Gas and Oil Home Heating Furnaces - Rheem Criterion II runs constantly, no heat

Hello,

We have a Rheem Criterion II furnace, model RGPH-10EAMR, dating from 1997, running on propane. Just recently it started running all the time, blowing unheated air (regardless of thermostat setting). Went through several troubleshooting steps, detailed below:

Shutting off power at the circuit breaker doesn't solve the problem; power back on = cool air blowing constantly.

The thermostat seems to work fine. It's a basic mercury unit, red and white wires. Open circuit resistance until heat is called for, then less than two ohms. Connections are good on the controller, and it didn't matter whether or not the thermostat was connected, nothing changed.

Plenty of propane in the tank, pipes all seem fine, no gas odor. (Although, even if there were no gas, it still shouldn't run all the time?)

The pressure proving diaphragm switch for the damper fan seems to work, or at least it clicks when mouth suction is applied. (Again, even if that didn't work, it shouldn't run all the time?)

Removed the cover for the controller. Upon holding in the safety switch for the outer panel, could see three LEDs, but couldn't read exactly what they are. They are vertically oriented. The bottom LED is amber, it illuminates briefly when the safety switch is pushed in, then goes dark. The top LED is green, seems to be a power on indicator, it lights steady green. The middle LED is also green. It also illuminates, but seems a little darker than the top one, maybe it is being pulsed on and off?

We had a new ignitor installed a few months back FWIW, but that seems very unrelated. The Rheem troubleshooting guide in the manual doesn't cover this problem.

Many thanks for any ideas!!

Maybe a high-limit tripped out from the last time it ran. When that happens, the blower keeps running, only with loss of heat function. If ignition process even tries to occur, then this is not it.

I would agree that it may be a limit has tripped for one reason or another. It is odd that it just started unless the unit had ran for some reason causing the limit to trip. There are 2 or 3 manually reset limits on your furnace, they are brown, round about the size of a nickel with a button on top, usually with a red spot on the button. There is 2 wires connected to them, one on each side. You can check these by pushing on the buttons to see if any of them click. You can hear the click when they reset. The are located on the burner tray, and on the combustion housing. Then restore power and see if that is the problem.

It that is not the problem, the only other solution would be some kind of power surge has shorted the intergrated fan control board. (circuit board).

I saw the limits when poking around, but didn't know what they were. I'm away from the furnace for a couple of days, but will report back. *Very much* appreciated!

Update on the high limits -- two of them *were* tripped, but unfortunately that didn't solve the problem. The furnace still blows cool air constantly when powered up, no matter what the thermostat setting is. Must be a problem in the controller ... $$. Thanks for the advice to check the limits, at least I know what those are now.

With regard to troubleshooting, if you can see the model and serial numbers, then you can google the model # which might be able to get a service manual on line to state what the problem is.

Hi Paverette,

Good advice; we did have the service manual for this unit, and followed its troubleshooting flow chart. Unfortunately, many of the problems end up with the "fix" being to replace the controller board. We had this done just recently, and it did solve the problem, at considerable expense. Thanks to all for your advice, at least I am much more educated now.