If I run the humidifier only when the heat is on, I get good evaporation. However, I want to consider also running it with just the fan running because our furnace doesn't run all that often. Will I still achieve reasonable evaporation in this case?

2 Answers 2


I ran the following tests with an ambient temperature of approximately 70 degrees F. I measured the (hot) water flowing into the humidifier and the water flowing out of the drain to calculate the % evaporation in each case (there is a reasonable margin of error).

  1. Furnace running in low stage. Approximate air temperature coming from register is 120 degrees F. Humidity was 25% @ 70F. Measured 24% evaporation rate.

  2. Furnace is off, fan running on low setting. Approximate air tempertaure coming from register is 70 degrees F. Humidity was 25% @ 70F. Measured 19% evaporation rate.

  3. Same setup as 2, but humidity was ~32% @ 70F. Measured 14% evaporation rate.

Unfortunately, this indicates a large amount of water being wasted, especially with the heat off. Aprilaire advertises that it uses 3 gallons per hour with a 0.7 gallon/hour evaporation rate (24%), which matches with my test for #1.


I'd add this as a comment on KyleL's response, but I don't have the rep to do so yet. You should check to see if your humidifier is fed with hot or cold water. A hot water feed will have an increased rate of evaporation over a cold water feed. If your humidifier is connected to a cold water feed, switching it to hot may fix your problem.

  • It's hot water. Sorry this wasn't super clear in the original answer.
    – KyleL
    Jan 25, 2016 at 15:16

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