A lot of the condos at Metro Crossing in Fremont have the same setup: a First Co. horizontal air handler hung in the ceiling, usually above a closet or hallway. And I keep getting the same call from them. Water is dripping from the unit, or there's a brown stain spreading across the ceiling below it.
When I open these up, the story is usually the same. You can hear and feel air rushing at the side of the cabinet, and the water isn't coming from the middle of the unit. It's running down the inside of the side wall, outside the drain pan. The pan isn't broken. The water is being thrown around it.
See it happen
The cutaway shows the edge of the coil, where the copper U-bends sit next to the cabinet wall. Compare the failure and the fix side by side, and tap the [?] targets for details.
Why the U-bends cause the leak
The coil's aluminum fins give water a continuous path to run straight down into the pan. The copper hairpin U-bends at the end of the coil don't. They're smooth and rounded, and they sit outside the fin stack, in the margin between the coil frame and the cabinet wall.
Air takes the path of least resistance. On these draw-through units, the blower's suction pulls a fast jet of air through that unsealed side gap. As condensation builds up on the cold copper bends, that moving air strips the drops off sideways and throws them at the cabinet wall.
From there the water runs down the wall, outside the pan, pools in the corner seam, and leaks onto the ceiling drywall. A dirty filter or a high blower speed makes it worse, because both increase the suction pulling air through the gap.
The three-point fix
- Return-bend shield. A factory return-bend cover where one is available, or a sheet-metal shield we fabricate, covers the exposed U-bends so the air can't reach them. Behind it is a dead-air pocket, and the water trickles straight down into the pan, undisturbed.
- Closed-cell neoprene gasket. High-density closed-cell tape on the coil's side flanges closes the gap against the cabinet rail and shuts off the bypass completely. All of the air has to go through the fins instead of around them.
- Lower blower speed. Moving the cooling speed to a lower tap, around 350 to 400 CFM per ton, brings the suction down below the point where it strips water off the coil.
What we do on site
For the technicians and property managers reading this, here's the checklist we work from:
- Install a U-bend shield. Use the manufacturer's return-bend splash shield if one is available for the unit, or fabricate a 24-gauge sheet-metal shield with about a 3/8" standoff and secure it over the copper hairpins with foil tape.
- Gasket the coil rail. Line the vertical coil rail with high-density closed-cell neoprene, about 1/2" wide by 1/4" thick. Don't use open-cell sponge foam; it soaks up water and lets air through.
- Set the blower speed. Move the cooling speed lead to a lower tap so the air speed across the coil face stays under roughly 450 FPM.
- Slope the unit toward the drain. About 1/4" of fall toward the drain outlet keeps water from standing in the pan at the far bends.
We also flush the condensate line and add a float safety switch to the pan, so if water ever does back up, the system shuts off before it reaches your ceiling.
What you can check yourself
- The filter. A dirty filter raises the suction across the coil and makes this leak worse. If it's grey and you can't see light through it, change it.
- Water on the ceiling? Turn the AC off at the thermostat until someone looks at it. Drywall soaks up water fast, and a wet ceiling can come down.
- Don't just replace the pan. The water is being thrown around the pan, not through it. A new pan will leak exactly like the old one.
Property managers and HOAs
When a complex has hundreds of the same unit, a leak in one usually means the same open gap is waiting to leak in the others. We're happy to look at a few units, show you what we find, and put together a plan for the building.
The short version
The copper U-bends at the end of the coil sweat like the rest of it. When the blower pulls air through the open gap beside them, that air strips the water off sideways and throws it onto the cabinet wall, outside the pan. Shield the bends, seal the gap, and slow the blower down, and the water falls where it's supposed to.
We wrote up three other leak calls with three different causes here: AC leaking water? It's usually not the drip pan.
Ceiling unit leaking in Fremont or the Tri-Valley? We'll find the cause before we sell you a part.