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
I'm able to recreate this consistently each day. If you have some ideas,
please let me know. Thank you.
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 :-)
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.
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
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
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). :-)
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
Can you please explain to me why this is so? Thanks again for your help.
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.
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