Boat Air Conditioning: One Cabin Not Cooling
One box, two cabins, and one of them wins. This is the most predictable complaint in marine air conditioning and it is almost always a ducting problem that was baked in on the day of the install — which is why the sizing pages spend so much time on it.
Uneven cooling: First Checks
Not urgent, but it rarely improves by itself. Hold your hand at each grille. If the near cabin gets a good blast and the far one gets a whisper, it is airflow. If both feel similar and one cabin is still hot, it is heat load.
Where this sits on the seawater loop
- Seacock
- Strainer
- Pump
- Condenser
- Evaporator
- Discharge
- Condensate
This is ducting. The seawater side is feeding the unit fine; the air is not reaching the cabin.Marine air conditioning is water-cooled, and that is what makes it different from a house. The full cutaway is in the guide.
Uneven cooling: 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.
Measure or estimate the duct run to each grille.
Airflow falls off with length and with every bend. A run twice as long with three more bends will never match, and no amount of adjustment will make it.
Look for a crushed or kinked flexible duct.
Flexible duct routed behind joinery gets crushed by stowage, and a single pinch can halve the flow to one cabin. This is worth actually looking at.
Check whether there are balancing dampers.
A properly ducted two-zone job has them. Choking the near cabin slightly is how you feed the far one — counter-intuitive, and effective.
Compare the return-air path from each cabin.
Air can only go in if it can get out. A cabin with the door shut and no return grille or undercut will not cool no matter what you send it.
Consider the heat load of the hot cabin separately.
A forward cabin under an uninsulated foredeck in the sun is a different problem from a duct problem, and the fix is shade and insulation.
When to Stop and Call a Marine Technician
No licensed work here. This is ducting, balancing and sometimes a second unit — all mechanical.
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 Uneven cooling
A duct run too long for the unit
Balancing dampers first; a second unit or a split if that is not enough.
A crushed flexible duct
Find it and re-route. Free, and often the whole answer.
No return-air path from the closed cabin
Undercut the door or fit a return grille.
No dampers fitted at all
Retro-fit them. A small job with a large effect.
A genuinely higher heat load
Shade, hatch covers, deck insulation. See the is-it-worth-it page.
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.