Marine Air Conditioning: No Seawater Discharge
The one check this site tells everybody to do first, and this is the page for when the answer is bad. No discharge means the unit cannot reject its heat, and it will shortly shut itself down on high pressure — which is the system protecting itself, not breaking.
No flow: First Checks
Stop the unit. Running it dry will cost you the pump.. Is the seacock open? It is the first thing to check because it is the most common answer, and because nobody wants to admit 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.
No flow: 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 seacock is fully open.
Half-open is worse than shut, because it flows enough to prime and not enough to cool. If somebody has been in the bilge recently, check this twice.
Shut the seacock and open the strainer.
The basket is the usual culprit. Weed, a plastic bag over the intake, barnacles growing in the skin fitting after a long lay-up.
With the strainer open and the seacock cracked, confirm water floods in.
If it does not, the blockage is outside the boat — the skin fitting or the scoop. That is a dive or a haul-out, not a plumbing job.
Listen to the pump.
Running but no flow is an airlock or a dead impeller. Not running at all is electrical — check its breaker before you condemn it.
Crack the highest fitting on the seawater line to bleed it.
Centrifugal seawater pumps do not self-prime. After any work that lets air in, the pump will spin happily and move nothing until the air is out.
When to Stop and Call a Marine Technician
Nothing on this page needs a licensed technician. If you get flow back and the unit still will not cool, move to the not-cooling page — that is where the refrigerant question starts.
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 No flow
A blocked strainer
Clean the basket.
A closed or partly closed seacock
Open it. Then label it.
An airlocked pump after service
Bleed the high point of the line.
Growth in the intake or skin fitting
Clear from outside — a dive or the next slip.
A failed pump
Replace. Keep the old one as a spare if the impeller is the only fault.
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.