Sailorlou has a great diagram.
Don't mix the 2 hot waters. Use a heat exchanger
. Most engines either have anti-freeze, or water pump lubricant in the water. Some diesels require an anti-cavitation mixture that probably isn't too tasty. Or engines have some other heavy metal residue.
I have a large hydronic system that uses a diesel
fired boiler and baseboard type radiators. The boiler has a coil that hot water passes thru to heat the potable hot water in the water heater. With the tank above the boiler, no pump is required. I plumbed both main engines into the circulation system. Only one engine
at a time is used for heat for safety
reasons. Also, I have a pellet stove with a self built coil that also can heat the boiler.
The boiler system was installed when diesel
was about 20¢/gallon. At $4/gallon I added the engine plumbing
and pellet stove. None of the original piping was insulated so much of the heat went into the bilge
and unused spaces. Also, I am changing the baseboard sections with a marine
forced air heater in each cabin/compartment. I went from 2 zones to 5. The car type heaters heat a cold boat hours faster than the baseboards. It lets me direct air into cold spots, too.
The engine fresh water pump moves the water even when the system isn't calling for heat. The engines run at about 175º, so the boiler doesn't over heat. I have the oil
blower/igniter disconnected and use the boiler's controls to turn on the circulation pump, thermostats and valves.
The way my system works now, the thermostat sends a signal to the boiler. That causes the circulation pump to come on and appropriate valve to open. The individual heating units have a 12V heat sensor on the supply line that triggers the fan on when it gets to 135º and off when the temp goes below 90º. That way no cold air on startup.
I've been changing over my electric
system to run thru an inverter
all the time so when off shore power
I don't run a generator
unless using something drawing high amps. So the 120V and 24V parts
of the heating system run when only cruising on my mains. Main alternators keep the batteries up and when on shore power
passes the 240V thru and also charges the batteries.