I had a replacement HVAC system installed in June of last year.
I won't go into the details of how disappointed I was in the installation after doing my darndest to be a knowledgeable customer by reading this forum and asking questions (thanks again to all the forum members that helped me).
Anyway, the system included an A/S HUMD200 bypass humidifier (I think it is the same as an Aprilaire 500M). As it never worked, I took a look and it was not wired correctly to the Honeywell - Equipment Interface Module (I am an Electrical Engineer - boo, hiss!). It was wired as a "powered type" device (i.e. the EIM only provides a contact closure), it should have been wired as an "un-powered type" where the contact closure actual routes the "juice" from an external power source to the water valve. I told the installing HVAC company about this and they explained that they would only have to do a minor rewiring at the EIM.
My question to them was if the 24VAC that was coming from the furnace (to run the thermostat) had sufficient capability to power the 24VAC@0.5A (12 Watt) valve solenoid. Their answer was that "they always did it that way with no problems".
As the humidifier did come with a transformer (which was not used), and the install manual for the furnace does not mention anything about 24VAC power sourcing for add-ons, is it correct to power this humidifier from the furnace?
Also, does anyone ever add an in-line water filter (such as used for refrigerators) to the humidifier water supply input in order to try and minimize "crud" build up inside the unit? I know that filters need to be changed regularly, but was wondering if adding a filter would increase the life of the water valve and keep the insides cleaner. I have iron well water and unfortunately I did not catch that they "saddle valved" to the wrong side of my softener - this has to be corrected anyway but could adding an in-line water filter help (as long as they are going to re-pipe the thing)?