Attic HVAC: fan doesn't work in AUTO mode and AC on

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furnace
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Attic HVAC: fan doesn't work in AUTO mode and AC on

Post by furnace » Fri May 18, 2018 2:15 pm

I have a vertical attic HVAC unit consisting of two components: furnace and AC coil mounted on top. The AC coil is installed on 2005, while the furnace is from 1986. My furnace works fine. The AC works if I switch the fan mode to "On." However, the fan *doesn't* work if the fan mode is in "Auto." In both modes, the AC blows cold air. So the problem is with the "auto fan" feature.

I'm manually turning the fan on/off when running the AC, but it's not a long term solution.

I've checked the thermostat and the battery. Since the fan works in "On" mode, the fan appears okay, but it's not receiving the signal to turn on. My internet research indicates it might be a bad "fan relay" or "control board."

My question is: Where is this control board? I've opened up the furnace access panel, but don't see it, or at least, it doesn't look like a circuit board. Do I need to open up the access panel on the AC coil? Currently, it's sealed pretty tight with silver tape and adhesive. I would have to pry it apart and tape it all back.

Gas Furnace: 1986 Comfort Maker
Refrigeration Coil: 2005 (brand / make unknown ... no owner's manual)

I've replaced the entire basement HVAC 3 years ago. I'm hoping I can just replace a part and keep the attic unit going a few more years.

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just frank
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Re: Attic HVAC: fan doesn't work in AUTO mode and AC on

Post by just frank » Fri May 18, 2018 3:47 pm

I'd vote for a bad control board OR thermostat.

Replacing the latter is (likely, if its non-proprietary) $20 and a trip to the hardware store.

Spirit Rider
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Re: Attic HVAC: fan doesn't work in AUTO mode and AC on

Post by Spirit Rider » Fri May 18, 2018 5:42 pm

I agree with just frank: This is most likely a thermostat problem. There is only one fan control line coming out of the thermostat. It is either on or off. However, you can have some more advanced systems that control the fan asynchronously from the control of the condenser. This is usually done for better humidity control.

The proper way would be to troubleshoot the thermostat/system. Given the low price of thermostats and generous return policies, it is just easier to swap the thermostat. This assumes you are reasonably handy, know your way around a terminal block, can follow instructions and are not color blind.

2comma
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Re: Attic HVAC: fan doesn't work in AUTO mode and AC on

Post by 2comma » Fri May 18, 2018 6:50 pm

I wouldn't bother removing that panel, I think all that's in there is the evaporator coil. I'd basically "follow the wires". Try to follow each wire to see where it goes. Depending on age there may not be a complete "board" per say, maybe just some relay's and other electronics.

If you look at any thermostat install manual you can identify what each wire does and then use a jumper wire to mimic what the thermostat would do. One of the terminals is power, you just jump the power to the correct terminal. It's only 24V so it won't shock you (unless you are standing barefoot outside at the compressor and grab the thermostat wire, ask me how I know).

I'd start with the thermostat for all the reasons given above but I'm not hopeful...

I just did a search and a HVAC tech explained your issue like this:

The "fan on" signal simply turns the fan on.
The "ac on" from the thermostat sends a signal to the AC circuitry to start. The "whatever is up there" starts the compressor and probably has a delay for the evaporator to cool before it starts the blower fan.
The "heat on" from the thermostat sends a signal to the heater circuitry to start. The "whatever is up there" attempts to light the burner, opens the gas valve, checks for ignition, checks for blow back, delays for things to warm up and starts the blower.

Bottom line is the thermostat basically just tell the heat or AC to "do your thing". All it's doing is measuring the temperature and saying "do your thing" or don't.
If I am stupid I will pay.

SittingOnTheFence
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Re: Attic HVAC: fan doesn't work in AUTO mode and AC on

Post by SittingOnTheFence » Fri May 18, 2018 7:40 pm

Did the AC previous work correctly, ie: fan on when AC on in Auto?

It could be that your system will only work with a thermostat that has heat & cool (that is how my original 3 wire thermostat worked). When I upgraded the thermostat I also upgraded the wire to 4 wire, the old furnace & AC worked in auto mode.

Check the thermostat installation wiring diagram. If you don't have that avail then google the manuf of the thermostat and download the installation manual.

Verify the correct wiring at the thermostat and also at the other end. The thermostat wiring ends at the "control board" (I presume).

strafe
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Re: Attic HVAC: fan doesn't work in AUTO mode and AC on

Post by strafe » Fri May 18, 2018 7:52 pm

Spirit Rider wrote:
Fri May 18, 2018 5:42 pm
I agree with just frank: This is most likely a thermostat problem. There is only one fan control line coming out of the thermostat. It is either on or off. However, you can have some more advanced systems that control the fan asynchronously from the control of the condenser. This is usually done for better humidity control.

The proper way would be to troubleshoot the thermostat/system. Given the low price of thermostats and generous return policies, it is just easier to swap the thermostat. This assumes you are reasonably handy, know your way around a terminal block, can follow instructions and are not color blind.
+1
In cooling mode, the fan is usually (though not always) controlled by thermostat.

If it's an electronic thermostat, make sure the cooling fan control setting in the installer setup has not been changed.

furnace
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Re: Attic HVAC: fan doesn't work in AUTO mode and AC on

Post by furnace » Fri May 18, 2018 10:17 pm

I replaced the thermostat last year. I swapped out a programmable thermostat for a dumb $20 thermostat. Everything worked well last season. Just flip the switch from "off" to "cool" and set the temperature. The AC would come on along with the fan. Once it reached the desired temperature (1 degree above the setting) everything would shut off automatically.

Thanks for the suggestion to replace the thermostat. I'll give that a try and hopefully that is all there is.

furnace
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Re: Attic HVAC: fan doesn't work in AUTO mode and AC on

Post by furnace » Wed May 23, 2018 12:13 pm

I swapped out the thermostat for a new one, and the everything works now. Thanks to everyone for your help :sharebeer

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lthenderson
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Re: Attic HVAC: fan doesn't work in AUTO mode and AC on

Post by lthenderson » Wed May 23, 2018 12:39 pm

furnace wrote:
Fri May 18, 2018 2:15 pm
I've checked the thermostat and the battery. Since the fan works in "On" mode, the fan appears okay, but it's not receiving the signal to turn on.
Did you check the tightness of the screws on the thermostat terminating the wires? Perhaps one has come loose especially since it was replaced last year.

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