Water Dripping From a Ceiling Vent: Which Kind

By Ray Bennett · Updated 2 October 2026 · 5 min read · Checked against the sources at the end

Water Dripping From a Ceiling Vent: Which Kind

Short answer

Dry the grille, switch the cooling off and watch. Water that keeps arriving is coming from above and is a leak. Water that only appears while it runs, forming on the outside of the metal, is condensation, and that is humid room air meeting a cold bare register rather than anything leaking at all.

Water coming off a ceiling vent looks like one problem and is really two, and from underneath they are impossible to tell apart.

Either water is arriving through the duct from somewhere above, which is a leak, or it is forming on the outside of the cold metal in front of you, which is condensation. The test takes an afternoon of not doing very much.

Before you start

What to carry: a dry cloth, a torch and a humidity meter if you have one. Ten minutes, then a few hours of watching.

Gas or electric? Neither, because this is the cooling side and no system makes a cold vent while it is heating. If you are seeing this in winter, something unusual is going on and it is worth a technician.

Find out whether it is one vent or several. One is usually that vent. Several across the house points at the humidity in the whole building.

Do not leave a wet ceiling alone. A spreading stain is not cosmetic. Plasterboard holds water, the insulation above it holds more, and mould gets going in a day or two in a warm loft. A vent that has dripped for a fortnight has usually already wet more than you can see.

And if the cooling has stopped at the same time, that may be a float switch shutting the system off to stop a ceiling coming down, which means a real leak rather than condensation: where that water is meant to go.

Which of the two it is

Above the grille or on the face of it. One is plumbing, the other is weather, and they need completely different work.

Check 1

Dry it, switch it off, and watch

Wipe the grille and the ceiling around it completely dry, then set the thermostat to Off and leave it a couple of hours.

Water keeps arriving with everything off. It is coming from above. That is a drain pan overflowing, a blocked condensate line, a duct that has come apart, or in a few cases a roof leak that has nothing to do with the air conditioning at all.

It dries out and stays dry, then starts again an hour after the cooling comes back on. That is condensation on the outside of the metal, and the rest of this page is for you.

Look at where the drops actually form. Beads on the face and the edges of the grille are condensation. Drops falling from inside, between the louvres, came down the duct.

Feel the ceiling around it. Damp spreading outward in a ring means water above the plasterboard, which is the leak case again.

Why a grille sweats

The box behind the grille is the cold part. If damp air can get at it, it collects water exactly like a cold glass does.

Check 2

Take the humidity out of it first

A metal register and the box behind it, which the trade calls the boot, sit full of cold air. If the air touching that metal is humid enough, the metal goes below the dew point and water forms on it. Nothing has leaked. It is a cold glass on a summer day.

So the first fix is the humidity, not the vent. Set the thermostat fan to AUTO rather than ON, because a fan running between cooling cycles evaporates water off the coil straight back into the house: the whole story on that switch.

Then the obvious sources. Bathroom fans that do not vent outside, laundry dried indoors, a crawlspace with bare earth, and a blocked condensate drain keeping a pan full.

Open the register fully. A nearly closed one makes the metal colder and slows the air past it, which is exactly the wrong combination.

Do not aim it at a wall or a beam. Cold air blown straight at a surface will start condensing on that too.

The boot behind it

Check 3

Seal the gap, then insulate the box

If the humidity is under control and it still sweats, the problem is that loft air is reaching the cold metal.

The gap round the boot. The hole cut in the ceiling is often bigger than the box, and that gap is usually open to the loft. Warm damp loft air pours down it onto the coldest thing in reach. Sealing that gap with mastic is the main fix here.

The insulation over the boot. It should be wrapped and covered. Very often it was pushed aside when somebody ran a cable and never put back.

The duct connection. A flexible duct that has pulled back off the boot leaves cold air blowing into the loft and warm loft air finding its way to the grille.

The grille itself. In a genuinely humid climate a plastic register, or a foam gasket behind a metal one, stops the last of it. That is a last step rather than a first one, because it treats the symptom.

Questions people ask

It only happens on the most humid days.

That is the condensation case confirming itself. The metal is the same temperature every day, so what changed is the air touching it.

Only one vent in the house does it.

Then look at that boot rather than the whole house. It is usually the one where the insulation got pushed aside or the gap round the box was never sealed.

Could it be a roof leak?

It could, and the test separates them. A roof leak drips after rain whether or not the cooling is running, and condensation only ever appears while it is running.

Will closing the vent stop it?

It makes it worse. Less air past the metal means colder metal, and a closed register raises the pressure in the ducts and pushes more air out of every leak.

The grille is rusty as well.

Then it has been doing this for a long time. Sort the humidity and the boot first, and replace the register afterwards rather than before.

Water is dripping from the unit itself, not a vent.

Different page. That is the pan and the drain: how to clear it.

Sources: EPA "A Brief Guide to Mold, Moisture and Your Home" (drying within 24 to 48 hours, condensation), DOE Weatherization Installer Job Aid "Seal Ducts With Mastic" DOE/GO-102023-5934 (gaps round duct boots), ENERGY STAR "Duct Sealing" brochure EPA 430-F-09-050 (registers, disconnected ducts, insulation), Lawrence Berkeley National Laboratory "Register Closing Effects on Forced Air Heating System Performance" LBNL-54005 (closed registers and duct leakage), and Lennox "Why Your AC Is Cooling but Not Removing Humidity" (fan ON or AUTO)