Most air conditioning breakdowns we attend are a drain
Not the compressor, not the controls. A 22 mm pipe run level into an untrapped stack, and nineteen litres an hour with nowhere to go.
We keep a rough tally of what we find when we are called to a system that has stopped. Over the last two years, roughly half of the call-outs on cooling plant came down to condensate — water the coil is supposed to make, sitting somewhere it should not be.
Where the water comes from
A cooling coil that runs below the entering air’s dew point takes moisture out of the air. That is not a fault, it is the job. On a 2,000 l/s make-up air unit taking outside air at 28 °C and 45% RH down to 12.7 °C, the coil sheds about 2.2 grams of water from every kilogram of air passing through it. At standard density that is 19.1 litres an hour, every hour the unit runs.
Nobody is surprised by 51 kW of cooling. Everybody is surprised by nineteen litres of water.
A 22 mm drain, laid level
Level pipe does not drain, it stores. The first time the pan fills past the outlet the water backs into the unit, and from there it goes wherever the base of the casing lets it. On a rooftop unit that is usually the ceiling of whatever is directly underneath.
The three things that fix it
Fall of at least 1 in 100 on the whole run. A trap deep enough for the fan’s negative pressure — for a unit at −250 Pa that is 25 mm of seal plus 25 mm of draw, so a 50 mm trap minimum. And an air break at the discharge, so nothing in the drainage system can come back the other way.
The trap depth is the one that gets missed. A draw-through unit sits at negative pressure, so a shallow trap gets pulled dry and the fan simply breathes through the drain instead of letting water out of it. The pan then fills while the pipe below it stays bone dry, which is a confusing thing to look at if you have not seen it before.
The one case where none of this matters
A coil whose surface is warmer than the entering air’s dew point cannot condense anything at all. Sealed switchrooms, most comms rooms, and any space with no occupancy and no fresh air are usually in that condition all year. Their coils run at a sensible heat ratio of 1.00 and their drain pans stay dry.
We still fit the drain, and we put a float alarm on it. If it ever trips, something about the room has changed — a door propped, a humidifier added, an extract fan failed — and somebody should hear about that before the switchgear does.
If your ceiling has a stain under the plant
It takes about twenty minutes on site to tell whether it is the drain, the pan or the trap. The survey covers the rest of the system while we are there.