Troubleshooting Your Boat’s Air-Conditioning System

Cool and in Control

Learn what to do when the air conditioner stops working.

Ah, paradise. You have traveled to the warmer climates for the adventure of a lifetime, a cruise aboard a bareboat. After months of planning this charter, YOU are the captain. 

Relaxing on the bow, you are first to arrive in this gorgeous anchorage. Remember, sometimes you watch the show and sometimes you are the show, so be kind as you observe varied levels of talent and maritime skill come in throughout the evening. 

It’s dinner time in paradise. The winds have settled, and the balmy weather has turned a bit muggy. No worries, mon. Start the generator to provide 110-volt power to the boat and turn on those lovely air conditioners. Life is good.

Uh-oh. The air-conditioner fan is blowing warm air. Strange that it worked last night at the dock on shore power. What to do? This is hardly an emergency and the charter company can not be reached for advice. So swap your captain’s hat for a detective’s badge and let’s go to work.

Welcome Aboard Checklist

Upon your arrival, the charter company usually does a quick onboard review, including belowdeck items in the engine room. Since this may be your first true bareboat without a captain, the charter company wants to make sure you know the basics: how to check fluids, locate the bilge pumps, operate the VHF radio, use safety equipment, and so on. There is often a test drive to show you the ropes. 

Your checklist is complete, and the boat is yours for the duration of the charter. You are in command. And yes, the air conditioner was on its best behavior on day one. Hmmm.

Time to Cast Off

Enjoying your first night relaxing in the balmy climate, listening to some Caribbean music, and watching the sunset, you are so ready for this big adventure. Your first sunrise in paradise means it is up and at ’em early. You remember to check fluids before starting the engines: main engine oil, transmission fluid, generator oil, and, of course, antifreeze in all engines. Fuel is full, waste tank has only one night’s use, domestic water tank is topped to the brim. Sea cocks are open, blowers are operating, and you start the engines. With the comforting purr of the mains, you shut down all other unnecessary systems and unplug shore power. Yup, one of the last things you did was turn off the wonderfully cool air conditioner.  

Your crew prepares to cast off and away you go. Cruising around the sunny, clear waters all day is amazing with a breeze to keep you comfortable. At the day’s end, after a successful mooring ball pick up, you shut down the main engines for your first night away from the dock.


The air conditioner worked perfectly while at the dock on shore power but is not cooling now on generator power. The generator is running and is providing power. You check other appliances, which are all working as expected. In fact, the air-conditioner fan is blowing, but cold air is not coming out. What is different?

Marine air conditioners are water-cooled and have a centrifugal water pump in the bilge that pulls raw water in from beneath the boat. Here is a great idea: Inspect the sea cock to confirm it is open. You may have bumped it closed while checking fluids in the engine spaces earlier. Okay, that’s open. Next, look into the strainer to see if it is fouled with any sea creatures or debris. That’s fine, too, and clean as a whistle. 

Finally, you check the water pump to see or feel if it is running. Ta-da! You found the culprit. The pump is NOT pumping water to cool the refrigerant compressor. 

Marine air conditioners need the constant flow of raw water to cool the hot refrigerant in the compressor unit. Without the cooling water flow, the refrigerant will heat up. When it overheats, you may see a High-Pressure Fault in the air-conditioning system. Some digital air-conditioner controllers show what looks like a HI P5 code that really means HI PS for High Pressure. Look for an overboard stream of air-conditioner water being pumped out after cooling the compressor. No water, no cooling and the air conditioner will fault in a High-Pressure mode.

Problem Solved

Now you discovered the problem—the water pump is not running. So how do you solve the issue? Go to the master circuit breaker panel. Look at the Alternating Current panel to see if the air conditioner water pump circuit breaker is in the ON position! Oops, it was still off. Flip it to the ON position, and the problem is solved! 

You are the hero. Take a bow. You and your crew are now enjoying a cool evening in air-conditioned comfort. Ah, paradise…. 

-by Capt. Chris Caldwell

Captains Chris & Alyse Caldwell are USCG 100-ton Masters and cruising coaches who offer personal boat training online or onboard your boat anywhere. The Caldwells help build your cruising confidence with hands-on training, with their training videos, through two-day seminars filled with tons of tips for the boater who loves learning, and now remote learning. If you have additional questions for Captains Chris or Alyse, please email them at

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