Hello;

I have a 30+ year old Rheem model RGAA forced air gas furnace that has a gas pilot light. When I set the thermostat to FAN, the fan motor turns on and runs until I switch it off. But when I set the thermostat to HEAT, either I get nothing, or the burner and motor turns on for a few minutes, then stops.

I measured the voltage across the gas valve. I read 25 volts a.c. no matter if the burners are on or off. Across the upper limit switch I read 0 volts because it remains closed. It is set to open when the supply heated air reaches 220ºF.

When I set the thermostat on HEAT and set the temperature above the room temperature, I measured zero ohms between the red and white wires at the furnace. When I set the temperature below the room temperature, I measure infinite ohms. Both readings are correct.

The only thing wrong I found was one loose plastic twist end cap that electricians use to hold multiple wire ends together. It was at the junction of power ground wires inside the furnace power kill switch box.

When I cycle the power kill switch, sometimes the burners and motor will activate for a few minutes, and sometimes nothing happens.

The pilot light looks blue and healthy and always remain lit.

I've run out of ideas and the house is getting mighty cold.

A schematic of the furnace appears at Furnace Schematic

The thermostatic fan control seems to be your problem.

I'm having trouble trying to identify the thermostatic fan control. There is a metal covered part labelled Honeywell R8239 that has a transformer and wiring board mounted on it. This entire part mounts on the back of the power source kill switch electrical box. According to the Honeywell web site, it states "The R8239 provides low voltage control of line voltage fan motors and auxiliary circuits in heating, cooling or heating-cooling circuits."
Is this whay you are referring to as the termostatic fan control?

Does your furnace have a item that looks like this?

That part that you posted, I have identified it as the Upper Limit Control on the schematic. It is physically mounted on the supply vent inside the furnace above the burners.
On this control, there is a metal part pointing to 220ºF and another one pointing to 100ºF.
Once I had this control's cover off while my furnace was supplying heat, when the furnace turned off, I heard a mechanical click and observed the round dial rotate.
So I need to replace this upper limit control?

Here is additional information. Last night when I got tired, I left the R8239 unmounted exposing the wiring, and went upstairs to go to bed. Surprisingly, the furnace ran all night supplying heat. Now I'm starting to believe my problem is intermittent.

When you set the thermostat to fan ON and set it for heat does the heat come on. In other words the fan will come ON with the thermostat turner to fan ON but if you turn up the thermostat dial for heat does the heat come on?

I tried as you suggested.
It is 60ºF in the house.
I set the thermostat for 80ºF.
I turn the switch to FAN.
The fan comes on but there is no heat.
The voltage across the gas valve is 26volts a.c.
The voltage across the upper fan limit control is 0volts a.c.

I forgot to mention that the pilot light is lit but the burners are off.

I measured the voltage across the gas valve. I read 25 volts a.c. no matter if the burners are on or off

Since you mentioned in your first post that the burners were working I thought just the fan test would be OK.

Careful on this one but if you have a jumper set connect them as to jumper out the limit controls one at a time then turn power back on and see if it run. We need to eliminate controls one at a time. Having the 25 volts across the gas valve is strange since it should be open then. Try this also remove the two wires from the gas valve and see if they show 24 volts or there abouts when the furnace should be running. It could also be a intermittent valve problem.

Oh my gosh!
After I finished the fan test above, I set the thermostat back to AUTO and left the temperature set for 80ºF even though the house is at 60ºF.
Fifteen minutes later, the fan turned on, the burners activated and now I have heat!! But for how long?

I'm getting confused with these symptoms.

Oops, even though there is a 20 degree difference in temperature needed to be overcome, the furnace ran for only about ten minutes, then shut off.

Do you have a new thermostat or a digital one?

I have a Honeywell CT87A round dial style thermostat

My Honeywell CT87A thermostat is not new. I installed it about ten years ago when I replaced my older thermostat.
I don't have a spare on hand.

That is a very simple thermostat. You might try to eliminate it from the circuit by jumping the R and W terminals on the furnace. This is not going to be a cure but it will eliminate it fror sure while testing the other controls. It might also be wise to disconnect the wires from the terminal strip before jumping. One other item that you mentioned was when you removed the transformer relay assembly the R8239 that the unit started working. You might also revisit that assemble since it does do the low voltage and also contains a relay. Check for loose wires and tap on the relay to see if it is intermittent.

I first started to unscrew both wires at the gas valve. When I was loosening the wire that connects the upper limit switch to the gas valve, the furnace suddenly turned by activating the burners. Then a few minutes later, the upper limit switch turned on the fan. And now I have heat coming into the house. Could it have been as easy as an intermittent screw contact at the gas valve? I will let the furnace run to see if it can make up the 20 degree difference before it shuts off. If it shuts off earlier, then I will try the test you suggested.
You have been so kind and patient with me so far. I can't thank you enough.

Anything is possible. LOL