I've recently been learning a lot about sealing our HVAC ducts to improve system efficiency. On my mission to seal every accessible leak, I noticed my air conditioning/furnace cabinet joints were leaking quite a bit of air. I started sealing them all up, but noticed my attached humidifier was one of the biggest culprits of air leakage.

The operating manual says it draws in ambient air through a humidifier pad via a fan into the duct to distribute throughout the house (picture from manual attached). That makes sense, but it seems the same air inlet used during winter operation lets air out when trying to air condition the house. The manual doesn't mention any sort of "summer operating mode".

So my questions are:

  1. Is there a way to prevent air leakage with this kind of unit (Honeywell HE300 if model-specific)?
  2. Am I going to a pointless extreme by trying to seal every last cabinet leak?

HE300 Owners Manual Image

  • You've got 2 good, but separate questions there. "Am I going to a pointless extreme by trying to seal every leak." is a great question, and the humidifier leak question is probably really irrelevant if the answer to that one is "no". I'd suggest splitting this into two questions, one of which might not even need to be asked.
    – FreeMan
    Jun 24, 2020 at 10:47
  • How much of a leak? You seem to be showing a closed system, so you'd rather not have leaks when humidifying in the winter either! Now, cold air sinks, so there will always be cold air in the "Return" when your A/C is running, unless there's a damper in the house to shut off the humidifier loop. Jun 24, 2020 at 16:36
  • Both of those are the wrong questions. If "it draws in ambient air through a humidifier pad via a fan into the duct to distribute throughout the house" then get one that doesn't do that. Should whole house humidifier be designed like that? No.
    – Mazura
    Nov 12, 2022 at 1:32
  • This comment is late but this particular humidifier does not draw "ambient air". It draws from the duct and returns to either the same or a different duct. There should be no exchange of air with the outside surroundings, and it should be possible to seal it well. Given that these systems are often in unconditioned space, I'd be surprised if any whole-house unit exchanged air with its exterior. If you really sense this is the main source of air leakage, it may have been installed incorrectly EG a bypass unit installed without a supply duct.
    – jay613
    May 15 at 15:25

1 Answer 1


Leaking duct work regularly costs users 10% when leaking into non conditioned air spaces but if the leak is totally enclosed within the living space envelope it really is not helping much. Every air leak in an attic for example is saving $ if the leak is in the air handler closet it may save some but is not as effective as the attic.

As for humidifiers I like them to be easy access ultrasonic ones to verify they are working and Venturi ones to verify they have not plugged or are dribbling so for that reason I do not go crazy when sealing these up as I check mine several times a year.

  • Ultrasonic humidifiers, unless you feed them distilled water, will coat everything around them in whatever impurities are in your water. (Ask me how I know...)
    – Matthew
    Mar 22, 2021 at 13:11

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