I have worked on a few of these systems over the years. You have several options if you
want to use propane. You can install a propane gas water heater and circulating pump and
use a thermostat and contactor to cycle the circulation pump. Your other option would be a propane fired boiler system with a circulating pump and similar control scheme for the pump. You also need to have an expansion tank installed on the system to account for expansion and contraction of the volume of the fluid as it expands when heated and contracts in volume while cooling.