Why is cold air blowing out of my AC drain line?
Cold air blowing from the condensate vents happens when the air conditioner is running. While covering these lines stops the airflow, it causes the system's air pressure to create a thumping noise in the drain.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1Installing a P-trap in the drain lineDid this help you?
- 2Putting caps on the drain pipesDid this help you?
- 3adding a tee-ed vent after the p-trap and capping the clean-outDid this help you?
- 4turning the secondary overflow pipe upward to hold waterDid this help you?
- 5taping up leaks found around the unit with foilmasticDid this help you?
- 6capping the secondary overflow pipe since there is an emergency panDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“Is it normal to get cold airflow blowing when ac is on from either of these vents?”source ↗
“If you cover them the air pressure makes a thumping noise in the drain.”source ↗
“While up there, I noticed cold air blowing out of the condensate drain.”source ↗
“no more air was blowing out of the primary drain but was blowing out of the secondary overflow.”source ↗
“While this helped some, it doesn't seem to hold enough water to stop the air flow.”source ↗
“On the front of the unit, I've also noticed that cold air is blowing around the flue and what seems to be an air supply for the furnace.”source ↗