I have a heat pump in the attic. The PVC condensate line has frozen twice when the temperature was in the low single digits. The HVAC contractor says that it doesn't freeze in the attic, but instead at the tail of the drain pipe that protrudes about 6" from the house. When that freezes, water backs up and the sensor in the furnace shuts it off to protect from flooding.

The HVAC contractor wants to reroute the drain line to a close-by vent stack, so that the drain line doesn't go outside into the colder air. This means that the water will flow into the septic system, an idea that gives me concern, especially in the summer months when the A/C is removing so much humidity from the air.

Is there another way to solve this?

  • How much actual water will be removed and flow into the septic system? I understand that septic systems are definitely different than city sewer, but is it really going to be enough to be a problem? If so, put in a Tee and valves so you can drain outside in the summer and to the septic system in the winter.
    – FreeMan
    Commented Jan 12 at 14:11
  • I'm with the camp that thinks condensate will affect your septic like you taking an extra pee. But—is there a way to devise a routing that drains to the vent stack when the exterior drain is clogged?
    – Huesmann
    Commented Jan 12 at 14:29
  • @isherwood Those were helpful to read. One suggestion to insulate the outside pipe sounds easy and quick. Several suggestions recommended attaching to an existing drain. The HVAC guy is talking about adding a trap to the side of a vent pipe. Is this ok since it is part of the drain system, but not a drain itself?
    – RetiredATC
    Commented Jan 12 at 15:51
  • Shouldn't the water be fed back to the condenser and evaporated? In the winter, to moisturize incoming dry air and in the summer to increase cooling efficiency.
    – Agent_L
    Commented Jan 12 at 16:53

1 Answer 1


Unless your HVAC system is massive, and for a residence I doubt that it is, the amount of condensate is going to amount to a gallon or two a day at the most.

As long as your septic system is operating normally, that is an inconsequential amount.

If you really don't want to do this, you could always install some pipe heat tape on the exposed part of the drain line to prevent its freezing.

  • I think it's a 3.5 ton system. I've never used heat tape, but the shortest I could find is 3' (for a 6" PVC stub) which doesn't sound like it would work, but I don't know. Also, I've read that not all heat tape is appropriate for PVC drains. It's supposed to get down to 5 degrees Tuesday, so heat tape seems like a great short-term solution, but maybe I'm wrong.
    – RetiredATC
    Commented Jan 12 at 14:26

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