What is wrong? › Commercial HVAC
Water from a diffuser is condensate with nowhere to go
Water in a ceiling means wet insulation and tiles now and mold within days. The EPA's guidance is to dry water damaged materials within 24 to 48 hours.
What is probably happening
- The condensate drain on the rooftop unit or air handler is clogged with algae or sludge, and the pan is overflowing into the duct or the ceiling.
- Duct or diffuser sweating: cold supply air meeting humid ceiling air, often after someone shut the unit off for a while or the economizer is stuck open.
- A roof leak arriving through the same opening. The RTU curb is a common leak point after a roof job.
- Frozen coil thawing after a filter or refrigerant problem and dumping more water than the drain can take.
Do this in the next five minutes
- Put a bucket down and move stock, paper and electronics away from the drip.
- If water is anywhere near lights, outlets or equipment, turn that circuit off at the panel before you touch anything.
- Turn the cooling Off at the thermostat. The drip usually stops within an hour if it is condensate. If it keeps coming, it is a roof leak.
- Push wet ceiling tiles aside carefully so water drains into the bucket rather than spreading.
- Photograph the stain and the tile for your landlord or insurer, then book a visit today.
Do not
- Do not run the cooling with a known overflow. It is your ceiling you are wetting.
- Do not leave wet tiles and insulation in place for days. Past 48 hours you are into mold remediation.
- Do not pour bleach down a drain line you cannot see the end of.
Who to call
A licensed commercial HVAC contractor
The drain, pan and float switch are part of the unit. If the contractor finds the water is a roof leak at the curb, then it is a roofer.
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What it should cost
$147 to $345
Professional AC condensate drain unclogging, national range
Source: Angi, clogged AC drain, updated July 2026. View source
National published range, not a quote for your job. The full cost table for this trade is on the commercial hvac page.
Ask this contractor, on the phone, before anyone comes out
Tap each one as you ask it.
- Is this condensate or a roof leak, and how did you tell?
- Does the drain pan have a float switch that shuts the unit off before it overflows? If not, what does adding one cost?
- Is the drain trapped and sloped properly, or is this going to clog again?
- Will you photograph the pan and the drain outlet for my records?
End the call if
- A drain cleared with no look at why it clogged.
- A 'roof leak' verdict with no visit to the roof.