i'm curious what the purpose is of that add on valve in the picture . on most HW heaters one of the engine hoses feeds into a sealed stainless tube in a loop inside the HW H. the outgoing hose takes the hot antifreeze out from the stainless tube loop.. having that valve which looks like it's between the intake hose and the outgo hose doesn't make sense. since the sealed stainless tube forms a sealed a loop inside the hw h, the heat transferred to the loop's stainless, from carrying the hot engine antifreeze through it , transfers its heat to the x gallons of h 2 0 in the tank. i'd find it unlikely that there's an air lock in the continuous loop from engine hose in, through the stainless loop, to engine hose out. that antifreeze, under pressure from the engine's 'fresh water ' aka antifreeze pump, is constantly circulating, under pump pressure , through the engine's counterflow heat exchanger, then goes back into the main antifreeze tank. Dx: can you measure the temp of the external surface of the engine to HW input heater hose, compare that to the operating temp of your engine, and then to the temp of the HW output engine hose?
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