Air handler blower running after A/C compressor shuts off.

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Old 07-13-10, 08:20 PM
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Air handler blower running after A/C compressor shuts off.

The blower does not always shut off after AC compressor shuts off:
I have a split system A/C. The inside unit is a Rheem Criterion II (installed in 2002) and the outside is an older Comfortmaker with a recently replaced Bristol compressor.
The malfunction is inconsistent in that it does not always occur but plenty often enough to drive me nuts.
For example: In the past hour or so just now Iíve had to manually shut the blower off twice and it worked automatically twice.

Typical scenario:
Compressor shuts off after reaching thermostat set-point but the blower stays running even after the usual built in 45 second delay and will not stop until manually stopped. When this happens the compressor won't restart unless the blower has been turned off then both can come back on.
In the meantime Iím recirculating increasingly warmer air.
I have to manually shut off the blower at the thermostat and even then there's a delay.
I did some research and ended up replacing the old thermostat with a new Honeywell RTH221B programmable one two days ago thinking that might fix it.
Worked fine for about 24+ hours then back to re-occurrences of the same problem.
Often when the blower has to be manually turned off I have to cycle power to the compressor at the junction box to get it to come on even though there was a delay of more than the required 5 minutes from the time the compressor shut off last.

Last year I had a problem with the compressor not turning ON when the blower came on and I fixed that myself by replacing the compressor relay that was worn out.
Again, this whole problem doesnít always occur and it might run and cycle on and off like itís supposed to for several hours in a row.
It tends to have this problem more when the system has to work more.

One of the fixes Iíve read about so far involves replacing the blower relay and/or the limit switch. I found the control board but have not been able to identify the relay or limit switch yet.
Iím not real knowledgeable about electrical or HVAC stuff but Iíve done some little jobs around the house and I know how NOT to get shocked. I have a multimeter if thereís something I need to check (provided I can actually find it).


Thanks for any input you might have.
Jeff in Florida
 
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Old 07-14-10, 09:41 AM
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Follow the conductors from the blower motor...

Follow the conductors from the blower motor, one of them will connect to the blower relay.
 
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Old 07-14-10, 11:29 AM
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I'm unsure what the A/C limit switch is.

Is there a separate inside air handler, or is this part of a furnace?

Some A/C units, like mine, have a built-in delay the runs the inside fan for a minute or two after the thermostat interrupts a cooling call. (The purpose is to purge the ducts of cool air.) Check that out.

I suppose it's possible that the fan relay is sticking closed. You can give it a rap and see if that shuts off the fan. Another possibility is that the 24V to the relay operating coil somehow isn't getting interrupted, possibly due to a bad "off" delay or an intermittent short somewhere. Check to see if there is 24V to the relay coil when there is no call for cooling.
 
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Old 07-14-10, 09:21 PM
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I'm not sure what the limit switch is either or what it looks like. It was referred to elsewhere as a potentially failed component in relation to blower problems.
My air handler is part of the furnace. I guess I should have mentioned that at first. Mine does have a built in delay of about 45 seconds after the compressor shuts off. Sometimes the blower shuts off after 45 seconds like it should and sometimes it never does on it's own which is the problem.

I didn't have time to do anything with it tonight because I got home late.
It's a matter of finding those components now.

I can probably find the blower relay per HVAC RETIRED's instructions but what about the component that supplies the delay? I wouldn't be surprised if it's not a soldered on chip on the control board and I've have to replace the whole board (like about $250) just to see if that was the problem.

Anyhow, thanks for your help.
 
 

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