Boat Air Conditioning Duct Condensation
Not a leak, and not the drip tray. Cold ducting running through a humid boat sweats exactly the way a cold glass does, and the water comes off wherever the insulation stops. It is a build-quality problem, and it is fixable.
Sweating: First Checks
Not urgent, but it will stain joinery and grow mould. Wipe a length of duct where it is dripping. If the outside surface is wet all over rather than dripping from one point, it is condensation, not a leak.
Where this sits on the seawater loop
- Seacock
- Strainer
- Pump
- Condenser
- Evaporator
- Discharge
- Condensate
This is insulation, not seawater — the loop is working exactly as it should.Marine air conditioning is water-cooled, and that is what makes it different from a house. The full cutaway is in the guide.
Sweating: Diagnostic Steps
Work down it rather than picking the step that sounds most likely. The order is chosen so that the cheap, common causes are eliminated before the expensive, rare ones — which is also the order that stops you paying somebody to clean a strainer.
Trace where the water is actually forming.
Sweating happens on cold surfaces in humid air. Follow the wet back to where it starts — usually where insulated duct meets uninsulated fitting.
Check the insulation is continuous at every joint and transition.
Duct itself is usually insulated. The collars, elbows and grille boots often are not, and that is where it drips.
Look at the grille bodies themselves.
A metal grille in a humid cabin sweats around its frame and drips down the joinery. Insulated or composite grilles fix it.
Consider whether the duct runs through an unconditioned space.
A cold duct through a hot, humid locker or bilge is the worst case. That run needs full insulation or a different route.
Raise the thermostat a couple of degrees and see whether it stops.
The colder the supply air, the more the ducts sweat. This confirms the diagnosis and sometimes solves it outright.
When to Stop and Call a Marine Technician
No licensed work. This is insulation and duct-work.
Handling refrigerant in Australia requires an ARCtick licence under the Ozone Protection and Synthetic Greenhouse Gas Management Regulations, and fixed 240 V work on a vessel is licensed under AS/NZS 3004.2. Both exist for good reasons. Everything above the line is yours; everything below it is not.
Possible Causes of Sweating
Uninsulated fittings, collars and boots
Wrap them properly. Fiddly, cheap, permanent.
Duct routed through an unconditioned humid space
Insulate the whole run, or re-route it.
Bare metal grilles
Replace with insulated or composite ones.
A supply temperature set very low
Raise the thermostat. Free.
Compressed or wet insulation
Replace. Wet insulation never recovers.
Ranked by how often each one is the answer on Australian boats, not by how often it appears in a service manual. The seawater side dominates this list because our water is warm, our marinas are full of growth, and the strainer is the part nobody opens.
Reporting This Marine Air Conditioning Fault
Tell us the boat, the unit if you know it, and what you found at each step above. We will tell you what we would check next, and whether it is worth a technician's call-out or a new machine. We do not charge for that and we will say if the honest answer is that the unit has had its life.