As the temperature drops well below freezing, your reverse cycle heater will be forced to work even harder. This will keep you very comfortable while inside your yacht, but can you keep this up all winter long?
It depends on the water temperature where you are moored. The reverse cycle heater works by extracting heat from the water you are floating in, and by default, cooling the water that is pumped out the side of your boat. As the water temperature gets colder, the unit will have to work even harder to cool the water and extract all the heat available.
If the water is too cold, the unit can cause the water to freeze while inside the tubing. When the water freezes, a few things happen: The water stops flowing through the tubing; and the water in the tubing will expand.
When the water stops flowing, there is no more new water flushing through the unit and it will not be able to continue to produce heat. You will notice this when cooler air starts to flow out of the air ducts and the boat will begin to cool down.
When the water freezes, it expands. The problem is most reverse cycle units have metal tubing that does not take kindly to any expansion. Freezing water will usually crack the tubing and wreck the entire unit!
As you can see, running your reverse cycle into the winter months may sound like a good plan for heating your yacht until you factor in the limitations of the reverse cycle unit.
A good friend of mine who installs and services marine refrigeration and air conditioners gave me some insight on the matter. He recommends shutting them down when the water temperature drops below 40F to play it safe, and an absolute minimum operating water temperature of 38F.
Excellent, now you know that when the water temperature gets below 40F, it's time to shut it down, and if you feel lucky, you can wait until the water temperature gets down to 38F before shutting it down. But how can you tell how cold the water is?
The simplest method is to pull up a bucket of water and stick a thermometer in it. This will tell you what the water temperature is right by your yacht without much involved. If you do not want to play around with buckets of water on frigid days, the other method is to look up the buoy data in your area.
If you are in the Baltimore, MD area this winter, you can find the water temperature by clicking on the link below
All other buoys operated by NOAA can be found here, and you can search the list to find the weather station nearest your yacht.
Keeping an eye on the water temperatures can give you plenty of warning when "shut down time" comes. The good thing about water temperatures is they don't change too quickly. If the temperature was acceptable this morning, it will be about the same this afternoon. Things that can make the water temperature fluctuate quickly are rain and snow. If you have a rain or snow storm, the water temperature where you float will change as the new water and its temperature flush out the old standing water where you are.
Shutting down your reverse cycle heater at the appropriate time this winter will keep you warm and the machinery in your boat operational for as long as possible.