My boat launched late March in upstate New York
with the Hudson
River water at 40 degrees and the air 50s during the day and 30s at night. This is common for us so here is our experience with reverse cycle ac, heat pumps, with cold water and cold air temps.
The system doesn’t get much heat from water below 45. Below 40, expect the water in the system to freeze and become inoperable until you attack it with a hair dryer. The boat is in fresh water so icing is more of a problem than in salt water
. With the water at 50 or above, keeping the boat warm is less of a problem. When the water gets to 55-60, the system blows hot air rather than warm and keeping the boat warm isn’t an issue. Be aware that the system adds heat to the air as it passes through the core
and only adds a few degrees to it. So, if the air inside of the boat is 40, the first pass through might raise it to 45 or 50 but then it mixes with the cooler air and may only raise the inside temp a few degrees over the first hour or two. Now the air, which is up to 45 goes through again and the inside temp is up another few degrees. The point is that for the first few hours, you may need a portable electric heater
to get the temp up to where it can be maintained by the heat pump. After the inside of the boat is comfortable, the heat pump can maintain it with 45-50 degree water. If the water is below 45 and the air cold, you may have to add an electric heater to stay comfortable.