I assume you want the humidifier to run only when the furnace is "in heat".
If you wire it to the blower motor it may run whether the furnace is heating
or cooling. A typical blower motor is multi-speed, with different motor
wires used to determine speed. Usually a slower speed for heating mode and
a higher speed for cooling, and you would connect it only to the blower
motor lead used for heating. However, you may find that the humidifier runs
regardless of which blower speed is selected due to a "back circuit" through
the motor.

One common way to get around this issue is to use a "current sensing relay
(avail. at HVAC distributors). It clamps around the low speed blower motor
wire and closes a switch contact only when the blower is in slow (or heat)
mode. These contacts can be used to activate the humidifier.

"Pat Coghlan" wrote in message
...
I used to have an Aprilaire bypass unit at our previous house. I liked
it very much, but they are only available through contractors. Our last
one cost about $450 installed.

I just purchased a Honeywell HE260 from Home Depot for $199 (there is a
$30 rebate offer in effect until Nov.). It's quite similar to the
Aprilaire in styling etc. I've got it bolted to the furnace (Lennox G2
- I believe) and the water supply connected.

Problem: The HE260 comes with a big honking 30VA transformer and a
kludgey "sail switch" (relay with a big "sail" that floats in the cold
air return and closes when air is flowing...Honeywell's way of sensing
blower activity). I want to avoid installing the transformer (24V) and
sail switch and, instead, just power the 24V solenoid water valve from
the furnace itself.

I doubt the solenoid draws more than about 500 mA. Is there any reason
I should NOT try to power it from the Y/T terminals (compressor
contactor) at the furnace's low-voltage terminal strip, which provides
24VAC when the blower is on?

-Pat