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Furnace Mounted Humidifier Won't Shut Off

Posted: Sun Jan 27, 2019 8:16 am
by rchesterton
I installed a Aprilaire 500M this weekend according to the instructions provided. It works fine but will not shut off. There's probably something in the instructions that I didn't understand completely. It came with a transformer to be mounted near the furnace to supply the power to the humidistat. I mounted it using the power to the switch box right on the furnace. I ran one leg to the solenoid and the other leg to the humidistat. Then there is a leg from the humidistat to the solenoid. Turned power on, everything works - water dumping into humidifier, solenoid turns on and off if I move the humidistat dial up or down. However, when the furnace/fan turns off after a call for heat, the solenoid does not close. It remains open and (i hooked it up to hot, as recommended) water keeps dumping down the drain line. I need it to shut off when the furnace is not running, turn on when it is.

Here are some pics:

You will see the transformer is not connected. I needed to disconnect to get it to turn off for now. There is a pic of my furnace panel - I probably need to tie into that so it sync's up with the call for heat/fan. I have a nest thermostat that is capable of controlling a humidifier. Ultimately, I would love to control it from that. I didn't mess with the furnace panel because the instructions stated NOT to wire to the furnace blower circuit. Honestly, I didn't know what that was. It did say that it was ok to go into the circuit before it enters the furnace, so I stuck with that.

How should I change my wiring so it will at least turn on and shut off with the furnace? And...if possible, can I wire it so my Nest will be able to control it?

Re: Furnace Mounted Humidifier Won't Shut Off

Posted: Sun Jan 27, 2019 8:18 am
by rchesterton
My pics didn't upload - here they are.

Re: Furnace Mounted Humidifier Won't Shut Off

Posted: Sun Jan 27, 2019 8:19 am
by rchesterton
Meant to include the other side of the instructions. Sorry for the multiple posts on this.

Re: Furnace Mounted Humidifier Won't Shut Off

Posted: Sun Jan 27, 2019 1:10 pm
by emtnut
You don't need the external transformer supplied unless you power it via the 'EAC' terminals in the furnace, or use the current sensor option.

You need to tap into the 'fan' terminal (usually green wire on G term), so it only runs when the fan is running.
I looked online to be able to see the diagram you posted. They say the wiring options are in the "Humidifier control safety and installation instructions"

DO NOT USE BOTH THE EXTERNAL TRANSFORMER AND THE FURNACE TRANSFORMER on the G contact, or you'll fry the furnace board !!!


What is this thing (see arrow in pic) ???

Re: Furnace Mounted Humidifier Won't Shut Off

Posted: Sun Jan 27, 2019 3:10 pm
by Aaron
That's a fuse holder.

Re: Furnace Mounted Humidifier Won't Shut Off

Posted: Sun Jan 27, 2019 4:16 pm
by emtnut
Aaron wrote:
Sun Jan 27, 2019 3:10 pm
That's a fuse holder.
Arghhhh <faceslap> :lol:

I was thinking it was something for the humidifier controls :oops:

Re: Furnace Mounted Humidifier Won't Shut Off

Posted: Sun Jan 27, 2019 11:38 pm
by Aaron
That NM cable should not be in the breeze like that, either. Furnace switch boxes should have EMT up to the joist space.

Also that single-gang handybox is probably overfilled. Those boxes are basically good for a single device termination, or a splice of three conductors. But not both. I recommend 4" square 1.5" deep boxes with switch covers. A chase nipple can be used for flush raceway into the furnace chassis.