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How to: Upgrading Your Icebox

Whether you decide to build a refrigerator box from scratch or buy a ready-made unit, you won’t regret installing a DC refrigeration system; a front-opening system (far left) is another option

Whether you decide to build a refrigerator box from scratch or buy a ready-made unit, you won’t regret installing a DC refrigeration system; a front-opening system (far left) is another option

The time has come when the prospect of cold drinks and long-term food storage has you thinking about upgrading your icebox to DC-powered refrigeration. Duncan Kent has been there and done that, and has some advice

Fresh food must be kept at a refrigerated temperature of 40 degrees F to stay edible for a week, whereas a freezer needs to be able to retain a constant 0 degrees F. The successful operation of any refrigerator is almost entirely dependent on the efficiency of the insulation surrounding the icebox. Insulation that is too thin or of the wrong material can double its operating time and hence its power consumption.

If you are planning a new installation and calculating the physical dimensions of the box, you must allow for the thickness of insulation required to construct a thermally efficient unit. If you already have an unrefrigerated icebox that you plan to convert, check the thickness and type of insulation first, as it will almost always require upgrading. The minimum insulation thicknesses recommended for the Northern Hemisphere are: (fridge/freezer) 2in/3in (sides), 3in/4in (bottom) and 1in/2in (lid). For tropical climates, add another 1-2in all round.

Proprietary insulated boxes, as stocked by numerous specialist agents and chandlers, will often fit your available space with a few minor modifications. However, many do not meet the recommended insulation standards, as most are designed for use in trucks where power consumption is not an issue, so you’ll often need to fix additional insulation around the outside of the box.

You can also make your own liner by constructing a mold, laminating it in fiberglass and then coating the inside with gelcoat or paint. It’s not a difficult project and can be a lot of fun. The WEST System website is a great resource if you want to try this.

An especially easy route may be to use one of the refrigerator boxes made by companies like Isotherm, where the fridge box and compressor are combined in a single unit. They are available in a number of shapes and sizes, and are increasingly used by production boatbuilders.

There are plenty of options for DIY fridge installers: (top left) a 12-volt cooler like this one from Dometic is a good choice for smaller boats; Technautics systems are popular (top right); high-end controls come with this Dometic unit (bottom left); a Frigoboat keel-cooler system (bottom middle); a drop-in box/compressor combo from Isotherm (bottom right)

There are plenty of options for DIY fridge installers: (top left) a 12-volt cooler like this one from Dometic is a good choice for smaller boats; Technautics systems are popular (top right); high-end controls come with this Dometic unit (bottom left); a Frigoboat keel-cooler system (bottom middle); a drop-in box/compressor combo from Isotherm (bottom right)

Cooling units

Your choice of cooler will make the difference between a “vaguely cool” and “properly cold” fridge. The two main methods of cooling are thermo-electric (aka: Peltier-effect, as found in many budget portable coolers) and the more effective compressor-driven units.

Peltier-effect fridges work by fanning air over a solid-state thermo-couple. They are compact and inexpensive, but won’t cool food sufficiently in hot climates and can only be used with iceboxes up to 1.25ft³ capacity. They are fine for short periods if you load them with pre-chilled food and/or throw in a block of ice. However, they will consume a considerable amount of power if the ambient temperature is high and food at room-temperature is loaded into them, making them less than perfect in many situations.

A proper marine fridge requires a compressor-driven cooling unit, as with a domestic fridge, where an inert gas is cycled through an evaporator plate inside the fridge, collecting heat that is then released via a condenser outside the fridge. (Once the heat is dispersed, the coolant is pumped back to the evaporator, after which the process is repeated.) Condensers are commonly air-cooled, so in a closed environment such as a boat it is vital to ensure it is adequately ventilated or it can lose up to 50 percent of its efficiency.

Boats in southern climes or travelling to the tropics may also want to consider fitting a water-cooled fridge. These most commonly utilize a hull-mounted, sintered-bronze condensing heat exchanger (aka: keel plate), which can be at least 30 percent more efficient than air-cooling. Alternatively, the Isotherm SP system comes with a special galley sink drain skin fitting with an integral cooling matrix for the fridge.

These days the components of a compressor-driven cooling system can be bought in kit form. They come with pre-gassed pipes sporting sealed end-valves, allowing them to be used as soon as the pipes are connected to the various other bits.

When you’re planning your system it is also important to know that the greater the surface area of the evaporator plate, the more heat it can absorb and the quicker it will cool the box down, so chose the largest size that will fit the box. The plate can also be bent during manufacture, which might allow you to cover more than one side.


Saving energy

Fridge compressors driven from a 12-volv DC supply usually draw between 3.5 and 5.0 amps while operating (usually around 20 minutes every hour in moderate climates), so a 48W/12V fridge can consume around 32 amp hours (Ah) of battery capacity (or 40Ah including battery inefficiencies) over a 24-hour period. Most new models incorporate a low-battery cutout switch, which will disconnect the fridge should the battery voltage drop below 11.5 volts. Many can also switch automatically between 110V AC and 12/24V DC to help preserve your batteries when you’re hooked up to shore power.

