When I plumbed in my new Atlantic hot water heater and Heatercraft cabin heater, both being in the the engine coolant circuit, all the fittings were 7/8 "standard". Perhaps your heater is sized for metric hose, which might be coincidently about 1/2 inch American...
Other than the hassle of putting in a pair of reducers somewhere, I doubt that the restriction is enough to cause a problem, but I am no engineer.
Another thing to consider: some Universals were plumbed by the factory to put the hose to the hot water heater coil into the circuit with the engine block, and some put it in the circuit that goes to the heat exchanger (directing hot water in that direction after the engine warms up enough to open the thermostat. I know of an E-33 ('83) with the the later setup, and my '88 boat has the former setup. Howcome?? Idono!!
When I was doing my "all new coolant hose" project a couple years ago, the Galley Store guy at Doc Freemans told me that boatbuilders tend to do it either way with no clear pattern.
FWIW, we have just returned from two nights afloat, and last night got down to the low 50's, and we were sure glad to warm up the diesel this morning and have some cabin heat within about 15 minutes!
Loren in Portland, OR
1988 Olson 34 #8, Universal M25XP diesel
