I recently swapped out an older Aprilaire 700 (manual mode) humidifier for a new Aprilaire 700 series. I tried using the humidistat from the old one, but the new one didn't work off of it, so I installed the new humidistat. I get 24V from the furnace to the humidistat, but was not getting any voltage out of the "H" terminals, so--after a good bit of hesitation (the wiring diagram with the unit is not great)--I connected another 24V transformer to a 110 line to feed 24V between the "H" terminals and the humidifier. The humidifier comes on when I force it on using the "test" mode, but it stays on after that. The control mechanism in the humidistat does not appear to be working, because the humidifier does not come on with the furnace. Any help with this situation would be much appreciated!
For those in the same situation, two 24V connections are required -- one to the humidistat for constant power, and the other--via the transformer provided with the unit--between the humidistat and the humidifier. That said, in manual mode, another connection between the "W" terminals in the furnace and on the humidistat is required for the humidifier to recognize when the blower trips on (and off).