Routine maintenance of a marine refrigeration system results in greater cooling power, maximum energy efficiency and longer-lasting food. Follow these steps to solve refrigeration problems before they occur.

Story by Jan Mundy

Most marine refrigeration systems are ignored until they fail. Provided foods remain cold and it's making ice, there's no great urgency for you to inspect the refrigeration system when there are so many other maintenance jobs calling your attention. Often systems are buried in a locker or under a berth - "out of sight, out of mind." The day will arrive, probably during a heat wave, when you open the icebox lid and are greeted with the nauseating smell of spoiled food. This might have been prevented had you added refrigeration to your maintenance log.

Two different refrigeration systems are installed in boats and which one you have onboard will determine the maintenance steps. Refrigeration components include: a compressor, controller, thermostat, grill or "radiator" if an air-cooled system, water pump and strainer for water-cooled units, filter, flat plate or bin evaporator or a cold (or holding) plate fitted inside the icebox. Holding plate systems also include a condensing unit.

Servicing Key Components

All evaporator systems use a compressor to cool the evaporator and all are the Danfoss brand. This is a completely sealed unit. Should it fail, replacement is your only option. Refrigeration systems contain a filter/dryer that dries and filters the air to keep moisture out of the system. Change this filter annually. Inspect brushes (some units are brushless) every 500 hours or so and replace brushes before they become worn. Check your owner's manual for the location of the brushes and the recommended frequency for service.

An electronic controller is found on many evaporator systems. It may fail but it is simple to replace. Pull off the wires, unplug the three or four-pin connector and install a new one (provided you have a spare). Older, pre-1994 controllers are no longer made and difficult to find. Spares are available from Frigoboat (www.frigoboat.com) and Adler Barbour, now called Coolmatic (www.waeco.com).

Check all wiring and terminals and service as needed. Be sure that all connections and mounting brackets are tight and there is no movement from any part. Manufacturers recommend replacing all hoses at least every 10 years and offer replacement hose kits. Again, check your owner's manual for specs. Some systems have a reset button. Check this switch if the compressor fails to start. Look for corrosion on the compressor housing. Saltwater can corrode aluminum casings, rot tubing and crack O-rings, creating a casing leak. Remove corrosion using Soft Scrub and a Scotch-Brite pad and paint any bare spots.

Frost that builds on the evaporator or the holding plate restricts airflow through the box. When ice gets 1/2" (12mm) thick, it's time to defrost. There are two ways to do this. Use an automotive windshield ice scraper or pour hot water on the surface to melt off the ice. Never use an ice pick, screwdriver or other sharp tool that can puncture the evaporator. After defrosting, squirt food-grade liquid grease (e.g., Pam) for easy ice removal next time.

Most systems include an analog or digital thermostat, two with separate refrigerator and freezer compartments, that mounts inside the box to control the temperature. Once set, the thermostat shouldn't need resetting. So, when the plastic turn knobs corrode and break off, either do without or purchase replacement knobs.

Holding plate systems have additional wear items. The belt adjustment on engine drive systems should be inspected routinely. Tension should be snug tight, just so it doesn't slip or squeal when the compressor starts up but not super tight like an alternator belt. After several years operating a refrigeration system in warm water, alkali scale builds up on the condenser. A compressor that runs longer than normal, uses more energy to exchange heat and creates excess heat are clues the condenser needs a thorough cleaning. On Glacier Bay units (www.glacierbay.com), the condenser is easily serviced. Remove end caps and then pass a wooden dowel through the tubes. Sea Frost (www.seafrost.com) recommends using this cleaning method for its condenser: remove the zinc, plug the bottom hole, remove the top hose and add 6oz (177ml) of a 5% to 7% muriatic acid solution. When foaming stops, reconnect the hose, open the thru-hull and start the engine. Run the engine for a few minutes to flush the system. Now, shutdown the engine, close the thru-hull and reinstall the zinc.

Leak Checks And Recharging

Connection leaks are the most common causes of failure. A gradual decrease in cooling performance, a system that cycles repeatedly or a build-up of frost over one area on the evaporator are all good indicators of a slow coolant leak.

All compressors produce vibration. Rubber dampening mounts help to isolate this vibration but often this isn't enough. Vibration can cause leaks at metal fittings where copper tubing connects to the compressor.

Leaks are easy to locate. If you suspect a leak, apply a soap-water solution and watch for bubbles forming on these connections. Use a flashlight and a small mirror (e.g., dental mirror) to examine the underside of connections. Also, wipe all connections with your finger. As oil is always circulating in a refrigeration system, where oil is present, there is a leak. Tighten the fitting just 1/16" (1.5mm) of a turn and then check for leaks. Don't over tighten. You want to tighten it just enough to stop a leak, not until it can't turn. Some older units have metal-to-metal fittings that tend to leak from contraction and expansion. These must be torqued as specified in your owner's manual. Other fittings have rubber O-rings that typically don't leak. Flare fittings also leak and should be finger tested frequently to check for oil residue.

Sea Frost and Technautics (www.technauticsinc.com) holding plate systems incorporate a sight glass for checking the charge level. When first starting the system, it's common to see bubbles or foam in the sight glass but they disappear after the condenser warms up. A sight glass that doesn't clear after running for 15 minutes indicates low refrigerant and a possible leak.

Older Glacier Bay systems have an elastomeric pad that cushions the motor and coupling to the compressor. Change this pad, which is located under the cover, every two to three years. Newer Glacier Bay units have vibration isolators to prevent vibration from being transmitted from the compressor to the connections. These isolators have corrugated bronze tubing and sometimes break. Carry spares onboard and if one breaks, unsolder the ends and install a new one. At this time, the coolant must be recharged.

