Armstrong Air Air Handler - Always on / Parts list?

Old 05-21-09, 10:11 AM
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Armstrong Air Air Handler - Always on / Parts list?


I have an Armstrong air handler that won't turn off -- I believe the fan relay is failing in the closed position.

It's an MB series blower/coil, with a MB12 blower paired with a MC36 coil. The line number for my unit family is 40592D093.

The unit is installed horizontally in my attic, and is about 12-13 years old (put in by prior owner). Fan will turn on when call for cool is given, but fan won't turn off when thermostat turns off; compressor cycles normally. Normally, the blower functions with immediate on, and a 90 delay prior to shutoff.

My questions:

I have the install and maintenance manuals, but where can I find a BOM/parts list?

Is there a good place to buy parts for this system online?

I believe there's only one board in this blower -- since it's 95 degrees in MA today, maybe someone knows enough to tell me from memory if this is true. Should I start with the fan control relay, or replace the whole board? Is this a common failure -- what's the best way to fix it?

Thanks for your help, guys.
Old 05-23-09, 05:53 PM
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What kind of heat do you have? There won't be any BOM available. You'll have to use aftermarket parts. Any chance you can scan and post the wiring diagram from the air handler? When you have the system off at the thermostat and the blower is still running, check for 24 volts from G terminal to the transformer common in the AH. If you have 24 volts, the problem is outside of the AH. If you don't read 24 volts the problem is either a bad fan relay or whatever device that delays the fan for 90 seconds is malfunctioning. Let us know what you find. Thanks.

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