Product review (4 stars): Using Aprilaire 50 sensing relay with BestAir Humidifier
I bought this to switch 24 volt power for a BestAir brand furnace mounted humidifier, using the furnace's built in 24volt transformer.
The furnace is from the 1990s and lacks a humidifier terminal on the control board and the BestAir solution in this situation is to add a second 24 volt transformer hardwired into the furnace fan circuit so the humidifier runs only when the fan runs.
This sensing relay achives the same outcome of having the humidifier run only when the furnace fan is running by connecting to the common lead for the furnace fan by means of an inductive pick up.
It installed easily and works great.
IMHO it is a better solution.