I am a bit curious, does it make any difference which side of the system a humidifier is installed on? Reason for the question is every house I've owned has had the humidifier on the return air side, with conditioned/heated air blowing across the pad.

Recently while helping my dad on his house, I noticed the humidifier on his furnace in the basement had the humidifier on the supply side of the system, with a duct blowing across the pad. Basically same setup as I am used to, but his is using the supply side of the system vs the return side.

I looked online and something I came across says it doesn't matter, but I question that because when you open the unit up to change the pad, air just blows at you. I just don't see how any moist air is getting introduced into the system.

When conditioned air is blowing into the pad and air is being sucked into the return, everything would be blowing in one direction. ie. one side of the pad is sucking air, one blowing. However, the way it appears to have been hooked up in my dads house is conditioned air is being blown at both sides of the pad. Does that make sense? How in the world is that air efficiently getting humidified???

BTW....the unit in question is a honeywell enviracaire elite humidifier and was installed 5-10yrs ago.

Also, how often should those pads be replaced? He said they never told him, so he's never replaced and his other humidifier appears to be an Aprilaire 220(?), which has probably been in the house the 15yrs they've lived there and never been replaced. My understanding is at least once a year, but I didn't think the one looked that bad, but maybe it's not operational...