[Doug Musick]

I have an older style aqauhot with the copper coils circling the exterior of th boiler tank.  When I add antifreeze should it be engine antifreeze or boiler antifreeze?  The aqauhot manual says boiler, but several post here say engine.

Which leads to a second question.  Can anyone explain the engine preheat plumbing loop to me?  Is it tied into the engine cooling system etc.

[Trainer]

Doug. The older heaters with a row of fuses and 3 clear relays use engine antifreeze. The fresh water coil is wrapped outside of the coolant tank.

The engine coil provides heat to the coolant tank going down the road. Turning on engine preheat heats the engine antifreeze for 2 hours when starting in frigid conditions. Only turn on engine preheat for that reason.

The engine loop is engine antifreeze and is isolated from heater coolant.