The Webasto is turned on when it is 32deg or lower. It is running on low speed with the reverse cycle heat pump keeping things stable. Figure I'll kick the Webasto into high gear the for the next few nights as we fall into the teens and maybe single digits.
Cy, how are you still running your reverse cycle? Most of those things don't work efficiently if the water is below about 40 degrees F, and have a mfg shutoff at 37. (Think about it - if they remove about 5 degrees of heat from the exhaust water, and the water temp is 36, you're makin' ice cubes...)
The seller from our boat put it this way: "Which would you rather put wear-and-tear on, a $1000 CruisAir; or a $40 oil-filled electric heater from Home Depot?