CASE STUDY 1: 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 5.5ft3 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 inject- ing foam into the apertures where the smaller coolant lines went through. I installed the Danfoss BD50 compressor in an underseat 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