Marine Air Conditioning Not Cooling
The compressor is going, air is coming out of the grilles, and nothing gets cooler. In Australian boats this is a seawater problem far more often than a refrigerant problem — which is good news, because the seawater side is the half you are allowed to touch.
Not cooling: First Checks
Not urgent, but do not keep running it. Go and look over the side for the overboard discharge while the unit is running. There should be a steady trickle of warm water leaving the hull. No trickle, or a dribble, and you have found it.
Where this sits on the seawater loop
- Seacock
- Strainer
- Pump
- Condenser
- Evaporator
- Discharge
- Condensate
Marine air conditioning is water-cooled, and that is what makes it different from a house. The full cutaway is in the guide.
Not 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.
Check the overboard discharge with the unit running.
A steady warm trickle means the pump is moving water and the system can reject its heat. Nothing, or an intermittent spit, sends you to the next three steps and you can stop reading the rest.
Shut the seacock, open the strainer, and look at the basket.
A strainer packed with weed, shell grit or a plastic bag is the single most common cause of a warm cabin in this country. Clean it, close it up, open the seacock, and see whether the discharge returns.
Feel the seawater pump housing.
Hot, humming, and no flow usually means it is airlocked or the impeller has gone. Silent and cold means it is not being told to run — a control or relay fault, not a plumbing one.
Look at the return-air filter and the evaporator coil behind it.
A blocked filter starves the coil of air, so the air that does come out is cold but there is far too little of it. If the grille air is genuinely cold and the cabin still is not, the problem is airflow, not cooling.
Put your hand on the air leaving the grille.
Cold air plus a hot cabin is an airflow or sizing problem. Air that is barely cooler than the cabin, with good seawater flow and a clean coil, is the point at which this becomes a refrigerant question.
When to Stop and Call a Marine Technician
If the seawater flow is good, the strainer is clean and the coil is clean, and it still will not cool, the next step is measuring refrigerant pressures. That is licensed work under the ARCtick scheme and needs a technician.
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 Not cooling
A blocked seawater strainer
Clean the basket. Ten minutes, no cost, and worth doing monthly in summer regardless.
A failed or airlocked seawater pump
A new pump is a few hundred dollars and an owner-serviceable job on most boats.
A choked return-air filter
Wash it. Most are foam and go back in wet-clean.
A scaled condenser coil
Descaling is a workshop job or a circulated acid flush — see the maintenance pages.
Low refrigerant charge
A leak, and licensed work. If a unit is low it leaked, and topping it up without finding the leak is money thrown away.
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.