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Old 12-19-2010, 03:27 AM   #1
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Rheem 80% Furnace Short Cycling


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My intitial issue:
I have a Rheem RGLH model (80% 75k BTU circa 1994). I just bought this 2100 sq ft house a year and a half ago and the furnace has been serviced every year except for the year that I bought it. The filters are the washable kind and I clean them good ever month.

Starting Saturday night I noticed the furnace was shutting down and restarting itself before even reaching the thermostat limit. It would run 5-10 minutes and then shut down, the fan would keep running and then the system would attempt to reignite. It wouldn't relight the first time this happened. So I took out the flame sensor and cleaned it with steel wool. It fired right up but went out again 10 minutes later. Checking the other obvious, the filter was clean. I only have a furnace filter. No filters at the returns. I have one big return and one in each bedroom (3). I called a tech out and they could not reproduce the problem. The only thing the tech could say was replace the board maybe. There were no error codes. The problem went away. When is replacing the board necessary? I know these furnaces have a history of having boards go bad, what should I look for?

The problem returned Tonight, I set the thermostat to 70 Degrees, and after a rise of two degrees (from 62-64) It short cycled again. No error codes. It has been running fine since, for stints of 25 minutes and now again for 30 minutes with no short cycling.

After this happened tonight, I replaced the flame sensor with a brand new one. A bad flame sensor wouldn't let a system start up right? Meaning a bad sensor wouldn't let the furnace run for 15 minutes and then shut it down, would it?

I would also like to point out that the supply vents in every area are open and there are no obstructions to the return vents.

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Status as of 12/18/2010

Just an update for everyone., knock on wood but the furnace has been running fine since we changed the fan speed. Without taking apart the furnace and using mirrors and accessing it underneath where the evap coil is, the heat exchanger looks like it's brand new. No soot, caked dirt or dust in there. We noticed that next to the limit switch there was a plugged 3/8th hole. Looks like it must have been serviced before? Because a temp guage fit right in there. Anyway, the measured temperatures in the heat exchanger area were 147 degrees with the fan on high and 156 degrees with the fan speed on low. The limit on this furnace is 200. After a couple hours of running the temperature doesn't get much higher than those figures if at all.

I took a measurement at the registers at varous spots around the house. The register thats about 10 feet from the furnace on a low fan setting its 131F and on the high fan setting it's 126F. At the furthest spot its 120F on low and 117on high. The heat rise on my unit is 40-70 degrees. So it's within spec according to those temps, I keep my house at 70 and 64 at night and during the day.
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Summary and Explanations

When the first HVAC guy came out, the first thing he looked as was my ducting. The return and supply ducts are of the right size, length and setup. I'm not sure this is a bandaid fix. I switched back to the medium fan speed and hasnt had any issues since.

If I leave the fan on the normal setting (med) the temperature gets nowhere near the limit of 200 degrees, it settles around 150-155 degrees after an hour of running on medium and around 145-150 when running on high.

Going over the previous owners maintainence notes, there were never any issues. The system was serviced 2 times a year until last year, when I bought the house. The filters have always been cleaned (rewashable) every month.



The 3-4 times that it did short cycle, there were no blinking LEDs, it would restart after cooling down and would run fine continuously after that. The evap coil, the heat exchanger etc are all free and clear of debris or dirt and dust.

My house is a two level house with my furnace being on the first floor. All my returns are on the second floor. One for each bedroom and one big one in the upstairs landing area. My second floor is roughly 600-700 Sq feet. The landing area is exposed to the first floor since I have vaulted ceilings.

One other note:

The returns in all my rooms were noisey. You could hear the air being sucked in, very audible. The returns had the dampers in them to close them. I changed them out to the fixed vent direction and now I can barely hear the returns. Is it possible that those returns were actually restrictive? As much as this experience has been crappy, I do enjoy the troubleshooting. However 2 well qualified techs couldn't find a valid reason why it happened. The latest HVAC guy said if it really was a limit switch issue due to air flow, it would happen more than this. The fact that the internal temperatures measured right next to the limit switch show the temperatures aren't anywhere near the limit of 200 shows that it's not a structural issue. The only other things that were recommended to change out were the limit switch itself and possibly the pressure switch. The inducer was checked out and really shows no signs of being bad.

Intermittent problems are a PITA. For anything really. You would rather that it just not work period.

Thinking back to the conditions when it first happened, my dryer, gas fireplace and furnace where all on. Not sure if that could have been a BTU supply problem. Furnace is rated at 75k BTUs. It short cycled 3 times in a span of time ranging from 6pm-8pm (a week ago today). The tech that cameout let the system run for close 45 mins and there was no short cycling. Voltages across the board were fine, gas pressure was fine etc. The second time it happened was last wednesday night around 11pm. The house was at 70 degrees, I called for the thermostat to raise it to 73 and within a few minutes it short cycled. I took off the cover and noticed that the pleated air filters I had in there were buckled down towards the blower. I had to cut the filter to the appropriate length (12x20x1) and one side on each of the filters had no support. I figured that could have been an instance where it wrecked havoc on the airflow. I then switched the filters back to the rewashable ones.

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