Wiring Aprilaire 700m humidifier

bfbeck · Dec 24, 2017

I have everything installed with my Aprilaire 700m except the wiring. I have a Lennox ml195 furnace which contains a 120 VAC HUM terminal. The wiring diagram shows that I need a current sensing relay so that the humidifier only turns on when the blower is on, but isn't that what the HUM terminal is for? The humidifier came with a 24 VAC transformer. I'm assuming I should wire the hot and neutral wires on the primary the transformer with the hot and neutral wires coming in to power the furnace. Is it okay to do this inside the furnace or should it be done at the furnace disconnect switch? With the secondary side of the transformer, I'm assuming one wire should go directly to the solenoid valve and the other wire should go to the humidistat and then wire the humidistat to the solenoid valve.

Are my assumptions correct? Do I need the current sensing relay?

The humidifier itself is going to be plugged directly into an outlet for power so there shouldn't be a need to hardwire that.

Thanks!

supers05 · Dec 24, 2017

bfbeck said:
I ... but isn't that what the HUM terminal is for? ...!

Depends on the furnace. For yours, it's a 120v output, which is on whenever heat is on. (It won't be on without heat trying to run.)

You can connect the primary wires of your transformer to the hum and neutral connections on the board.

Your plan for the low voltage will work.

Cheers!

bfbeck · Dec 25, 2017

Thanks! One more thing, the Aprilaire installation documentation says not to wire the transformer into the furnace blower circuit. Is that what I'd be doing if I used the HUM terminal or is that safe since it turns on only with the heat? I used my multimeter to check voltage of these different terminals and the EAC turns on at 120 VAC when the blower turns on. What is the Aprilaire documentation referring to when it says not to wire the transformer directly into the blower circuit?

supers05 · Dec 25, 2017

You're not directly wiring into the blower circuit. That's what the terminals are for. EAC is for an air cleaner, don't use that.

They say not to connect directly because it's much more likely that someone will screw it up.

Cheers!

user_12345a · Dec 25, 2017

you can use humidifier terminal with a transformer or go direct between W and C on the control board.

W and C wastes water before the blower comes on, transformer is preferred.

COLDIRON · Dec 26, 2017

You can also wire it through a sail switch that when airflow is established the humidifier circuit is made/on. No airflow no humidifier.

supers05 · Dec 26, 2017

Yes you are correct, but that's adding extra parts that isn't required for his situation. 1 less thing to break.

Cheers!

user_12345a · Dec 26, 2017

diy style humidifiers have pressure switches so people don't have to wire humidifier to furnace. they come with plug in transformers.

they used to use sail switches.

frankly though even pressure switches can fail and people who aren't comfortable wiring to furnace probably shouldn't be installing humidifiers.

Wiring Aprilaire 700m humidifier

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