The HVAC Duct Is Gurgling in My Son’s Room—And How I Fixed It
Last Updated on June 18, 2025 by Chad Peterson
A customer recently called me with a strange complaint: In my son’s bedroom, his HVAC duct is gurgling.
Every time the HVAC fan kicked on, I’d hear a bubbling noise in the ductwork in my son’s bedroom—like an old coffee pot percolating. It didn’t sound like refrigerant noise—refrigerant lines don’t usually make that kind of racket. This was louder, more like water moving through the duct itself.
What I Found: A Backward P-Trap and a Flooded Duct
It was obvious when I went into the garage and looked over the installation of the condensate line. As you can see in the image below, the journeyman didn’t oversee the apprentice when he installed the P-trap backward.
Water overflowed the pan and flooded the duct below, causing the HVAC duct gurgling noise.
Result:
- The trap couldn’t hold water properly
- Condensate backed up
- The pan overflowed
- Water drained directly into the supply duct below
- And every time the fan kicked on, it bubbled and gurgled through the standing water
How I Fixed the Gurgling Duct
To solve the issue, I first shut down the system and disconnected the condensate line at the AC coil. Once removed, I rerouted the P-trap and drain line to exit the house. But first, I had to core drill the foundation to get the pipe to flow a minimum of 1/4 inch per foot to get the right slope.
Here’s What I Did
- Removed and reinstalled the P-trap in the correct orientation
- Flushed the condensate line to ensure it was clear
- Reassembled and tested the system—no more gurgling
It was a quick fix, but one that could’ve easily led to mold or damage if it had been ignored.
Checklist: If You Hear Gurgling in Your HVAC Ducts
- ✅ Is the gurgling sound coming from one room or all over the house?
- ✅ Does it happen only during cooling mode, or when the fan runs alone?
- ✅ Check the condensate drain line orientation
- ✅ Is the drain line draining water outside
- ✅ Look into the AC coil pan: is the pan full or leaking?
- ✅ Do any low-hanging flex ducts have signs of moisture or water?
When to Call a Pro
If you find standing water in a duct, don’t wait—it can lead to mold, insulation damage, and air quality problems. If you’re not sure what to look for, a quick service call can save a bigger repair down the road.
