Boat Air Conditioner Leaking Water into the Bilge
A 16,000 BTU unit pulls litres of water a day out of humid air, and all of it has to go somewhere. If your bilge only gets wet when the air conditioning is on, this is condensate, and it means the drain was never finished properly.
Condensate: First Checks
Not an emergency, but it will make the boat smell and wear out the bilge pump. Taste it, or just think about it: condensate is fresh, seawater is not. Fresh water appearing only when the unit runs is condensate every time.
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.
Condensate: 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.
Find the drip tray under the air handler and its drain spigot.
Every unit has one. On a self-contained box it is under the coil; on a split it is under each air handler, which means several of them.
Check the drain hose actually falls all the way to its outlet.
Condensate drains by gravity. Any rise, sag or loop in the hose holds water, and a hose full of water backs the tray up until it overflows.
Blow the line through.
Slime grows in a warm, permanently damp hose. A blockage halfway along produces exactly this symptom and clears with a shop vacuum or a gentle blow.
Find out where the drain is supposed to go.
A sump or a shower drain with its own pump is right. Straight to the bilge is a shortcut somebody took, and it is why you are reading this page.
Watch the tray while the unit runs on a humid day.
If the tray overflows even with a clear drain, the tray is not level. Boats sit bow-down at rest more often than people expect.
When to Stop and Call a Marine Technician
None of this is licensed work. It is plumbing, and it is the item most often left off a cheap quote in the first place.
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 Condensate
The drain was plumbed to the bilge from new
Re-plumb to a sump or shower drain. A few hundred dollars and worth every cent.
A blocked or slimed drain hose
Flush it. Add it to the annual list.
A sag or a rise in the hose run
Re-route with a continuous fall. Cable-tie it so it stays that way.
The drip tray sitting out of level
Shim the unit, or move the drain to the low corner.
Uninsulated ducting sweating
Not the tray at all — see the sweating-ducts 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.