HVAC Blower fan shuts off when I start AC

My HVAC system's blower fan shuts off when I start AC. The blower fan works fine when I turn on heat or turn off heat and turn off AC. AC is working fine by itself. I can say this because out side AC unit fan is running and the AC unit freezes the A coil (evaporator coil).

When I start AC the blower fan stops immediately. After few minutes frost built up on the A coil. When I stop AC, the blower fan starts and clears up the frost on the A coil. The A coil is clean, the filter is new. The blower fan stops the moment I start AC... way before frost built up on the A coil.

Any help is appreciated.

Thanks

 

Has the thermostat or wiring been changed or disturbed recently? Have you tried replacing the batteries in the thermostat? Most thermostats use small relays that get get energized when enabling that function. Your thermostat likely has a separate relay for the blower and the outside unit. If your batteries are marginal, it's possible that it doesn't have enough current to energize both relays.

If changing the batteries doesn't help and you can get to the thermostat wiring (with the thermostat connected), check the voltage between the R and G terminals on the thermostat. With the thermostat off (not calling for cooling), you should measure ~24VAC. With the thermostat calling for cooling, you should measure ~0VAC between the R & G terminals. If you can't access the terminals at the thermostat, you might try at the air handler, where the wires coming from the thermostat connect to the air handler (blower assembly).

 

Welcome to the forums.

Doesn't make a lot of sense. So if the thermostat is not in heating or A/C mode.... the blower fan runs ?

 

Bob,
I changed the batteries and kicked off AC. The blower fan lasted for 10 minutes and now I am back to original. At least your prediction look like the area to investigate further. Later today I will change batteries one more time to be sure. If that don't work, I will buy a voltmeter and see how far I can go before calling a mechanic. Thank you very much for your help.

PJ,
Nice to be on this forum.
System has tree modes Heat, off, Cool. I selected Off. Fan has two modes Auto and On. I selected On. Now blower fan runs but no system is on.

 

You can turn the fan to ON with the mode set to COOL. The compressor will still cycle on and off as the stat calls for cooling, but the fan will (or should) remain on all the time.

 

There is one more thing you can try that doesn't require a voltmeter. When the blower stops running (with the thermostat calling for cooling), try jumpering the R to G thermostat wires together (this is what the thermostat relay does). If the blower turns back on and runs (while jumpered), it would indicate a bad thermostat (or intermittent thermostat wiring if jumpered at air handler end of thermostat wiring).

 

When I opened air handler end to jumper R and G, I notied a socket named COOL did not had wire. For the socket information please see PCB diagram on page 6 of following manual. The HVAC has CEPL13043801 control module (the circuit board that controls operations including turning on/off).
http://www.lennoxproducts.com/wp-con...325878-751.pdf

I located the wire that was supposed to go to the socket labeled COOL and that is it. Now everything is working.
Following is the picture of the socket COOL (red arrow) where wire was missing.
https://drive.google.com/file/d/0Bzn...it?usp=sharing

Thanks for your help Bob.