I'm trying to understand why a whole house humidifier is placed on the cold air return where water is injected into the air flow and then it has to pass through a five inch filter before entering combustion area and hot air piping.

Would the filter not stop the condensation from going past it?

My own home has this and I'm lucky to ever get a humidity reading above 37% when it sounds like it's working constantly, I understood 40 % + is recommended. I just assumed the filter comes into play?

  • 1
    Warm air in the furnace plenum or any warm air duct will be much more effective in adding moisture. Nov 20, 2020 at 22:03

3 Answers 3


The biggest reason is probably "because that's where it fits."

It's true that warmer air can carry more grains of water than colder air can. That's a good physics-based reason for locating the humidifier on the supply air duct.

But lots of homes have an air conditioning evaporator sitting atop the furnace. Furnaces don't often get installed in a premium room; instead they end up stuffed in a closet somewhere that's lucky to have an 8 foot ceiling. Then the supply ducts have to remain below the floor joists, and the ducts are about a foot deep, and next thing you know there are only 7 feet of vertical height to fit the furnace and A/C into. There often isn't vertical space to accommodate a humidifier. Oh, and there are one or two ducts for the exhaust flue and maybe intake air too that come down in front of the furnace. And so on..

A return air duct, on the other hand, is often a large blank canvas. There is lots of unobstructed surface where the humidifier can be mounted. It might become routine to some installers to place a humidifier there "because that's how we always do it" even if there were room on the supply duct on a particular job.

A humidifier should be able to get the job done regardless of which duct it's installed on. If yours isn't, maybe you could ask a question about that.

  • 1
    You're probably right about it being more of a "put where it fits" decision. I don't think I've ever seen a humidifier on the supply side, even where there is adequate room, so maybe there's more to it than that.
    – SteveSh
    Nov 20, 2020 at 23:02

No. There is no significant condensation. If that was the case your filter would be dripping wet. Water vapor molecules are much smaller than any filter will catch.


If the humidifier you are installing is a by-pass humidifier it does not matter a lot where you mount it because the bypass duct has to be mounted into the other duct to yield a pressure differential. That pressure differential is what allows the air to be forced through the media pad to remove moisture from the water supply.

If the humidifier you are installing is not a bypass type unit, then it should be mounted on the supply because that is where the hot air is located and the hotter the air across the media pad the greater capacity of the unit. In some instances you may need to run the fan longer or even 24/7 to increase the humidity in the home. Those units on produce humidity when the blower fan is running. You can also increase the capacity pf the humidifier by using a hot water supply but first check with the MFG to see if they allow that.

The size of the humidifier is determined by the size of the home, and the homes construction. Is it an older home or newer home? Is it well insulated and well sealed against outside air infiltration? Do you have the unit serviced and cleaned yearly? Is the media pad replaced yearly or as recommended by the unit's MFG? What type heating system do you have? Who sized that humidifier or was it chosen on price alone? Post the name and model number of the unit and a description of your home. All these items must be known when sizing a humidifier.

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