My 3500 came with a 20 amp inline car style fuse in the power line. Webasto recommends attaching to a nonswitched power source as the heater runs the fan to cool down after stopping the fuel
. Electronic damage can occur if shutdown isn't followed.
Mine can only be switched off by the main battery
switch, so I'm living dangerously, I have to remember to turn the heater off, let it run til the fan turns off before turning off the main. Not an issue with the way I use my boat.
I don't understand why the insistence on outside air for combustion. The only thing I can think of is it lowers cabin
pressure drawing in outside air, which might contain exhaust
gases. I brought this topic up here before and no one had a good reason they thought of.
Most drip pot heaters draw combustion air from the cabin. Seems this is a good thing as you draw in some low humidity air from outside the cabin.
Don't follow internet
advice, follow manufacturers recommendations, (unless you know more than they do)
Edit: Found my original post with this question
Why combustion air from outside?