Aprilaire 56 humidistat not turning on for low humidity

Question

My humidifier is mounted in the return air duct and the humidistat is mounted in the same duct a few feet before the humidifier.

I did two tests while the air handler was running and calling for heat:

1. I moved the humidistat dial to “Test”. the water control solenoid opens,and water flows through the humidifier as expected.

2. I removed the humidistat from the duct and used a hair dryer to blow warm air on the back of the humidistat. Within 10 to 20 seconds, the water control solenoid opens and water flows normally.

So it seems like the humidity sensor is miscalibrated because it won’t open the water valve until it is exposed to the very dry air from the hair dryer.

Does my conclusion sound valid? If so, is there a way to recalibrate the humidistat so it turns on under normal dry winter conditions.

Any other suggestions about what is wrong or further diagnostic steps?

The pics below show the installation and also the measured humidity inside the return air duct where the humidistat is mounted.

Answer 1

Test your outdoor temperature sensor with an OHM meter it should have some resistance. The amount of resistance is posted in the instruction manual for the controller. The exact resistance changes with outdoor temperature. If it has resistance, you need a new control panel. If it's open you need a new temperature sensor. I've been through several 56 controllers in 7 years. Not sure why they wear prematurely. FYI, the furnace must be on and calling for heat in order for humidifier to operate.

Just noticed you're using manual mode with no outdoor temperature sensor. It's either the control panel for the humidifier or something in the furnace is not telling the control panel to fire.