I've got a bypass humidifier mounted on my gas furnace, and an ecobee 3 thermostat driving both furnace and humidifier. What I like about having the ecobee drive the humidifier, is that it knows the outside temperature and so is able to adjust the target humidity level to avoid condensation on my windows.

The problem is that here in Colorado, the air is so amazingly dry that even with the humidifier running 100% of the time that the furnace is running, it can't get the humidity over about 28% - even when target humidity (based on outdoor temperature) is over 40%.

The ecobee has a setting for "type of humidifier" where the options are "steam" or "evaporative". Right now it's set to "evaporative" but according to their tech support, setting it to "steam" would allow the humidifier to run while the fan is running, even if the furnace isn't (and I have my system set to run the fan 75% of the time, to try to keep the different parts of my house at the same temperature).

Does running the humidifier when the furnace isn't heating have any risks? Does the humidifier somehow depend on the fact that the air flow it's dumping into is recently heated?

1 Answer 1


I have had to replace ductwork on systems that allowed the humidification while the fan was not running. In one case the duct work buckled and collected enough water to come through the Sheetrock, several others the ductwork rusted out. There needs to be water sensors and if moisture is detected below the humidifier it needs to be shut down as the water is collecting there, if the air is not circulating a in duct humidifier will only cause problems if on with no flow in my experience.

  • 2
    Agreed, running the humidifier while the fan isn't running is a terrible idea - and it's not what I was asking about. I was asking about running the humidifier while the furnace fan was running, but the furnace wasn't heating. Jan 31, 2021 at 19:04
  • As long as there is air flow and that is what I said not furnace but fan!
    – Ed Beal
    Jan 31, 2021 at 19:49

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