Engine heat when running, diesel heat when not

So I'm a bit of a newbie to heating in a boat. I am going through a repower to either a Merc 300 HP gas I/O or 350 HP I/O and in the process I discovered the engine can provide in cabin heat (Thank you Jesus). I was in the process of having my refit guy instal a diesel heater to my gas boat, with its own smaller tank. Is there any way I can use heat off the engine, then when at anchor I can use the diesel heater basically using some common units of the system, rather than two complete seperate systems.

Called Bus Heaters. Will work while your engine is running. Similar to your car.

https://shop.hamiltonmarine.com/departments/hydronic-heaters-|HT|BHR.html

rsn48 said:

Is there any way I can use heat off the engine, then when at anchor I can use the diesel heater basically using some common units of the system, rather than two complete seperate systems.

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Yes there is an easy way. My setup with a Hurricane II is identical to what you want to do. Put a heat exchanger into a valved coolant loop off the engine with the diesel heater loop on the other side of the heat exchanger. The valves would be open when you're cruising and off when you're at anchor or the dock.

A decent heater install guy should know all about this setup.

You can power a "bus heater" easily as well a power a hot water heater that has a coil inside .

The only cautions are to power both in parallel , from the engine , to the device , then back to the engine.

The caution is if in series , one after the other, many HW heaters have a thermostat that slows down the circ water , in the hope of preventing scalding .

It doesn't work but will stop much heat from coming out of the box heater if in series..

IF you travel with kids , or old folks an anti-scald valve in the Hot water circuit is needed, as eventually the water heater will get to the engine coolant temp usually 180f or so.

NOT a "tempering valve" . At local box stores.

sunchaser said:

Yes there is an easy way. My setup with a Hurricane II is identical to what you want to do. Put a heat exchanger into a valved coolant loop off the engine with the diesel heater loop on the other side of the heat exchanger. The valves would be open when you're cruising and off when you're at anchor or the dock.

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I did what sunchaser did above, but with Webasto. The only trick for webasto is that you need to cut the wires for the coolant pump and splice in an extra 12V switch. This allows you to run the coolant pump even when the diesel heater is off, so that the coolant can circulate through the boat. Passive circulation isn't enough. A diagram of our system is shown below. We used one of those stacked plate heat exchangers built for pool heating to pull heat from the engine like sunchaser is saying.

Make sure you get a fresh water cooled (aka closed cooling system) mercruiser as the direct cooled ones don't work well with bus heaters.

Westiculo said:

I did what sunchaser did above, but with Webasto. The only trick for webasto is that you need to cut the wires for the coolant pump and splice in an extra 12V switch. This allows you to run the coolant pump even when the diesel heater is off, so that the coolant can circulate through the boat. Passive circulation isn't enough. A diagram of our system is shown below. We used one of those stacked plate heat exchangers built for pool heating to pull heat from the engine like sunchaser is saying.

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My Hurricane is setup similarly to as you describe. The diesel heater pump (but not the burner) comes on when the aqua stat on the heat exchanger rises to around 160 F. No manual switches at all.

Our “bus heater” is small and noisy.
Underway we just run the Wabasto. Don’t usually realize it’s on until I step out on the afterdeck.

Boating buddy's Red Dot "bus heater" was very noisy. Heatercraft in our 26-footer (ours was called Accu-Heat) was super quiet, and worked very well. Never turned it up past the lowest of three settings.

We have another Heatercraft for when our Nordic Tug is underway, and an Espar forced-air diesel heater for when not underway. Great combination.

I have hydronic heating with a boiler or plumbed to my engines. I use the heaters shown thruout the boat instead of the original radiators. The advantage to air heat over radiators is much faster heating from a cold boat, about 4 times faster to the same temp, but uses more fuel. I use a water coil in a stove as auxiliary heat to the boiler. It also pays to insulate the lines and the boat in general.
Heaters have 3 position speed plus off, and the boat is divided into 5 zones.

Make sure you get a fresh water cooled (aka closed cooling system) mercruiser as the direct cooled ones don't work well with bus heaters.

Yes, I'm getting the fresh water system, my old Merc though low in hours (before I purchased it) suffered being an adapted automotive engine rather than a marine engine from the top down as the new Mercs are.