"FAN ON" switch causes AC to be on all the time

[tac0967]

Hello

I had a neighbor install a new furnace in my house 2 years ago. He's been in the A/C business since at least the 1970s. The new furnace is one of the high efficiency furnaces that doesn't use the chimney (a Concord 90+ efficiency). Both the AC and furnace work properly, except for one problem. Ever since he installed the furnace, whenever I turn the switch on the thermostat to "Fan ON" (instead of AUTO), the Air Conditioner will also turn on and it will STAY ON, regardless of the temperature setting on the thermostat. In other words, I could leave the house for 2 hours after turning the fan switch to FAN ON, and the AC will run the entire time and the temp in the house will keep dropping regardless of any of the settings (and regardless of if the other switch is turned to heat or cool). My neighbor told me tonight that the green and yellow wires are switched and that he will be over Saturday to fix it. I just wanted to post this message here to see if he is correct...I don't want anything else to break if he is wrong. Any advice will be helpful. thanks. Todd, Omaha

[RoBoTeq]

Doubtful that those wires are switched, but they are shorted somewhere in the system.

[HVACTechNC]

Possibly a bad Tstat. Doubt that the wires are switched. Update us once you find the problem, Im curious to know what it was.

[heaterman]

Have your neighbor (contractor) check the low voltage wiring.

[hceptj]

Like they said...it doesn't sound like switched wires...you would have noticed other issues with the AC system since it's been 2 years. Let him check it and let us know what he finds...

[RoBoTeq]

A question for the OP; have any optional devices been recently added to your HVAC system? Humidifier? Air cleaner? UV lights?

[trouble time]

Why is everyone saying it is not the wiring? Sounds like it could be the problem to me.

[HVACTechNC]

If the wires were just switched when he puts the thermostat to "on" for the fan the condenser would come on, but not the blower fan. he said they both do.

[trouble time]

If the wires were just switched when he puts the thermostat to "on" for the fan the condenser would come on, but not the blower fan. he said they both do.

The G back feeds through the stat to Y.

[HVACTechNC]

The G back feeds through the stat to Y.

Whatch'a talkin' 'bout Willis?

If that were the case then why wouldn't the voltage "back feed" through the stat even if it were wired properly and still bring on the condenser? The thermostat doesn't know or care which color wire is connected to which terminal and to what piece of equipment it goes to. I'm not following your logic on this one, please explain further.

[trouble time]

Whatch'a talkin' 'bout Willis?

If that were the case then why wouldn't the voltage "back feed" through the stat even if it were wired properly and still bring on the condenser? The thermostat doesn't know or care which color wire is connected to which terminal and to what piece of equipment it goes to. I'm not following your logic on this one, please explain further.

How does the thermostat turn on the fan on a call for cooling? If you think about this, you will get to the point I am making. Think about the relationship of the Heat-off-Cool switch and the fan. You will get there. Willis?

[jpsmith1cm]

The G back feeds through the stat to Y.

Please be careful not to get too technical in open forums.

This is starting to border on DIY, guys.

[HVACTechNC]

I dont wanna break the rules, but I am curious about what he is talking about. I'm not following his logic, and if I am missing something I would like to learn.

[beenthere]

If the wires were just switched when he puts the thermostat to "on" for the fan the condenser would come on, but not the blower fan. he said they both do.

many new furnace boards automatically bring on the blower when they receive 24 volts on the Y terminal. So it could easily be that the wires are switched.

[tipsrfine]

Problem probably lies in a spider web of low voltage wires somewhere above the basement where somebody wire nutted the outdoor unit to "G".

[54regcab]

Some furnaces have a "Y" terminal that activates the cooling speed regardless of if the "G" is energized. My Carrier works this way, YMMV. If the "Y" and "G" were reversed at the stat it would work normally until you turn the fan to "On", then it would run the A/C.

[HVACTechNC]

That makes sense. I work on more heat pumps than furnaces, so I wasn't thinking about the furnace control board and its impact on this situation...

Eureka

[tac0967]

Thanks for your help guys. The problem actually was the green and yellow wires being switched! I watched him fix it; the fan on now works like it's suppose to. I'm still trying to figure out how I never had this problem with the old furnace... Anyway, thanks again!

[RoBoTeq]

Thanks for your help guys. The problem actually was the green and yellow wires being switched! I watched him fix it; the fan on now works like it's suppose to. I'm still trying to figure out how I never had this problem with the old furnace... Anyway, thanks again!

Uh, yea....some of us would like to know how that worked as well

[Milk man]

Uh, yea....some of us would like to know how that worked as well

Are you still doubting?

Maybe the yellow wire never went to the where we assume it went to, maybe it just landed on the thing "G" usually controls. Maybe.