Every summer we get the same call. The air conditioning worked well in June, and by mid-August one or more units trip in the afternoon, cabins won't cool, and the guests have noticed. The cause is rarely mysterious. It's usually the seawater, and the warmer the sea gets, the less margin the system has.
How a yacht's air conditioning gets rid of heat
Whatever type is fitted, a yacht's AC moves heat from inside the yacht into the sea.
In a self-contained (direct-expansion) unit, refrigerant picks up heat from the cabin air at the evaporator coil. The compressor pumps it to a seawater-cooled condenser, where the heat goes into the sea.
In a chilled-water system, a central chiller cools a loop of fresh water (often with glycol), which is pumped to fan coils in each cabin. The chiller's own refrigerant circuit then rejects that heat into seawater through its condenser.
Either way, the last step depends on seawater flowing through a heat exchanger, and on that seawater being cool enough to take the heat away.
Why August is harder
As the sea warms through the summer, the condenser has to work at higher pressure to get rid of the same amount of heat. Air temperatures, sun on the deck and guests opening doors are also at their highest. A system that had some margin in May has little left by August.
When the refrigerant pressure goes too high, a safety switch stops the compressor to protect it. That's the high-pressure trip that shows up as a fault on the panel, often in the hottest part of the afternoon.
Anything that reduces seawater flow or makes the condenser less efficient pushes the system over the edge:
- Blocked strainers. Weed, jellyfish, plastic and loose Posidonia leaves.
- Air-locked seawater pumps. Most AC seawater pumps are centrifugal and not self-priming. They lose prime after a haul-out, when heeled under sail, or if air gets drawn in. The pump runs but moves no water.
- Weak pumps. Worn impellers or failing motors.
- Scale and growth inside the condenser. It builds up gradually and cuts heat transfer. Condensers need descaling from time to time, which is a job for a technician with the right chemicals and procedure.
The quick checks
When a unit trips or blows warm air:
- Look at the overboard discharge for that unit or the chiller. You should see a steady flow of water. If it's weak or absent, it's a seawater problem.
- Clean the strainer. Close the seacock first, then open the strainer, clean it, refit the lid, reopen the seacock.
- Bleed the pump if your system has a bleed point, or follow the manufacturer's procedure for re-priming.
- Reset the unit once flow is back.
If seawater flow is good and the unit still won't cool, the cause is more likely the refrigerant charge, the compressor, or airflow at the evaporator: a clogged filter or a failing blower. That needs a technician.
Signs worth acting on early
- Ice on the evaporator coil usually means poor airflow or a low refrigerant charge.
- Water dripping from a unit or ceiling usually means a blocked condensate drain.
- A musty smell comes from a dirty coil and drip tray.
- One cabin warm on a chilled-water system suggests air in the loop, a stuck valve or a fan-coil fault.
- Chiller flow alarms point to the chilled-water pump, a strainer, an air lock or the flow switch.
Getting through the summer
A few habits make a real difference:
- Clean the strainers often in summer, not just when something trips.
- Close doors and hatches and use sun covers. Every bit of heat you keep out is heat the system doesn't have to move.
- Cool the yacht down before guests board, rather than expecting the system to pull a hot saloon down in an hour.
- On generator power, start units one at a time. Several compressors starting together put a big load on the generator.
- Don't run the set points very low. A huge difference between inside and outside temperature keeps compressors running flat out with no margin.
Most of all, get the seawater side serviced in spring: pumps, strainers, hoses and condensers. It's the cheapest way to avoid August call-outs.
We work on Dometic systems (including Cruisair, Marine Air and Condaria), Webasto, Heinen & Hopman, Frigomar and Technicold, from cabin units to full chilled-water installations. See our air conditioning service page, book online with photos, or call +34 971 251 374.