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 
better idea.
Wiring in a Aprilaire 500m to an older (11-86) Trane furnace w/o A/C
humidifier
Related Solutions
There's two ways to wire this up. The first method is to take advantage of the HUM terminal on the furnace control board. This terminal will supply 120 volts when the furnace is running in heat mode, so you'll have to use an additional transformer.
You'll hook the primary side of the transformer to the HUM, and Neutral terminals in the furnace. Then you'll connect the 24VAC terminals of the humidistat, to the secondary of the transformer.
The other method, is to use the furnaces transformer to provide power to the humidistat. However, you'll have to make sure the transformer is large enough to supply the load.
With this setup, you'll connect the 24VAC terminals of the humidistat to the C and W terminals on the furnace control board. That way when the thermostat calls for heat, the humidistat will be powered by the furnaces transformer.
You could use the EAC-1 terminal to power a step-down transformer. This is common with furnaces that energize the HUM terminal to line voltage.
Check the humidifier manufacturer, they should have a properly sized transformer to fit this exact need. They should also have a wiring diagram for this type of setup, in the installation instructions.
Basically, the primary side of the transformer will connect to the EAC-1 terminal, and a neutral terminal on the furnace control board. Then the secondary side will power the humidifier.
Whenever EAC-1 is energized, it will power the transformer, and the humidifier will turn on.
As noted in the comments on the question, you may not get the expected results.
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Best Answer
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.