Marine Air Conditioner Runs Constantly
The opposite of short cycling, and much less damaging — but it means the boat is losing heat as fast as the machine can remove it, and the running cost reflects that. Usually the boat is the problem, not the machine.
Never satisfies: First Checks
Not urgent, but expensive. Is the air leaving the grille genuinely cold — ten degrees or so below the cabin? If yes, the machine is working and the boat is losing it. If no, read the not-cooling page.
Where this sits on the seawater loop
- Seacock
- Strainer
- Pump
- Condenser
- Evaporator
- Discharge
- Condensate
Nothing on the loop is wrong. The boat is losing heat as fast as the machine removes it.Marine air conditioning is water-cooled, and that is what makes it different from a house. The full cutaway is in the guide.
Never satisfies: 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 thermostat setting.
Set to 16 °C on a 35 °C day, most units will run forever and never get there. This is not a fault; it is an instruction the machine cannot carry out.
Look for open hatches, ports and companionways.
Obvious, and constantly the answer. A boat with the companionway open is being asked to cool the marina.
Feel the deck above the hottest cabin at 3 pm.
An uninsulated deck in Australian sun is the biggest single heat path on most yachts, and shade over it beats any amount of extra compressor.
Check the hatches and windows for reflective covers.
Glass and acrylic are the second biggest path. Cheap covers, remarkably effective, and they let the unit finally catch up.
Compare the unit's capacity against the sizing figure for the boat.
If it is genuinely undersized for the climate, it will never satisfy on the worst days. That is a sizing conversation, not a repair.
When to Stop and Call a Marine Technician
Nothing here is licensed. If the air is not cold, that page is where the refrigerant question lives.
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 Never satisfies
Heat coming in as fast as it goes out
Shade, hatch covers, deck insulation. Cheaper than any hardware.
The thermostat set unrealistically low
22–24 °C. It cools no slower and stops eventually.
The boat not closed up
Free.
A genuinely undersized unit
See the sizing pages. A second unit is often cheaper than replacing the first.
Degraded performance from scale or a low charge
Service. If it used to satisfy and no longer does, this is where to look.
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.