cleaning. Fan and limit control may look like this one

https://www.supplyhouse.com/White-Rodgers-5D51-35-Fan-And-Limit-Control-Fan-50-To-265-Degree-F-Range-Limit-100-To-300-Degrees-F-Range-5-Element?utm_source=google_ad&utm_medium=Shopping_ntm&utm_campaign=Shopping_NTM_New_users&gclid=EAIaIQobChMIxfO9gf79-gIVhQ-tBh3YvwAJEAQYByABEgKd8fD_BwE

 

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[JerrB]

Have a volt meter? Burner module has two 24 volt terminals. Need to check power on those terminals. Power drops out units going out on the hi limit. Need to inspect the blower wheel the cups on the wheel might be full of dirt. Would need to be.pulled and cleaned. If unit has ac the coil could also need cleaning. Fan and limit control may look like this one

https://www.supplyhouse.com/White-Rodgers-5D51-35-Fan-And-Limit-Control-Fan-50-To-265-Degree-F-Range-Limit-100-To-300-Degrees-F-Range-5-Element?utm_source=google_ad&utm_medium=Shopping_ntm&utm_campaign=Shopping_NTM_New_users&gclid=EAIaIQobChMIxfO9gf79-gIVhQ-tBh3YvwAJEAQYByABEgKd8fD_BwE

Thanks for the quick reply!! Yes I'll check those voltages and things you mentioned and let you know later today. The control module looks like attached.

 

[JerrB]

Okay,
So I checked the operation of the Fan and Limit Control operation. The 24V at the control box goes away after a few minutes because the temperature reached the high limit of 190 degrees. The burners then shut off as they should. The circulation fan is still running. Then the 24V comes back as the air temp goes down below 190. The pilot light then lights but the burners never turn on. Finally when the temp goes below 100 degree(fan Limit) Everything turns off. If I wait for everything to completely cool, it then turns on again and repeats the same cycle.
If I take off the bottom panel so the air is coming from the basement rather then the return ducts, the unit stayed on. So it may be a flow problem causing the limit temperature switch to trip.
I have the filter out and the AC coil is only a year old, also the squirrel cage fan looks clean, so I dont know why the flow is being limited. Should the hi Limit switch cycle on and off? Or in normal operation, should the air temperature stay below 190 degrees(Hi limit setting). It seems to me that the Fan and Limit control is working properly. Could it be the flame starter module not working when hot, or the gas control valve not sensing pilot?
BTW, I only see a ground wire and an aluminum tube for the gas going to the module. How does the gas valve know that the pilot is lit?
Tx

 

[Breplum]

I agree, the hi temp limit is working properly. It should never be the control.
That is one old beast you have. You'd save a lot in the long run to go 96% AFUE modern model.
what you call a ground wire near the aluminum pilot tube is likely the flame sensor.
I don't know your unit but generally the rod senses flame. In a modern furnace it functions as a pick-and-hold to control burner staying lit.

 

[JerrB]

Thanks for the reply. It definitely is just a ground wire, green and shows on the schematic as GND. The only connections are the Hi Voltage for spark, the pilot light tube and the ground.

https://www.supplyhouse.com/Heil-Quaker-1149944-Pilot-Burner

Somehow, the control module senses the mv signal maybe from the tube connection to the gas valve? Anyhow, yes, it is an old unit(Whirlpool not Westinghouse) made by Heil-Quaker probably 70 % efficient. It's working now. I'd like to change to hot water since it's a ranch and I have basement access for the piping, but the AC is using the ducts. I read that HW heat is more efficient than Air, but I also see Hot Air systems up to 98% AFUE?

 

[JerrB]

I just wanted to update this. The pilot sense uses that ground wire to detect a small voltage 2uV or so that is rectified through the pilot flame if there is one!

 

[Fitter30]

One of these pilots are more compatible than the one you shown. U will notice some burn left some right some both directions. Spark comes out of the flame rod and so does the ma signal