We had our AC replaced and have been having some issues. Currently it seems like the drip pan isn't draining until I pop open one of the clean out ports on the higher level on the p-trap that was added during install.

At that point there will be a large gush of water so vigorous that it almost escapes the siphon drain, and lasts for a minute or so until receding to a gentle trickle.

If I put the cap back on and leave the AC running, the water will again gush the next time I pop the cap open, in say, an hour.

I should note that once the cap is open the pan drains quite well. There don't seem to be any clogs or such in the condensate line.

The technician was here just yesterday, and removed the cap on the lower branch, saying that this was causing a vapor lock issue in the trap. This doesn't seem to have helped and I see the same gush when I open the port.

I should also note that the water doesn't gush out until the fan is turned on and the cap is removed. Could my drip pan be tilted away from level such that a puddle is accumulating?

It seems like a lot of the AC problems we've been having could be in some part traced to a failure of the system to remove the condensation water. Lack of cooling power and de-humidification ability, icing over of the cooling coils, etc...

This is a blow through system, and the high end of the trap is at higher pressure when the fan is running, so I don't see why it would be vapor locked, etc...

One suggestion is that the original AC has second p-trap already somewhere downstream of the new one. But I don't understand how lowering the pressure at the drip pan end could help. And the ultimate exit is some three stories lower, so hard to get at...

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  • If filter is dirty excessive negative air pressure will hold condensate inside. Likewise if there is inadequate size return air opening for the cfm of the blower the same will happen. What is the dimension of the cold air return vent and what is the cfm of the blower? Often a new air handler will need additional return air vent to accommodate greater air flow capability
    – Kris
    Commented Jun 29, 2021 at 23:22
  • This is a blow through system, meaning the blower motor is upstream of the cooling coil. When I pop the cap off, a stream of air rushes out of the trap and continues blowing while the fan is running; presumably that means the system is at higher than atmosphere pressure? Commented Jun 30, 2021 at 0:02
  • As an experiment on the top remove the red cap and put a piece of tubing about as high as the standpipe you are plumbed to. Decrease the tubing size to maybe 1/2". The water should drain but you will lose a bit of air at the furnace. If that doesn't work chalk it up to a bad hypothesis.
    – Gil
    Commented Jul 30, 2021 at 4:09

1 Answer 1


According to hvac-talk


You don't need a p-trap on coils in the positive side. If you Instal one for code purposes, it needs to be vented on the coil side. The water in the p-trap will not allow plumbing odors to escape out of the pipe. If you cap it, the condensate will not drain properly. Based on my experience the air pushes the water out when it is not capped. If you cap it, the air can't push the water out. I've seen some installs where they drill a hole on the cap to allow enough air to escape and push the water out.

If the coil is on the negative side it will need a trap and needs to be capped. If you don't cap the trap on the coil side. The primary line on the pan will suck air not allowing the water to drain while the system is running.

  • Pretty much the opposite of consensus on that thread. But this pull quote is consistent with my experience but still strange. The pressurized air from the furnace is perhaps pushing the water away from the drain when the vent is left off? Presumably this means that the drain pan is not sloping toward the drain or least relatively flat? Commented Jun 30, 2021 at 1:13

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