2005 Aprilaire 700 humidifier. Replaced a non-working circuit board. Now, the humidifier works fine when the furnace is running. It starts and stops as it should based on humidistat setting. Problem: If the humidifier is running and the furnace shuts off, the humidifer fan stops, but the solenoid valve remains open until I switch off the power completely to the furnace (which secures the 24V power from the humidistat to the circuit board input and therefor the solenoid valve) . I am thinking my "new" circuit board is bad, but I'm not sure. When this condition occurs, I tested the output of the board to the solenoid when the furnace is not running and I'm showing about 22V to the solenoid. I'm assuming that is what is keeping the valve open.

I solved my problem. In the course of making changes I had changed from an automatic humidistat to a manual humidistat. I had the new manual humidistat wired incorrectly.