Rheem blower runs occasionally

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Old 01-18-11, 09:14 AM
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Rheem blower runs occasionally

I am curious why the blower runs occasionally on a Rheem classic 90 plus : Model RGRA.
Sometimes the blower runs without the burner on since it was installed brand new. Here is the scenario:
1) Thermostat calls for heat and the blower and gas lights to make heat come out of the registers.
2) Room gets up to temp and thermostat kicks off and roughly 1-2 minutes later the blower kicks off. All seems fine.
3). At random sometimes the blower will run from 30 seconds to 3 minutes any where from 20mins to many hours after the heat was shut off. It will also do this during the spring and fall seasons when the heat at the thermostat is off.
Is there a moisture or temp sensor on the control board to run the blower? And why does it do this? Is it to remove moisture or just circulate air without heating it?
 
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Old 01-18-11, 11:01 AM
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Could be a defect in the thermostat or a defect in the circuit board of the furnace.


Often the thermostat can be used to switch the furnace fan on, and often that turns the fan on to a higher speed than is used for heating. You might want to inspect and experiment with your thermostat to see if it has that kind of feature which might be switching on the fan as you describe.

If the thermostat switches the fan to a higher speed, does the fan turn on to that higher speed when it is operating erratically?
 
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Old 01-18-11, 11:56 AM
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I did experiment with the thermostat months ago. I had the mechanical thermostat replaced with a programmable thermostat. The furnace behaved the same way with both thermostats.
Then the thermostat was disconnected and the furnace still did the blower fan on every once in a while.
Yes, both thermostats have the option to turn the fan on if desired.

It never does it during the summer. Only during the spring a little bit and mostly in the fall and winter. It is like there is a temperature sensor on it and if it sees a differential or moisture it runs the fan for a little while.

So this is not normal and would be a board defect? It did it since it was new.
 
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Old 01-18-11, 12:07 PM
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You've eliminated the thermostat as the problem.


Furnaces these days operate the fan off the circuit board, and they don't have a way to operate the fan as you describe.

Erratic operation of the kind you describe is usually due to an electrical defect someplace. So I'd call it a circuit board/control module problem.
 
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Old 01-18-11, 01:06 PM
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Thank you. This Rheem furnace has 3 wires for heating, cooling, and fan. The fan can be activated independent of the cooling or heating. I thought that was usually a standard feature for a FHA gas furnace.

I will look into having the control board diagnosed for turning the fan on sometimes when the burner is off and the A/C is off.
 
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Old 01-18-11, 02:01 PM
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Probably tough to diagnose definitively if I'm correct. Intermittent problems are the bane of repairmen.

Even so, often there are ways to cause an intermittent failure to manifest itself.

But I don't know how you'd do that with a problem as random as the one you describe. You are probably left to ruling out other possible problems (such as the thermostat) and then saying "well, the only other thing that could cause it is a bad circuit board." ---which is what I did.

Of course, a repairman on the spot might observe some odd defect causing a problem ---you never know.


I'd invite you to post whatever your do that winds up correcting the problem.
 
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Old 01-18-11, 10:12 PM
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The strange thing is that it reports no error codes. The two green LEDs always stay lit. From a control board point of view everything is operating as designed.
 
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