Rheem Imperial 90 Plus on the fritz...ideas?


Old 11-06-04, 06:47 PM
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Rheem Imperial 90 Plus on the fritz...ideas?

Hey all. My furnace started acting up a couple of days ago. Of course, it has to happen right when it gets cold. Here's what it's doing:

-Room temp is UNDER 60*F and the t-stat is set for 70*F

-Furnace will not come on unless I go to OFF at the t-stat and then back on OR if I turn the power off at the furnace and then back on.

-Once the induced draft blower comes on, the furnace will go through it's 1 minute or so cycle before gas starts flowing into the burner.

-Sometimes the gas makes a "huffing" or "banging" sound as it starts flowing into the burner.

-Once the the gas is ignited the furnace will blow warm air all through the house for a short while and then the gas will stop flowing into the burner.

-The gas cycles on and off at random intervals while the furnace is running.

-After the gas cycles on and off 2 or 3 times the furnace will keep running for a couple of minutes before shutting off.

-After is shuts off, the room temp will still be below whatever is set at the t-stat.

-The furnace will not come back on without me turning the power off and then back on at the t-stat or at the furnace.

I called the HVAC guy to come out and take a look at things. He was perplexed. His company originally installed the unit back in '93 and they have always been the one's to service it. The only previous problem it experienced was a bad igniter/flame sensor. I believe that the igniter also serves as the flame sensor on this particular furnace. Outside the house, he found CO readings at the exhaust pipe greater than 500 ppm. This initially led him to believe that the heat exchanger was bad. He pulled it out and there was nothing wrong with it. Then he jumpered various wires and checked continuity at various places but the furnace still behaved the same. He pulled the igniter/flame sensor out and it was fine. I pulled it out again later myself and tried to lightly sand it to see if that would help. It didn't. He thought that maybe the igniter/flame sensor was sending a bad signal to the integrated control board but his test equipment showed that the signal was good and the board was in good working order. All water drain lines were clear. There are no obstructions to airflow anywhere in the system. What else can I try? He says that I'm looking at around $600 to replace parts and hope it works. My other option is to have him put in a new unit for $2450 installed. I don't want to spend $600 for troubleshooting and new parts and still have problems or high CO levels. But I don't want to spend $2450 for a whole new unit either if the problem is being caused by an inexpensive part. Thanks in advance for any tips or advice.
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Old 11-06-04, 07:02 PM
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Dear Waveoff,

Sounds like you've covered all your basic diagnostic points. However, if the "control board" is acting up, you will indeed experience these problems. Your CO ppm has nothing to do with your furnace shutting down. In fact, I find that reading alot with gas furnaces. The 2 things I would tell you to look for, you already covered (Flame sensor, or Ignitor). If these 2 items are in good working order, you may want to replace the control board. The 600.00 must include alot of labor charges?! Most control boards are within 250.00. Power outages, moisture, and power surges are the biggest reasons a control board will fail. Your model may have a specific board for ignition purposes, and you should start there. Good luck, and stay warm.
Old 11-07-04, 06:17 AM
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Thanks for the tip! I'll try the new board and see how that goes.
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