All refrigeration systems must be recharged after fixing a leak. Older systems contain R12, which can only be evacuated by a certified refrigerant mechanic. Compressors now contain 134A, the same earth friendly refrigerant used in automotive air conditioners. You no longer need a mechanic to recharge a refrigeration system. Purchase a charging kit at Kmart, Wal-Mart or most auto parts stores. Locate the service ports on the compressor (they all have them). Attach the refrigerant charging hose to the port on the low-pressure side, open the refrigerant can and start the compressor. Always charge from the low-pressure side, never from the high-pressure side. If you're not sure which one to use, hire a technician to do this job. Put in 1/4 of the can and then check the charge. Refrigeration systems with sight glasses make charge checking easy. Add refrigerant in 1/4-can increments and run the compressor until bubbles not longer appear in the sight glass.

Charge Levels

A system operating at peak efficiency has an even layer of frost on the entire evaporator or holding plate. A partial frost line indicates an undercharge condition. If frost creeps down the copper lines, then the system is overcharged. When this occurs, lightly depress the valve in 5-second intervals to gradually release refrigerant. Continue doing this until the frost line is no longer visible on the copper lines. A higher charge results in higher operating temperatures. Moisture on the compressor housing or rust build-up on a metal base indicates one of two conditions: that it's been overcharged or undercharged for many years. Overcharging can cause compressor lines to sweat. A compressor that is warm to the touch may be the result of an undercharged system.

Cool Running

All refrigeration systems use either air or seawater to remove heat and cool the compressor.

Air-cooled units have a wire grill or a radiator-like unit mounted on the compressor. Dust, lint and pet hair can restrict airflow and cause the compressor to overheat. Keep the grill clean to obtain maximum cooling efficiency. At least once a year, clean with a brush, compressed air or a vacuum. Do this carefully to prevent damaging the grill or the delicate radiator wire. Fans fail, so carry a spare one, which is a standard 4" (10cm) computer box fan available at Radio Shack.

If your refrigeration system is water-cooled, you should pack a replacement pump. Routinely check seawater strainers and clean as needed. Check hoses and all connections for corrosion. Replace any suspect parts, especially on pumps mounted below the waterline.

How do you keep foods cold with a water-cooled refrigerator when you haul out? Here's a tip from Frigoboat. Mount a portable battery-operated fan so it blows air across the compressor. This should keep it humming for a few days until you relaunch. Caution: Never use a fan if the compressor is mounted in an area that requires ignition protected electrical devices (e.g., engine compartment). Few, if any, readily available household-type fans, will be provided with this critical safety protection.

Certain refrigeration systems utilize a keel cooler mounted below the waterline on the hull. Some units have a sacrificial zinc anode, which should be checked frequently, or at least when you inspect drive train anodes. A keel cooler without zincs usually has a bonding strap to connect to the boat's bonding system or the battery negative to prevent stray current corrosion. Check the bonding strap and make sure all connections are tight throughout.

All engine drive systems that are seawater cooled have a sacrificial zinc in the condenser. Remove and inspect the zinc monthly. Replace every six months or sooner if inspection reveals excessive wear.

Performance Enhancements

If your refrigerator box has a drain, plug it. The drain was put there to drain off melted ice, but your refrigeration system, like a household fridge, has minimal moisture build up. Since cold air sinks, the drain becomes an escape duct for cold air (bad) and a passage into the bilge for any moisture that condenses or contents' spills (smelly). Plug the drain opening with a rubber or soft wood plug or silicone sealant or cover it with duct tape.

Where tubing exits the box, make sure holes are well sealed. If the box is divided into refrigerator and freezer compartments, check that the divider is entirely sealed. When excessive ice forms on the evaporator or holding plate, or the compressor runs longer than usual, warm air may be gaining entry under the lid or through the door. Check the gasket for air leaks between the top of the box molding and countertop, or along the door. To do this on a front opening door, slide a bill or piece of paper under the door and observe if the gasket grips it all the way around. Another method is to shoot an infrared thermometer at the door or lid. You can also use this device for checking engine temperatures and air conditioning leaks. Replace gaskets as needed. Consider replacing a poorly insulated lid with a Ready-To-Mount hatch from Glacier Bay. Consisting of a vacuum insulated panel, hatches are available in three sizes for top or front-opening installation.

It's good practice to stir up the air inside the box to eliminate cold air settling at the bottom and possibly freezing delicate foods. A Fridge Mate battery-powered fan, available at marine stores, circulates air to give an even temperature and better box efficiency.

Poor box insulation is a common problem with refrigeration systems. Many boxes are inadequate, which greatly reduces cooling performance. Technautics and Glacier Bay offer thermal insulation panels that are easily retrofitted inside an existing box with sufficient space. Technautic's 1" (2.5cm) thick panels are equivalent to 4" (10cm) of closed-cell urethane foam insulation; Glacier Bay's Barrier Ultra-r vacuum insulation panel apparently delivers R-50 per inch (per 2.5cm) insulation value, the equivalent of 10" (25cm) of closed-cell urethane foam.

Take the time to add these service procedures to your maintenance log. While I cannot guarantee you'll always arrive at your destination with cold steaks, chilled beer and frozen ice cream, these preventative measures certainly increase the odds in your favor.

How many vented loops are there on your boat? Where are they located? When were they last serviced? If you've answered, "What's a vented loop?" or "I don't know," read on. Doing so could save you considerable heartache and big bucks.

This article appeared in DIY 2003-#4 issue, which is available as a back issue or on the annual CD-ROM.