Why a Clogged Drain Line Can Shut Down Your Commercial AC
The rooftop unit is off. The compressor is fine, the fan motors are fine, and the breaker hasn’t tripped. The condensate drain line is clogged, and a safety switch shut the unit down to keep it from overflowing.
It’s an easy one to misdiagnose, because the equipment looks healthy and the building is still warm.
Where the water comes from
Cooling a building also pulls moisture out of the air. Warm, humid air crosses the evaporator coil, the water condenses on the coil, and it drips into a drain pan. A drain line carries it from the pan to wherever the line terminates, away from the unit.
In Central Florida, units run most of the year in humid air, so that line carries water nearly every day. We covered the building-wide side of this in our post on commercial building humidity control.
How a drain line clogs
A drain line is dark, warm, and constantly damp, which is where algae and slime grow. Over months the growth narrows the line. Dust and debris that get past the filters collect on the wet coil, wash into the pan, and add to the buildup. Once the line drains slower than the coil makes water, the pan fills.
Why the unit shuts off
Most systems have a float switch in or near the pan. When the water gets high enough, the switch cuts power so the unit stops making water before the pan overflows. The switch is working correctly. The tenant just sees a unit that won’t cool.
What happens when it overflows
If the switch is missing, has failed, or has been bypassed, nothing stops the pan from overflowing. Water goes into the ceiling tiles, insulation, and drywall below and beside the unit.
EPA’s guidance for commercial buildings is to dry wet materials within 48 hours to prevent mold growth. That’s hard to meet when the leak is above a ceiling nobody is looking at. It’s also how a drain problem turns into a building maintenance problem, with the HVAC vendor and the maintenance vendor both involved and the tenant waiting on both.
Signs to watch for
- Water stains or damp ceiling tiles near a unit or air handler
- Standing water in the drain pan, or slime in the line
- A musty smell near the equipment
- Tenants reporting a warm space when the equipment looks normal
If you see any of these, call your HVAC vendor. Don’t bypass the float switch to get the unit running. It’s the protection against an overflow. Other warning signs are covered in 7 Signs Your Commercial HVAC System Needs Attention.
What maintenance does about it
During a maintenance visit, a technician can inspect the pan, clear the line, and test the float switch to confirm it cuts power when it should. It’s a small task inside a routine visit. For the rest of what a visit should cover, see our commercial HVAC maintenance checklist.
No single interval fits every building. A unit that cools year-round in a humid space builds up faster than one that runs part of the year, so the schedule should reflect how hard each unit works. Ask your vendor how they set it.
Two things are worth doing now. Ask whether drain lines and float switches are in your current maintenance scope, and whether they show up on the service report. And log which units have shut off or leaked, and when. A unit that keeps repeating is a sign that clearing the line once didn’t fix the cause, so that unit needs a closer look from your vendor.
Why does my commercial AC keep shutting off?
A clogged condensate drain tripping the float switch is one cause. Dirty filters or coils, electrical faults, refrigerant problems, and control issues can also shut a unit down, so the cause has to be diagnosed, not assumed.
What does a condensate drain line do?
It carries the water that condenses on the evaporator coil out of the drain pan and away from the equipment and the building.
Can a clogged drain line cause water damage?
Yes. If the pan overflows, the water reaches whatever is below and beside the unit, usually ceiling tiles, insulation, and drywall.
Who should clear a commercial drain line?
A qualified HVAC contractor. The work often means roof or ceiling access, and the float switch is wired into the unit’s controls, so the line and the switch should be checked together.
If drain lines and float switches aren’t in your maintenance scope, IAH can walk through it with you. IAH handles the HVAC side and IMS handles general building maintenance, which is the idea behind vendor consolidation. If a unit leaks into the ceiling, one team can work through both.