Residential AC blower fan not starting on cool but working for heat

[zorobabel]

Hoping this is an easy fix...
Residential AC blower fan not starting on cool but working for heat. Outside compressor seems to work (fan spinning, noise - including swooshing at the radiator/blower). Heat works fine (electric).
Anyone had this problem before?

 

[Dinoburner]

The HVAC tech will probably check to see if 24v is available at the fan relay to start with.


















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[Rand]

does it work if you set it to fan on?
then turn off when the A/C cycles or what?

 

[zorobabel]

I opened up a box at the unit and found a diagram and electrical "things" inside. Following a friends advice, I tapped on the fan relay and it started shortly after. But that should have affected the heating too, so possibly coincidence...

 

[cpayne5]

If the fan comes on for heat, but not A/C, I'd suspect the thermostat.

An easy test would be to jump red to green at the thermostat. If the fan comes on, then you know everything downstream is OK.

Generally, here's what the colors at your thermostat are...
red - 24v
green - fan
white - aux heat (resistance strips, propane, nat gas, etc)
yellow - compressor/condenser
orange - reversing valve (this is what controls whether the system is a/c or heat)

Jumping red to the other colors triggers the relay(s) for that function.

 

[97prizm]

A sensor or relay tied to the A/C.

 

[Skippy722]

I had this problem at my first house. Initially thought it was the thermostat so I replaced that but it didn’t help. The board had separate relays for the fan motor depending on what mode it was in.... heat and fan it worked fine, AC had issues. Ended up replacing the board myself and it never gave me another issue for the time we lived there.

 

[PandaBear]

I had this problem at my first house. Initially thought it was the thermostat so I replaced that but it didn’t help. The board had separate relays for the fan motor depending on what mode it was in.... heat and fan it worked fine, AC had issues. Ended up replacing the board myself and it never gave me another issue for the time we lived there.

This is the likely answer.

If the fan come on with heat then the fan itself is fine. If the compressor turn on but not the fan when in cooling then the thermostat is fine. Now the fan only switch may go to the heat or cooling relay, so the possible scenario is one of the relay is bad. You likely need to use a volt meter to probe all over the place to check the voltage on the input and output of relays to see if they are bad for sure.

 

[mk378]

A unit old enough to have a separate fan relay (not a circuit board) would likely only use that relay for A/C. There is a thermal switch that controls the fan for heating.

 

[Yah-Tah-Hey]

If the fan comes on for heat, but not A/C, I'd suspect the thermostat.

An easy test would be to jump red to green at the thermostat. If the fan comes on, then you know everything downstream is OK.

Generally, here's what the colors at your thermostat are...
red - 24v
green - fan
white - aux heat (resistance strips, propane, nat gas, etc)
yellow - compressor/condenser
orange - reversing valve (this is what controls whether the system is a/c or heat)

Jumping red to the other colors triggers the relay(s) for that function.

Me too cp. That is where a change takes place.

 

[wag123]

The blower motors on most home HVAC systems have two speeds, lower speed for heat and higher speed for A/C, consequently there are two different circuits controlling them. As mk378 stated, on older furnaces there is a thermal switch that controls the fan for the low speed heating side. Either the controller circuit or relay on the higher speed A/C side of the blower motor has an issue. The fan switch on your thermostat usually actives the higher speed side of the blower motor. You can easily test these with a VOM or test light.