[Richard]

Do what John said.

The only difference is that with your era AH, the engine coolant IS the coolant inside the AH. That era AH has the potable hot water loop wrapped on the outside of the AH boiler.

AH changed the design in the early 2000s to put the potable hot water loop inside the boiler which necessitated a change to propylene glycol as the AH fluid. The internal loop could crack and contaminate the hot water. If ethylene glycol (engine coolant) were used in the latter unit, it could poison the potable hot water. Propylene glycol is not toxic to humans.