Air handler not running on AC start up

[A747]

I started up the central air conditioning this morning, the first time this year. I heard the compressor unit fire up but after checking why the house wasn't getting any cooler, found that the I had not air flow. Went down to my sub basement and the air handler was not running. Up at the thermostat, I found that placing the fan switch in ON rather than in AUTO will start the fan. Before I call the HVAC pros, is there anything I can check on my own to figure out why the fan won't run in Auto. The central air was installed back in 1985 when the house was built.

 

[bonneyman]

Being the system is 40 years old I'd have a reputable company come out and check it over. But it sounds to me like the thermostat has an internal problem. If the blower runs in the fan "on" position but not "auto", you've proven everything in the system (motor, fan relay, and interconnecting wiring) is functional. Some connection inside the thermostat is not making the independent action of energizing the fan relay when in "auto". After 40 years of switching on and off that circuit is just not working. Just my SWAG.

 

[PoorUB]

I started up the central air conditioning this morning, the first time this year. I heard the compressor unit fire up but after checking why the house wasn't getting any cooler, found that the I had not air flow. Went down to my sub basement and the air handler was not running. Up at the thermostat, I found that placing the fan switch in ON rather than in AUTO will start the fan. Before I call the HVAC pros, is there anything I can check on my own to figure out why the fan won't run in Auto. The central air was installed back in 1985 when the house was built.

Was the thermostat replaced recently?
With an electric air handler the thermostat commands the fan for AC. There should be a setting for the type of heating system. It needs to be set for electric heat thermostat controls the fan.

 

[A747]

Thanks, I was wondering if there was any type of interlock between the air handler and AC compressor similar to the gas furnace setup. I'm suspecting that the thermostat may be the problem. The slide switches are starting to act a bit problematic. I'll replace the thermostat first before calling HVAC.

 

[fitter30]

Thanks, I was wondering if there was any type of interlock between the air handler and AC compressor similar to the gas furnace setup. I'm suspecting that the thermostat may be the problem. The slide switches are starting to act a bit problematic. I'll replace the thermostat first before calling HVAC.

No there isn't. Fan runs in the on position thermostat isn't set up right or is bad. In cooling auto fan should send voltage to G wire on call for cooling.

 

[A747]

Well, it looks the thermostat is the culprit. I just turned on the AC and both the compressor and fan kicked on with the fan in the AUTO mode. Time to dig out the replacement thermostat I bought a couple of years ago.

 

[PoorUB]

Maybe flicking the switch from auto, to on and back to auto cleaned the contacts and it will work fine.