Some years ago I installed a "hydronic" cabin heating
system consisting of a closed loop of fluid heated by an Espar diesel
heater (with an integral pump) and a couple of radiators in the cabin
. It works well.
A bit later, in order to recover engine heat for cabin heating
when under power, I installed a separate heat exchanger in the engine cooling
circuit that would heat the fluid in the hydronic circuit from the heat in the engine cooling
circuit. (I also installed a separate pump in the hydronic circuit for use when under power and when the Espar furnace is not operating.) This arrangement works well, too.
I use Zerex G-05 ethylene glycol antifreeze in the engine cooling circuit (as recommended in the Practical Sailor article referenced above). I use "non-toxic" Sta-Clean propylene glycol in the hydronic circuit (http://stacleanantifreeze.com/
)--in anticipation of installing a potable water heater in the hydronic circuit at some point in the future (when my desire for creature comfort overcomes my inherent laziness). Presumably, any leakage of a propylene glycol antifreeze into a potable water system would be less toxic than an ethylene glycol equivalent.
The two circuits are completely isolated and independent--mechanically connected only at the heat exchanger.
Sta-Clean is advertised as a heavy duty antifreeze suitable for use in diesel engines--but I have no way of evaluating its relative merits compared to Zerex G-05 and chose to stick with Zerex for engine cooling.