My son bought a condo last winter. During the winter, the gas furnace worked very well and the air handler blower motor came on when heat was called for and shut off when the desired temperature was reached.

Now that there have been a few warm days, when the system is put in the AC mode, it pumps out plenty of cold air, the blower motor on the air handler comes on but will not shut off even after the condo reaches the temp set on the thermostat. Thermostat Fan setting is set to AUTO mode, not ON.

The system is a Carrier 58STA070 gas furnace with the air handler blower and a Honeywell RTH111 thermostat. Sorry, I don't have the info on the exterior AC compressor unit or the interior AC coils mounted above the furnace, but I think they are Bryant.

Thermostat is a pretty bare bones model and is set to Cool and the blower is set to AUTO.

Any thoughts on what wiring or other items I should check that might cause this air handler blower to continually run when the AC is on?

Just FYI, I am about 45 minutes from my son's condo, so I won't be able to check any wire colors or connections until I go down tomorrow, so any suggestions (which are greatly appreciated) will have to wait until tomorrow to see if they resolve the issue.

Thanks for any input.

 

Does the blower motor and outside unit keep running or just the blower only ?

 

Only the blower motor on the inside Unit

 

I was able to find the installation manual for your son's furnace which includes the schematic (pg. 33). It appears that there is a Blower Motor Relay used on the control board. It's possible that this relay is sticking. There is also a time delay setting that allows you to change how long the blower stays on after the A/C shuts off. This can be adjusted from 90 seconds to 180 seconds. Here's the link to the manual: https://www.questargas.com/ForEmploy...A,%2058STX.pdf

Although this is unlikely, you can rule out the thermostat by temporarily disconnecting the wire from the G terminal on the thermostat. When the thermostat calls for the blower to come on, an internal relay connects the R to G terminals, enabling the blower to run. If you remove the wire from the G terminal, the control board will never get the signal to enable the blower.

 

Thanks Bob,

I guess my question is what can be done with a defective relay on a printed circuit board other than replacing the entire board?

I have heard on other threads that a electro/mechanical relay can be given a good whack that might open the circuit but I'm curious what you do on a relay on a printed circuit board.

Any suggestion? What would indicate this relay might be bad? Burnt area?

Again, thanks and any more advice would be greatly appreciated.

 

You could try giving the relay a gentle tap to see if it turns off. Obviously, with the relay mounted on the circuit board, you don't want to hit it too hard, but a gentle tap shouldn't hurt it. I'm guessing that you'll find the relay is encased in a housing, so you won't be able to see the terminals. I'm expecting that they'll be pitted and "sticking" closed. If it turns out that the relay is the problem, you may be able to find a replacement relay if there are markings on it that indicate a generic part number and you're handy with a soldering iron.

 

Thanks again Bob.

Heading to my son's condo tomorrow morning so I'll see if my 3lb. sledge can give it a "gentle tap".

Supposed to be near 90 degrees tomorrow, so hoping this fixes the issue.

Regardless, he will have the air on and can just manually turn the system on and off to keep the blower from running continuously.

 

I would check that budget thermostat.

Does the indoor fan turn off if you place the thermostat mode to Off?

Does the indoor fan turn off if you pop off the thermostat?

 

Thanks to everybody who replied with suggestions.

First thing we did was pull the front of the thermostat off and quickly noticed that the nimrod who installed the thermostat got 75% of the four wires on the right terminals.

Instead of having the green wire on the "G" terminal, this wizard put it on the "O" terminal.

A quick swap out and the blower fan turns off (after a 120 second pre-set delay) once the room gets to pre-set temp and the AC unit compressor shuts off.

Thanks again guys.

 

I have a Rheem Criterion II Furnace and no matter what position the Tstat is in, blower runs continuously. I am pretty sure the issue is in the PC board. And rather than putting out $100-$200 for a new board, considering buying a fan relay and wire around the board. Any thoughts of issues I may have???

 

Possibly a tripped limit or shorted wire.

Do you have any blinking error codes at the furnace?
Does the heat work?

Has the thermostat been replaced in the last year?