Aprilaire 400 only works in test mode

I have a two year old aprilaire model 400 humidifier installed on my electric heat pump in the northeast, where current temps are around 30 degrees. It worked fine in past years and kept the humidity level in the house around 38-40%. This year when I turned it on for the season, it would not run. It only runs in test mode and shuts off immediately when returned to normal operation. I have the humistat set for 40% RH in manual mode (dial turned all the way up), and hydrometer upstairs reads from 28-35% RH. 35% is fine, but there is no indication that the humidifier has ran at all this season. The evaporative pad is bone dry and I never hear the selenoid click on when I'm in the basement. I tried trouble shooting anything I could think of. The humistat has 24+v to the RC terminals and .7 to the HH terminals with the blower off. With the blower on, there is 24v running through the HH terminals, which leads me to believe there is nothing wrong with the humistat. There is an A50 relay, which I believe is working because selenoid will only open when the blower is running. I would greatly appreciate any help you could provide. I am trying to avoid a costly service call, and don't need another reason to throw this away and get the new Honeywell steam humidifier. Thanks.
Ray M.

 

The water is flowing in this mode?

I find it odd that you are getting reading at H H. You take a reading at the selenoid valve??

When is this opening in test only?