It's an air lock in the engine heater that I'm mostly concerned about preventing. The heater lines run from the Yanmar engine, down under floorboards, then up several inches to the heat exchanger/blower. I need to make sure there isn't air there. I'll look into the bleeder valve SG pointed out, or perhaps just fill each hose and the exchanger, plug the ends, then connect them quickly before any liquid escapes ala Todd.
Q
The Refleks and Dickinson diesel-burning heaters specify a fuel filter. Dickinson says the fuel must be filtered before the oil metering valve to prevent dirt and debris from plugging up the needle and seat in the valve. I have about $1.5 boat units into this project, spending another $70 for a filter per the instructions seemed to make sense