Had an issue wiring it up and the support line pointed me in the right direction.

My furnace never runs long enough to allow my humidifier to reach the set point.

This allows the humidifier to turn on the fan regardless if the furnace is running . The unit is mounted to the outside of the furnace and has a red light indicating when the humidifier is on.

great idea easy to install

Bypass humidifiers can put out a lot of humidity, but only if set up in the following way:
1) hot water must be used (and insulate the copper piping all the way to humidifier solenoid)
2) The humidifier must be able to work independently of the furnace/heater, ie activate the blower and itself when needed... and this part performs #2 perfectly.
The humidity in my house in winter is 25-27%, and get dry itchy skin, dry eyes, nosebleeds. With humidifier only on when furnace is on, hovers around 30-32%. But after installing this part, humidifier can run long enough to keep humidity constantly at a comfortable 40-45% (needs to run about 75% of time to do so). I am using 1 600M model for 3200 sq feet, and I need about 9 gallons per day output to keep humidity above 40%.

My newer home does not run the heat much when >32 degrees. This keeps the fan running to reach humidity setpoint. Works great.

wiring diagram is for new units with built in relay - will not work if wired as shown to older models with built in transformer; instead wire 4851 w/y and w/blue wires into solenoid, not humidifier wires. these wires need to see 24V only when hum is on - older units have 24V on hum wires when off!

This relay helps to humidify the house all the time, not just when the heating system has a call for heat. Any time the humidity in the home drops below a predetermined level the relay kicks on the humidifier and furnace blower. Humid air circulates through every room! The only downside is early in the heating season the humidifier doesn't seem to keep the air "moist" enough. Aprilaire claims this is common and has to do with the outdoor temp and your body becoming used to less humidity in the air than summer air.

Works Great

nice rleay

I installed this on a gas system where the heat is constantly on/off/on/off and the humidifier doesn't run long enough. This works great because it will keep the fan going until desired humidity is reached.

Service and delivery comments:

Arrived within a week in perfect condition