Air handler blower stays on


Old 04-22-06, 05:47 PM
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Air handler blower stays on

Hi, I have a Goodman GMP075-3 furnace whose air handler blower has recently started to stay on, even though the temperature is cool enough and the outside unit has stopped running. I've tried setting the thermostat temp all the way up (90+ degrees), and switching it from "cool" to "off", but the blower continues to run regardless. The thermostat (a GE 3AAT70, I think) has a "fan" switch that can be used to turn the blower on, but it's set to "auto."

Now, while I know the general concepts of how A/C and heating work, I don't know the details... in particular, how does the thermostat control things? My thermostat has 4 wires: Red, Green, Yellow, and White. From what I can tell, R has 24VAC or so, and the thermostat shorts G to R to turn on the blower, Y to R to turn on the A/C outside unit, and W to R to turn on the furnace. Is that right? How does the blower know to turn on when the A/C is on? As far as I can tell, the yellow wire only goes to the outside unit; it's not connected to the air handler. Or does it need the thermostat to short G to R in addition to shorting Y to R?

In any case, while leaving all the thermostat wires connected, I measured about 28VAC between G and R while the blower was on, which seems to imply that G and R were not shorted.

Any suggestions on what I should look at next? (I did take a look at the furnace/air handler unit up in the attic, but had no idea what to look for specifically... the thermostat wires are connected to a circuit board with a microcontroller and relays and other stuff).
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Old 04-22-06, 05:54 PM
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Fan stays on

To eliminate the possibility of the thermostat, disconnect either the R or G wire & allow the furnace up to 5 minutes for the fan to shut down. If the fan stays on, go to the furnace & disconnect the wire from the R or G terminal on the circuit board & wait another 5 minutes. This will eliminate the possibility of a shorted wire in the wall or elsewhere. If the fan still stays on, you need a new circuit board.
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