Bruce, the engine loop inside the water heater is usually copper with no check valves so it should not matter how it’s connected, it’s just a loop. You can tell which hose is the supply with the engine running, it’s going to be the warmest one. The little bit of air inside the loop will be removed by the engine in its normal cycle and coolant will be drawn back in from your coolant reservoir. The air inside the tank will come out the hot side of a faucet. I avoid bleeding air using the pressure temperature relief valve because they are notorious for dripping after you do, probably less of a risk at boat pressure compared to household pressure. If it does it’s usually made by Watts and they aren’t too expensive or hard to replace with a small pipe wrench. You haven’t said why you suspect air but if it’s been disconnected a while the engine inlet or outlet can become plugged and not circulate. Best way to tell is if you have warm water circulating through both hoses with the engine running. Len Mitchell Crazy Legs 1989 37+ Midland On.
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