HVAC blower weirdness

[Jack]

Good morning, I've got some weirdness with my forced hot air furnace blower when the HVAC goes on. Thought you all might have some input before I either attempt to fix it or call a tech. Here are the symptoms.

1. Turn on AC. Outside unit goes on. Furnace blower does not.
2. Turn on heat. Furnace blower goes on as expected.
3. Turn off heat. Furnace blower turns off as expected.
4. Turn on AC. Outside unit kicks on. Furnace blower then starts up. AC runs fine.

I'm able to recreate this consistently each day. If you have some ideas, please let me know. Thank you.

[jamesgangnc]

It's likely that a different speed is being used by your furnace blower when in ac mode. Start troubleshooting by finding those connections at the control board inside the furnace and checking for

115vac when it should be running. I agree it's a little weird. Knowing if the control board is hanging up verses the fan is the first step. When you open the furnace you'll probably find a copy of the schematic. This is presuming you have a vom and some basic electrical skills. Otherwise call the guy :-)

[Tony Hwang]

Hi, Blower control lead from 'stat is same for heat or cool. First you have to see if G lead from 'stat is enabled when it acts up. From there you move to control board inside furnace. You didn't mention what kinda furnace. Any way in cool mode blower runs at higher speed.

[Lefty]

"Tony Hwang" wrote>>

Not necessarily. There's not nearly enough information here to make that call. Many furnaces use the thermostat and subbase to "call" the blower in AC mode, yet the blower is called directly by bonnet temperature in heating mode.

You don't know that it has a board, or thet it runs at a different speed for AC either. It may be just a fan center relay. Can't really tell without more information. I could make many guesses, such as motor, cap, wiring issue, board, fan center, weak disconnect, etc. etc., but they would be just that, guesses.

He should probably call a guy in to look at it, unless he wants to just follow his own steps 1) through 4). :-)

Sorry, Lefty

[ransley]

Why not a defective thermostat, thats the cheapest thing to try before calling a tech.

[Tony Hwang]

Hmmm, Still my logical approach is sound. Another lefty.

[Steve Barker]

switch fan to on, instead of auto, leave running.

[Jack]

Thanks to all for the input. Here was the solution. I feel a bit silly telling you all the solution. A very simple one but it would be nice to know why it works this way. I flipped the fan switch to 'auto'. Fan blower kicks on immediately when I turn on the AC.

I tried my sequence below to test this with the fan set to 'ON' and got the same results as before. Fan in 'Auto' mode starts blower for AC just as it should.

Can you please explain to me why this is so? Thanks again for your help.

[Bob]

You have a problem with your thermostat or the the control unit in the furnace. With the fan switch at ON, the fan should run at high speed regardless of whether the thermostat is calling for heat or cooling. In the AUTO position, the fan operates with the compressor. To test the furnace, cross the red and green leads from the thermostat. The fan should run. If it does, your thermostat is defective.