I have a residential 16-SEER four-ton air handler/heat pump unit with a built-in humidifier. Both summer AC condensate and winter humidifier over-run feed into a condensate pump which is on the basement floor next to the air handler. The discharge from the pump feeds a 3/4” plastic tube, which travels into the basement overhead and then outside to a drainage area. There is a finished bathroom in the basement and NO utility floor drain. Laundry is on the first floor, so limited access to that drain. The problem is that my outside drain line freezes and the condensate pump backs up and shuts down. The float valve NC contacts in the pump shut down the humidifier so no more water comes in, but then the humidifier is off. How can I drain the pump discharge inside the house to an existing basement DWV pipe?

  • What does the end of the tube look like? I had a similar setup with 1/2" tubing, and as long as the terminal end was off the ground a bit it would drain out from gravity and not freeze shut. Maybe post a photo.
    – isherwood
    Jan 21, 2019 at 19:30

2 Answers 2


If the drain line is accessible you could plumb it with a valve so that it goes into a tank during freezing weather. When not freezing it could go outside. What flow rate is coming out? If this is not possible or desirable, figure out how to direct the drain line into the drain under the lavatory.

Alternatively, you could put electric heating tape on the the part of the line that freezes.

  • I like @Jim Stewart s idea about adding heat tape to part of the line. It could be on a switch too.
    – Lee Sam
    Oct 19, 2019 at 16:16

If the unit has a condensate pump as you say it does, run the discharge to a laundry sink or other sink. If you can't run the line have a contractor do it for you. The discharge line could also be connected to any accessible drain line as long as it is plumbed correctly.

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