Though these are not so common on sailing boats anymore, you can also install an engine-driven compressor, powered by a belt from the crankshaft, to bring the fridge temperature down rapidly. Although these ideally require the engine to be run for one or two hours a day, the addition of a holding (eutectic) plate (such as the CoolBlue system by Technautics), or a thermo-electric cooler will allow it to retain the cold for the long periods in between. This can be especially useful at night when a noisy compressor would otherwise keep you awake.

Holding plates work by using a fluid that freezes at a temperature above that of water. The coolant runs through a matrix within the plate, freezing the fluid and turning it into a “cold reservoir,” much like a regular freezer block.

The downside with engine-driven compressors is that they’re not as controllable as a purely electric fridge and the temperature in the box tends to vary from very cold to cool between engine boosts. However, most new electric fridge compressors, such as the Isotherm ASU and SX coolers, also incorporate a “Smart Energy” voltage sensor that automatically switches the compressor pump to full speed whenecer it detects the engine running or the alternator charging. When the engine is switched off it then drops back down to half speed, which in turns halves the power consumption. This system works exceptionally well when combined with a holding plate.

Of course, cold air sinks, so top-opening, rather than front-opening fridges don’t lose nearly as much cold air when opened—the exception being the very latest “drawer style” front-opening fridges (Dometic and Vitrifrigo for instance), where the food is stored in a sealed drawer, as in a domestic freezer, to stop the cold air spilling out every time the door is opened.

A fridge also needs a drain, but a simple open hole at the bottom will allow the cold air to quickly seep out. To prevent this drains should have a pump, tap, plug or “U” bend filled with water.


Like many “marine” items, DC fridges are expensive when compared to their domestic AC counterparts. In fact, some opt to install the latter on board using a DC-AC inverter, despite the inherent inefficiencies of such a method. Domestic fridges, however, are not designed to operate when heeled over and can run inefficiently or even fail completely if powered up when sailing. If you only daysail and switch your fridge on once you’re settled at anchor or in a marina, you might just get away with it, but you’d have to restrict the number of times you open it during the day—particularly if it is front-opening with no drawers.

Motorhome and RV fridges are cheaper and can often run on AC, DC or sometimes even propane. But once again, these don’t like working on an incline and using them on propane would require an external flue. Also, their insulation is usually inadequate.

Portable iceboxes, either thermo-electric or compressor-driven, are popular too and usually come with both 110V AC and 12V DC leads. Ideal for weekends or short trips, they can be filled with cold food from home, kept cool on the journey via your car’s 12V auxiliary power socket and then plugged into the boat’s AC/DC system when aboard. They do, however, consume a lot of power (5 to 8 amp[s) when on DC continuously, especially models without a thermostat. 

Top tips

• The fridge should not be installed near to the engine or stove.

• Installing it below the waterline will improve performance as the ambient temperature will be lower.

• All insulation should be closed-cell polyurethane foam and definitely not expanded polystyrene, which isn’t dense enough and absorbs moisture.

• Ideally, the lid should have a double seal, one top and one bottom.

• Never leave an open drain at the bottom as the cold air will seep out.

• If you install an air-cooled compressor in a locker it should have a free-flow of air, preferably fan-assisted.

• Compressors can draw up to 10 amps at startup and 4 to 5 amps continuously, so the correct size power cable is important. Too small and it will reduce the voltage reaching the compressor, noticeably reducing its efficiency.

• Keep the fridge full and open it as little as possible.

• Remove any cardboard packaging from the food as this will retain heat.



Upgrading to a 12V system

The engine-driven reefer in our Pearson 39-2 project boat was state-of-the-art in its day, but that day was long past. The boat had sat on the hard for a number of years before we bought it, and the individual components had corroded and/or seized up. Given the potential cost of repairs, it did not take long to decide to replace it with a 12V system (nor did it make sense to me to have to run the engine to keep the fridge cool) and only a little longer to decide to go with a holding plate system, preferably watercooled.

My starting point was the Pearson’s deep and narrow fridge, which I calculated at 5.5ft to arrive at the size of the evaporator or holding plate that would replace the existing plate. Looking through the array of tempting options from various manufacturers, I kept coming back to Isotherm’s ASU unit with the SP water cooling option. The ASU (Automatic Start Up) controller monitors battery voltage. When the state of charge is below 12.5 volts, the compressor runs at half power. When it senses more than 12.5 volts, whether from engine, solar or wind power, it runs at full power. It also shuts down the compressor if the state of charge drops below 11.5 volts. A small control panel gives you the option of automatic or manual temperature control.

Installation was easy; basically, I just swapped out the old holding plate with the new, slightly smaller 14in x 11in x 2.4in unit, filling in the old mounting holes with thickened epoxy and injecting foam into the apertures where the smaller coolant lines went through. I installed the Danfoss BD50 compressor in an under-seat locker adjacent to the galley; with no requirement for ventilation, I don’t need to worry about venting hot air.

