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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22624
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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Had a problem with furnace humidifier fed from the 24V Aux

Customer Question

Had a problem with furnace humidifier fed from the 24V Aux furnace line - took the humidifier control out of the equation and put the 24V leads straight to the 2 leads on the humidifier solenoid. Now the AC compressor kicks on when the furance kicks on and the humidifier is also does the AC control even play into this problem - I am baffled.
Submitted: 7 years ago.
Category: HVAC
Expert:  BillyHvac replied 7 years ago.
are the units wired through a pump safety at a condensate pump on the common wires?
Customer: replied 7 years ago.
We do not believe we have pump - by the way the furnace is a Bryant model 330aav Series B - the leads come off the Humidifer and ground terminals in the furance
Expert:  BillyHvac replied 7 years ago.

The only ways this symptom can happen is..


wrong wiring

short in wiring transferring power to other wires

back feed through a common wiring point.


The actual best way to begin eliminating is to replace the thermostat wire from the furnace to the thermostat, furnace to the ac.


I know it does not sound fun...but you have 2 choices...keep hunting for the culprit , or start making certain options go away.

Customer: replied 7 years ago.
one other item - this whole time - the thermostat has been sitting on HEAT - we have not moved it to Cooling ......the problem begain with the humidifer not "kicking in" this winter ( we are in Minneasota) we started to troubleshoot that.

Just so I understand ...somehow when we straight wired the humidifier - 24V is getting to the AC controls to kick in the compressor......correct?

More info
Right now we just disconected the humidifier and the furnace/AC problem goes away. The furnace, humidifier, and AC had been working fine for >5 years so something is now malfunctioning....
Also we have a programmable thermostat with no known issues....

Would the first step be hooking up the humidifer control again and taking some voltage readings?
Expert:  BillyHvac replied 7 years ago.

Ok, if you have a Bryant the Hum terminal is only live while the gas valve is energized. So to test the humidifier requires the burners to be on.


I have had humidifiers backfeed depending on how it is wired. The issue is if nothing has been changed and just popped up. The humidifier should be wired to HUM terminals, AC to Y and C....It is when we start combining items on the common side that this feed back can happen.


Take everything off like ac and humidifier and just have R and W from stat hooked. Run that a couple cycles, then put a volt meter on HUM terminals for 24 v during heat cycle. If so hook up to HUM. Run a few cycles. Then instead of putting the yellow from thermostat and the red to ac on the Y terminal, just wire nut them together. So Y from thermostat and Y to ac wirenutted . Then the last ac wire to C.

Customer: replied 7 years ago.
Ok - we are checking the wires back from the AC and HUM to the correct terminal blocks - at first glace it looks right but we will check again and also follow your instructions.

Sounds simple - hook up only the furance and test - measure for correct 24V on HUM terminals (labeled HUM and G) when the gas value is energized - if yes - hook up HUM and test....

but your last item on the AC wiring - reading them - doesn't that disconnect the AC? Or does this re-wiring simply avoid a backfeed from the HUM terminals to the AC terminals but will still allow us to test the AC ....sorry if I am using incorrect terms :-)

Expert:  BillyHvac replied 7 years ago.
The Y terminal function through the board is redundant. We get the same thing by wirenutting the wires instead of putting on the board. The correction to last reply is the humidifier is not HUM and is HUM and C.
Customer: replied 7 years ago.
The terminal leads are labeled:

Houston I think I see the problem - HUM was wired from HUM and G terminal leads - I'll get back once we hook them up to the right leads (Hum and COM24V) - if that was the case - how this thing has worked is a mystery
Expert:  BillyHvac replied 7 years ago.

Hopefully onto something there.....The key is to breathe slowly and not get frustrated...or you keep starting over....dont ask how I know that....

Customer: replied 7 years ago.
and typo - that was a Y/Y2 terminal

The AC is wired from the Com24V and Y/Y2 which looks to be correct - yes?

Still working with the hubby on the testing this all out
Expert:  BillyHvac replied 7 years ago.

Ac is Y/Y2 and 24vcom



Customer: replied 7 years ago.
Ok - wired the HUM to the correct terminals and disconnected the AC

HUM still was not engaging even with the 24V present - we reverse the leads going to the solenoid and now the HUM appears to be engaging properly when the gas valve is engerized.

Hubby is leaving AC disconnected until a bit warmer day to test out if the compressor comes on when the HUM is on (a bit chilly out tonight) - the Y/Y2 and 24VCOM do not have 24v present when the furnaces is running so it looks like all should work when we hook them up later to the two AC wires.

If all this wiring stuff makes sense and you don't see anything hooked up wrong - I think we are clear.
Expert:  BillyHvac replied 7 years ago.
Sorry for delay. All looks good to me. I think you should be all set.
BillyHvac and 2 other HVAC Specialists are ready to help you
Customer: replied 7 years ago.
No problem - thanks for the help