How to Maintain Your Boat's Air Conditioning Seawater System

With a flashlight, trace the A/C seawater circuit: the intake seacock, the strainer, the pump, the hoses to each A/C unit and the overboard discharge fitting on the hull. Photograph each part and note it in your BoatMend boat profile.

An intake valve with its hose. The handle here is turned across the hose, which means closed; in line with the hose means open.

With the A/C running, look at the discharge on the outside of the hull. You should see a steady stream. Learn what normal looks like when everything is clean so you notice when it gets weak. A trickle or no water means a flow problem: go to the strainer first.

Turn the A/C off. Close the intake seacock and open it again. Dometic's maintenance advice includes checking that the handle actually shuts off the water. A seacock that won't move or won't close needs attention at the next haul-out.

With the A/C off and the seacock closed, open the strainer lid, lift out the basket and clean out weeds, shells and growth. Rinse it, check the lid gasket for cracks or flattening, and reassemble firmly. Open the seacock again.

Many A/C seawater pumps are centrifugal and not self-priming: if air gets in when you open the strainer, the pump spins without moving water. After reassembling, Dometic says to prime the system to avoid air locks. Usually that means letting water fill the strainer and pump before starting (some installations have a vent or need the strainer lid loosened briefly); check your manual. Restart and confirm good flow overboard.

With the system running, look for leaks at the pump, fittings, hoses and the pump's shaft seal. The pump should run smoothly without getting very hot. Dometic's checklist also includes examining the pump's impeller; on pumps where it can be reached, follow your pump manual to inspect it with the A/C off and the seacock closed, and replace it if vanes are cracked or missing. Hoses below the waterline should have stainless clamps and no cracks or soft spots. If several A/C units share one pump, turn each unit on alone and confirm each can start the pump by itself.

Look at the unit in its cabinet. Dometic suggests checking for oil spots (a sign of refrigerant oil leaking), seawater corrosion, standing water in the condensate drain pan, damaged copper tubing or insulation, and loose wiring or bonding connections. Pour a little water into the drain pan to confirm it drains away; a blocked condensate drain makes mold and water damage below.

Remove the filter behind the return-air grille. Vacuum it, or rinse it in the direction opposite to the airflow, then let it dry completely before refitting. Vacuum dust from the evaporator fins gently with a soft brush; don't bend them. While you're there, look at the ductwork for crushed or tightly kinked sections, and check that the insulation around the ducts is intact; bare duct "sweats" and drips condensation into lockers and headliners.

Measure the air temperature going into the return grille and coming out of a supply vent in cooling mode. A Soundings contributor says a difference of roughly 15 to 20°F is normal. Much less suggests low refrigerant or a flow problem; much more can mean a clogged evaporator. If you have reverse-cycle heat, switch to heat for a minute to exercise the reversing valve.

Check the controller too, as Dometic suggests: the room temperature it displays should be close to your thermometer's reading, and the unit should stop cooling (or heating) once the cabin reaches the set point. A unit that never shuts off, or short-cycles on and off, may have a sensor or control problem for a technician.

If the strainer is clean and primed but overboard flow is still weak, scale and growth are probably restricting the condenser coil. Owners commonly use a marine A/C descaler pumped in a loop from a bucket through the seawater circuit with a small pump kit:

• With the seacock closed, connect the kit at the point your A/C maker specifies (often at the strainer or pump inlet), and route the discharge back to the bucket, not overboard.
• Circulate the descaler for the time on its label, watching for debris coming back into the bucket.
• Flush with fresh water, reconnect all hoses with good clamps, open the seacock, and check flow. Use only a product made for your condenser's material (copper-nickel or titanium), and follow both the descaler label and your A/C manual. Some pros use acid solutions; BoatTEST recommends leaving those to a technician because of the handling and legal disposal.

Water trapped in the condenser, pump or low spots can freeze and burst them. West Marine explains the common method, and says to follow your A/C maker's procedure first:

• With the seacock closed, introduce a non-toxic propylene glycol antifreeze solution at the point your manual specifies. Dometic's procedure for its seawater pumps disconnects the hose at the strainer outlet and keeps it raised so the pump stays primed.
• Run the pump until undiluted antifreeze comes out (Dometic's endpoint for some systems), and collect what comes out instead of letting it go overboard or onto the ground.
• Drain the strainer and the hose between strainer and seacock if your manual says to. Some pumps (for example certain Dometic magnetic-drive pumps) have an impeller that should be removed and stored for winter.
• Never use automotive ethylene glycol antifreeze. Some makers also allow blowing the circuit out with low-pressure air; use only the pressure your manual allows, since too much can damage parts and air can leave water in low spots.

Reinstall anything you removed, reconnect hoses, open the seacock, check for leaks and confirm steady overboard flow before running the A/C normally. Flush out leftover antifreeze into a container and dispose of it properly.