I recently had a brand new addition added to our house include a new master bath, plus a new HVAC system. The unit sits above the bathroom in the attic. The HVAC installer connected the drain for the AC to one of the sinks in the bathroom. So the drain basically runs straight down the wall behind the vanity and comes out at the sink drain. When the AC defrosts there’s a good 2 mins worth of water draining into the sink. Not a few drops every now and then, but like someone is continuously slowly pouring a huge glass of water out.

How can I reconfigure it to drain silently? What are my options here?

In theory I could reroute the AC drain pipe to the laundry room, but that would likely require drywall work.

drain connection under sink

drain from AC coils

UPDATE: It looks like the refrigerant line is frosted over at the coil, even worse so at the condenser.

frosted refrigerant line

  • @JimStewart besides needing to redesign where the sidearm enters the trap it sounds like you’re saying the system is probably frosting over, is that right? That might also explain why it doesn’t seem to cool correctly, I can leave the thing on all night and it won’t reach 72.
    – Matt
    Jul 10, 2018 at 13:11
  • replace the "plastic" pipes with metal; you can barely hear toilets flushing in the basement in old homes with iron pipes, whereas w/PVC it's whoosh whoosh trickle...
    – dandavis
    Jul 10, 2018 at 17:06
  • @dandavis are you suggesting replacing the sink trap with a metal trap and assembly? I believe that’s where the sound is coming from rather than the pvc coming out of the unit. Certainly an interesting idea
    – Matt
    Jul 10, 2018 at 17:09
  • 1
    yes: metal is harder for sound to penetrate and can be replaced w/o the aforementioned "drywall work". Pipe wrapping might help as well, no matter what the material, so it's likely worth a try up-front...
    – dandavis
    Jul 10, 2018 at 17:18
  • @dandavis is talking about thick walled cast iron piping for drains inside walls, which are quieter than plastic, but these are not used for traps under a sink. Jul 14, 2018 at 19:42

1 Answer 1


The sound is coming from the fall of the condensate into the water standing in the trap. This is a dishwasher sidearm designed to allow high flow and not obstruct a kitchen drain. Some have a hood in the center that directs high flow down the drain. This hood may be causing the condensate to accelerate in the air down to the standing water.

  1. You may be able to cut out the center hood inside the tailpiece. This might allow the condensate to drain down the side of tailpiece rather than fall into the trap.

  2. But if that doesn't work, replace the sidearm tailpiece with a standard tailpiece. Fashion a makeshift sidearm that enters the trap just above the standing water level. You could just drill a hole in the trap and cement in a sidearm just above the level of the standing water.

In any case change the opaque hose to clear so you can observe the flow of condensate.

Note that if you ever have to use a plunger in this lavatory you should pinch off the flexible condensate line (with a shut-off clamp or locking pliers). See Siphon Hose Shut Off Clamp Small Plastic Clamp Fits 7/16 in O.d.tubing Plastic Tubing Mid-range Clamp. There are other designs for a shut-off clamp, one out of metal with a thumbscrew which actuates a bar which compresses the hose.

EDIT Should you decide to drill a hole into the trap above the standing water level you could possibly use a press fit of tubing into the hole. You'd insert a short piece of rigid tubing inside at the end of clear plastic tubing and force that into the hole. If you use silicone grease around the outside, it will make insertion easier and help seal the hole. Petroleum grease would cause deterioration of the PVC over time. Soap would lubricate, but would not promote sealing, but if you didn't have silicone and wanted to see if the sound was reduced to acceptable levels you could use soap. Candle wax would be OK, I think.

Additional EDIT

You could leave the sidearm tailpiece in place and cap off the sidearm. Then redirect the flow to go into the trap. drill into the trap above the waterline. If you would drill from the side, you'd be able to do this without disconnecting anything. The new transparent condensate drain line could be smaller in diameter and more flexible than the existing one.


You may be able to fix the sound without any surgery or drilling the trap. In this solution you would use a small enough clear flexible hose that it could be inserted in the sidearm and pushed down so the end of the hose was just above the standing water in the trap. This would enable you to try this idea without modifying the trap or even disconnecting the trap. This would partially obstruct the drain, but it's worth a try.

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