The old evaporator
When you buy a 20 year boat you know you will eventually replace everything. So the time has come to replace our refrigeration system. After searching the Internet, talking to other sailors and visiting the boat shows I decided on Isotherm's unit. This system has an intelligent controller that reduces power when the ships voltage is low. I don't really know if that's important, but it sounds good. It seems to work on the theory that sooner or later you will charge the batteries and when the voltage rises the refrigeration system will kick into high gear. There is also a manual setting that turned out to be really handy.
Replacing a refrigeration system isn't very hard. Ours came in three parts, the compressor unit, the holding plate and a control unit. You simply tear out the old unit and bolt in the new one. The connections between the compressor and holding plate keep the refrigeration fluid from leaking when you screw them together. You don't need any special tools and you don't have to mess with charging the system.
Our "Cold Machine" was 20 years old and had worked great up to the last season. We could have possibly had it serviced. It probably just needed a charge, but it was past the end of it's advertised life so we decided on a new one. Friends recommended holding plate technology over the evaporator type. Now that ours is installed I have to agree, holding plates rock!
The new compressor is much smaller
There are some things that the experts don't tell you and they can make the difference in performance. First, the manufacturer recommends putting in a divider to separate the freezer section and the refrigeration section. Our ice box is not that well insulated so we also made a soft cover for the freezer. This worked better than expected. We can open the refrigeration side and get a beer without disturbing the freezer, so the compressor has less work to do.
Getting things to actually freeze was a bit complicated. It turns out that the temperature sensor was attached to the back side of the holding plate.
That part got cold quickly and the unit would shut off before anything actually froze. To get it to work correctly we lined the freezer section in construction aluminum. You can pick it up at any hardware store. It helps conduct the heat away from the food and back to the holding plate. Then we moved the temperature sensor to about an inch in front of the holding plate and attached it the the aluminum liner. Perfect!
After a trip to the supermarket we pack the freezer solid with things that must be frozen. We then put the control unit in manual and turn it up to maximum cold. In less than 24 hours the food is frozen through and we can put the control back in the normal "power saver" mode. Once everything is frozen the temperature in the freezer section will slowly drop to about 15 degrees. This acts like a super holding plate and keeps the whole refrigerator cold while only running a couple of hours a day.
New holding plate
In conclusion we recommend the Isoterm refrigeration unit. It performs well and the manufacturer's representative was most helpful. You should expect to experiment a bit with the installation. We had to move the temperature sensor a couple of times before we got it right. The aluminum liner is very important. It evens out the temperature across the freezer section. Without it you should expect the food furthest from the holding plate to have a hard time freezing.
One last note. The holding plate will try to get to about 9 degrees farenheit no matter what. If your ice box is super insulated it will take the whole box to that temperature. On the other hand if your ice box isn't insulated it won't be able to keep the food cold and the compressor will run continuously. So you have to balance your use, the size of your box and the compressor/holding plate against the water and air temperature in your area. And you may have to make adjustments with changes in season and use.
The freezer cover