New here. I recently purchased a Aprilaire 500m with a humidistat installed on the return. It's working fine, sure... But I don't want water pouring down the drain while the furnace/blower is not working. Yes, I wasn't aware of this ahead of time and thought the humidistat would handle it. A big duh on me. So, I have an idea that I'd like to bounce off the community here. The humidifier came with an additional step down transformer. I'm thinking of wiring it in just before the blower for it's 115v power supply and then just hooking up the 24v blue and red terminals to the humidifier. It's an older (11-86) Trane furnace w/o A/C and I've attached a copy of the wiring schematic if anyone is interested in giving me their opinion or a perhaps a Trane wiring diagrambetter idea.

1 Answer 1


I would use a Honeywell sail switch like the S688A-1007 from SupplyHouse.com for about $50.00 bucks and wire it in series with the power source for the humidifier. You could also use an air pressure switch from the same supplier. Those sail switches work very well.

  • Great idea! Thank you so much!
    – Flywood
    Jan 7, 2021 at 18:56

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