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How to Troubleshoot Your Boat's Air-Conditioning System

Be Cool

Don’t let the dog days of summer make you swelter.
Here’s how to troubleshoot that air-conditioning system.

It’s July, and a skipper’s thoughts turn to his air-conditioning system—especially if the darn thing stops working and his family moves into the Holiday Inn next door to the marina. (Nice vacation, huh?) But here’s the good news—if your system conks out, getting it going again will often be pretty simple. As with virtually every other maintenance project onboard, step one of just about any fix will entail finding and reading the owner’s manual: Everything you can do yourself is explained in there; for everything else, call a pro and join mom and the kids at the hotel pool. While you’re looking for the manual, by the way, here are a few things that may come in handy at some point in the near future.

Dometic split-system-type air conditioner
This Dometic split-system-type air conditioner separates air handler/evaporator (left) and condenser/compressor (right). It uses environmentally correct R410A gas.

Air-conditioning systems come in different configurations, but all share three major components: The evaporator, where boiling refrigerant absorbs heat from the surrounding air; the compressor, which pushes the refrigerant through the system; and the condenser, where the refrigerant is cooled and liquefied before being pumped back to the evaporator. Most air-conditioning systems are “direct expansion”: they pump refrigerant directly to air handlers in the boat’s living spaces. A chilled-water system, on the other hand, cools fresh water that, in turn, circulates through the air handlers. Maintaining a chilled-water system isn’t much different than a direct-expansion system, except that there’s more plumbing. In all cases, reading and studying the aforementioned manual before opening your toolbox or calling for help is very important. But, if there’s not enough time for that for some reason, just look at the controls—many modern systems include digital panels that display error codes.

The best way to understand how to care for whatever type of system you’ve got is to take a trip through the system, starting at the expansion valve, located just upstream of the evaporator. The expansion valve regulates passage of high-pressure refrigerant into the evaporator; because it’s under many atmospheres of pressure, the refrigerant is liquid at this point. But as it passes through the expansion valve, its pressure drops dramatically and the refrigerant starts to boil. When any liquid boils—scientists call this “phase conversion”—it absorbs heat. In this case, “boiling” doesn’t mean “hot”: The boiling point of a common refrigerant, R410A, is around minus 55 degrees F. At normal temperatures and pressures, it’s a gas.

Boiling Cold

Air conditioners don’t really produce cold, they remove heat. As the phase-converting refrigerant passes through the evaporator coils, it gets colder but more gaseous (because it’s boiling), and absorbs heat from the surrounding air. A fan blows warm air from the cabin across the evaporator and discharges it, now cooler, through ducting back into the cabin. The evaporator/fan/ducting assembly is called an “air handler.” The warm air that’s pulled in from the cabin is called “return” air, while the “supply” air is the cold air coming out of the air handler.

Efficient heat exchange at the evaporator demands a free-flowing supply of hot or warm air from the cabin. Behind the return-air grille, there’s a filter to screen out dust and other junk. If the filter gets dirty, the airflow is restricted and it’ll take longer for the cabin to cool off. The first thing you should do if it’s way too hot belowdecks is to clean the filter—vacuum it, wash it, do whatever the owner’s manual tells you. Even if your air-conditioning system is working great, clean the filter once a month. If you need to clean the filter more often, try cleaning the cabin.

An award-winning Dometic self-contained turbo unit.
An award-winning Dometic self-contained turbo unit.

Need a way to check whether your air-conditioning system is working up to spec? Julio Ramirez, a technical advisor with air-conditioning maven Dometic (, recommends taking the temperatures of the supply and return air with a digital thermometer—it’ll set you back about 20 bucks if you don’t already have one. Put the system’s fan or blower on high speed before testing, Ramirez says. “There should be anywhere from 12 to 20 degrees difference between them,” he adds. If the difference is less, the problem could be a loss of refrigerant charge; although the system is supposed to be a closed loop, sometimes refrigerant leaks out. Checking and recharging the system is a job for a licensed air-conditioning technician.

While you’re cleaning that filter, incidentally, check the condensate pan under the evaporator for water. Air conditioners remove humidity along with heat, and the water has to go somewhere. In most installations the pan drains into the bilge, but if the drain gets clogged and the pan overflows the water will go somewhere you don’t want it to. Check the drain by blocking it temporarily and filling the pan with fresh water—it should be gone in 30 seconds or less when you unblock the drain.

What’s The Deal With Reverse Cycle?

Most marine air conditioners have a reverse-cycle mode, which changes the flow of refrigerant and turns the system into a heat pump. Many skippers never switch to reverse cycle, but they should: There’s a reversing valve in the system that can stick if it’s not activated regularly. If it does stick, when you want heat, you’ll be left out in the cold. 

Part of your scheduled air-conditioner maintenance, along with cleaning the air filters and raw-water strainer, should be to switch to reverse cycle for a couple of minutes periodically, even if it’s already hot as Hades in the cabin. The point? Eventually the autumn chill will send you reaching for the heat switch, and when you hit it, you want warm air to flow. (If your reverse-cycle valve sticks, by the way, tapping it lightly may free it; listen for a hiss as it activates. Don’t whale on it too hard, though.)

Reverse cycle uses the same principle as air conditioning, except the refrigerant flows in the opposite direction. The compressor pumps hot, pressurized refrigerant into the air handler where it transfers its heat to the cabin via the flow of cool cabin air over the coils. The refrigerant then flows through the expansion valve to the condenser, where the now very cold refrigerant picks up heat from the seawater and carries it through the compressor to the cabin.

Reverse-cycle units can produce heat down to seawater temperatures of 40 degrees F or so, but they work better if the water’s warmer. So, although the technology is fine for autumn boating, if you plan on an Arctic exploration, you might want to install a dedicated heating system.

Pump It Up, Cool It Down

After leaving the evaporator, the refrigerant, now a low-pressure gas, travels to a compressor which once again pumps it up to many times normal atmospheric pressure. If the system is working properly, you should see condensation on the piping between the air handler and compressor because of the cold refrigerant; dry piping here could mean the system needs recharging. 

Pressure equals heat, so to cool the gas on the downstream side of the compressor it’s run through a raw-water-cooled condenser, which returns the refrigerant to a liquid state. The colder the water, the more efficient the cooling, but most air conditioners will work fine to water temps into the 80-degree range, sometimes warmer. Then the refrigerant returns to the expansion valve and the cycle begins again.

Efficient heat exchange at the condenser demands a free flow of raw water, of course, which means keeping the supply side clear. Clean the relevant sea strainer once a month (or more often in tropical waters), and whenever your air-conditioning system is functioning, keep an eye on its raw-water discharge to ensure the flow is unrestricted. Most systems have a safety switch that shuts off the compressor if the water flow is insufficient, which causes system pressure to increase beyond a preset limit. If you fire up your system and it runs for a little while and then shuts off, it could be that the safety switch has tripped and you need to check the water supply.

If the strainer is clear but an insufficient amount of water seems to be running through the system anyway, the cause could be a bad water-pump impeller, an easy fix for most skippers. Or it could be the condenser coil. “Marine growth and scale builds up and clogs the coil,” says Wayne Wright (, a Tampa Bay marine air-conditioning specialist. The fix is to flush out the cooling system with a weak acid solution, a job for a technician in most cases. “It should be done every two to five years, depending on where the boat is located,” advises Wright.

If you take good care of your air-conditioning system, it’ll take good care of you, maybe for decades, says Wright. “A self-contained marine air conditioner,” he adds, “has a life expectancy of about 15-20 years, depending on how much it is used, with many older units still cooling effectively since the 1980s.”