Location of the compressor was limited to the 6ft reach of the coolant lines on the SP unit, which replaced the galley sink drain seacock and through-hull. The existing through-hull was too small, so I hammered a wooden bung into it to give the pilot drill something to bite into and cut it out with a holesaw sized for the SP fitting.

The most difficult part of the installation process was snaking the 12AWG power cable though the boat’s nether regions to reach the compressor. After that it was a simple matter of connecting the pre-charged refrigerant lines together and flicking on the breaker. The compressor hummed into life and almost immediately, frost began to form on the holding plate. Success!

A year later, I have only one regret. The fridge has performed pretty much as advertised, proving both efficient and frugal in operation; power draw is 5 amps at startup and once the box has cooled down it varies from 3 to 4 amps when the compressor is running, which is perhaps 10 to 20 minutes an hour on the hottest days, and so far a single 100 watt solar panel has easily kept up with its demands. The regret? No freezer; Isotherm doesn’t recommend this unit be used as such. When the time comes, I will add something like a small Engel. — Peter Nielsen

Starting from scratch: the old coolbox has been pulled out (left); Mocking up the new fridge box with the Sea Frost unit in place (right)

Starting from scratch: the old coolbox has been pulled out (left); Mocking up the new fridge box with the Sea Frost unit in place (right)

Case Study 2:

Building a unit from scratch

When we decided to refit our 38ft sailboat for full-time cruising, my wife, Tara, insisted that we add pressurized hot water, a cabin heater and a refrigeration/freezer system. The first two I felt comfortable installing, but I had no experience with marine refrigeration and had read enough articles on the subject to know that there is no single right way to go about installing such a system on a boat. Refrigeration is one of the top energy draws on a small boat, and I did not want to get this wrong.

The first thing we had to decide was whether to install a self-contained pre-made unit or build our own refrigerator/freezer box. After taking measurements where we wanted the system to go, we decided building our own box would make the most efficient use of the space. Once that decision was made, we now had to find the best 12-volt refrigeration components for our needs. Much online research ensued.

With insulation installed, it was time to make the final measurements for the refrigerant lines (left); The finished product has served well for several years (right)

With insulation installed, it was time to make the final measurements for the refrigerant lines (left); The finished product has served well for several years (right)

We soon zoomed in on the Sea Frost BD system. Owner Cleve Horton became an invaluable resource while we built the box, helping to determine the dimensions and amount of insulation to make the system as efficient as possible. We chose the BD system because it would be sufficient for our box size and can be air cooled. Water cooling is more efficient than air cooling, but if the box is well insulated and not too large, the difference in efficiency is not large and air cooling simplifies the system and lowers maintenance.

In addition to the basic BD system, we chose a freezer bin and an electronic thermostat with compressor speed control options. The freezer bin consists of two direct evaporator cold plates set on opposing sides of the bin, creating a small freezer. The rest of the box becomes the refrigerator as cold air spills over from the freezer bin. The electronic controller allows us to set different compressor speeds for when we are in different climates. It also allows us to monitor refrigerator temperatures as well as cold plate temperatures, which makes it easy to adjust plate temperature for a desired refrigerator temperature.

The hardest part of the job was building the box and then deciding where in the boat all the other components would go. This needed to be done so Cleve would know how long to make the coolant lines and wiring harnesses. We had enough room to build a 4.4ft top-loading box, which would allow us to use 6in of Dupont TUFF-r insulation around all four sides and bottom. We used 4in of the same material for the top and hatch doors. This would give us an “R” value of 39 on the sides and bottom and 26 on the top. The freezer bin set in the back of the box would be just under one cubic foot. This sounds small, but it has worked well for us living aboard full-time.

I used marine plywood to form the box and laid four layers of fiberglass cloth on the inside, wetted out with WEST System epoxy resin. As I did this, I also mixed a white dye with the epoxy so the inside of box would not need to be painted. It is critical to make the box waterproof to keep moisture from getting to the insulation and keep out any air circulating outside the box. Next step was building mockups of the freezer bin and compressor/condenser box. After I had the three major components of the system, the box and mockups, I cut the insulation and set everything in place, so I could make the required measurements for Cleve.

After receiving the mechanical components from Sea Frost, it was just a matter of putting all the parts together. The whole system came fully charged with refrigerant; the coolant lines between compressor and evaporator plates also remain sealed until the connectors are torqued together, so assembly was easy.

It took a few days of adjusting the thermostat to get the refrigerator compartment to our desired temperature, but once we got a feel for the controller and a baseline of cold-plate temperatures it became a simple procedure. We have found no need for a circulator fan inside the box. Power draw varies, but the maximum is approximately 40 Ah per day.

This refrigeration unit is simple and has run continually for almost three years without a hiccup. The only upkeep is defrosting the freezer based on the humidity and ice build-up, about once a month. But at the end of the day, we always have ice for our sundowners!

—Brian Flanagan






Sea Frost





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