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When my heat or AC is running, I hear dripping down a duct in my wall. What could cause this?

This is a Payne split system with natural gas furnace and the issue occurs when the AC or furnace is running.

Here are some things I have checked:

  • I'm using a brand new cheap filter that doesn't cause air resistance
  • I cleaned the evaporator coils while we were still running the AC
  • I cleaned the condensation drain line and replaced part of it to make it match the install instructions for that evaporator coil
  • the condensation drain connects to a trap and runs to the sewer line. I verified this wasn't clogged
  • previously there was water in the float switch and the condensate drain was clogged, but now it's clear and no water in the float switch
  • there has never been water in the drip pan below
  • I left the original duct that dripped unhooked and now it's dripping down a different duct

I had an HVAC tech come out over the summer and he said it could be water sitting in the evaporator blowing up through the vents, but then said if it's dripping out in one spot, it could be a plumbing issue and not the vent. I had verified it was the vent by unhooking that duct work. Then he suggested the plumbing to the sewer was the problem, as the trap is below attic flooring and not visible. I have poured water down it and it drains fine.

What am I missing here? I'd like to exhaust my efforts before involving another HVAC tech who may or may not be able to tell me what is wrong. If I need one, I'll pay for a second opinion vs using my home warranty.

The red is from the water heater. The blue is a trap below the flooring to the sewer. The green is replaced condensate drain line.

HVAC unit in attic

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  • I had this once and it was the humidifier spraying too much water into the duct.
    – Hot Licks
    Commented Dec 20, 2022 at 18:16
  • what is the humidity at your place
    – Traveler
    Commented Dec 20, 2022 at 18:32
  • Outside humidity is only about 40% right now. I have a sensor in the attic showing 44%. My Thermostats are showing low 30s, but 4 other sensors throughout the house are showing 38-40%. I don't have a humidifier.
    – Daniel
    Commented Dec 20, 2022 at 18:42
  • Thanks, so it is not that, and you only hear it when the heater is running
    – Traveler
    Commented Dec 20, 2022 at 18:53
  • 1
    We have 3 units and this is the only one with a problem. Could the blower motor speed be too high, causing it to suck water in from the drain line? I'm just throwing stuff out there at this point, because I have no idea..
    – Daniel
    Commented Dec 21, 2022 at 19:07

1 Answer 1

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When my heat or AC is running, I hear dripping down a duct in my wall.

I assume you've actually seen this water? Dripping water should come out someplace.

Water in the evaporator can't be a bigger problem in the winter. If there is water in the air handler it's left over from the summer. Heating up air doesn't make water condense.

I had verified it was the vent by unhooking that duct work.

Verified what was the vent?

I cleaned the evaporator coils while we were still running the AC

Is it possible you sprayed water into your ductwork? Seems unlikely to have caused a long term issue tho.

I left the original duct that dripped unhooked and now it's dripping down a different duct

Unless you have a pipe hooked up to your ductwork, the only way for water to get in is condensate from the A/C (or a hose from cleaning evap coils). Follow the ductwork back until it's dry.

Your photo shows something "new." The trap is in the new portion. What you show as the trap is just the drain. The trap (traps water there) keeps the system from sending air from the air handler to the vent. It looks good to me, it shouldn't be backing up into the evaporator, your down leg is plenty long enough to prevent that. Is it possible you're hearing drips fomr your water heater because the relief valve is leaking?

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  • We had a water stain on the ceiling below the original duct that was dripping. I said vent, but meant duct. It was going to a specific vent, but the water would drip to a 90* and then leak into the ceiling below. We don't have damage yet where the new duct is leaking.
    – Daniel
    Commented Dec 20, 2022 at 19:40
  • The section labeled new, is a new trap and vent feeding to the section that drains to the sewer. The section labeled trap, is a trap below the flooring that then drains to the sewer.
    – Daniel
    Commented Dec 20, 2022 at 19:42
  • I let everything sit a while after cleaning and didn't see any additional water before closing up the evaporator housing. It's possible some water sat there, but I don't see how it would have for this long.
    – Daniel
    Commented Dec 20, 2022 at 19:44
  • Unless water is being sucked up from the drain, I don't see how it could make it into the duct work. The physical location of the water heater and the drain are not near where the duct is dripping.
    – Daniel
    Commented Dec 20, 2022 at 19:47

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