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Old 22-07-2021, 11:19   #1
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Using hot water heater as an engine warmer?

Curious if anyone is using their hot water tank water to keep their engine warmed?

Everyone talks about how they use the engine to heat water, but not the other way around?

Winter time in the PNW it'd be kind of nice to be able to circulate the engine coolant through the hot water heater as my diesel heater heats the hot water and keep my engine warmed(to remove condensation from it and stop it from being a complete heat sink(sucking the heat out of the air).
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Old 22-07-2021, 11:51   #2
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Re: Using hot water heater as an engine warmer?

We use our diesel fired hydronic system to heat the boat, water, and, if we wish, the engine; we can also use engine waste heat to warm the boat. I suspect it would work with electric heat but at what cost per kw/hr? This diagram may help: https://www.suremarineservice.com/si...ts/Olympia.pdf
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Old 22-07-2021, 12:37   #3
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Re: Using hot water heater as an engine warmer?

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Originally Posted by Scott Berg View Post
We use our diesel fired hydronic system to heat the boat, water, and, if we wish, the engine; we can also use engine waste heat to warm the boat. I suspect it would work with electric heat but at what cost per kw/hr? This diagram may help: https://www.suremarineservice.com/si...ts/Olympia.pdf
Well electricity is generally cheaper to heat water than is diesel, atleast where I live. That said, I dont have a hydronic heater, but I do have a dickinson bulkhead heater that I plan on integrating into my hot water system and will likely install a hydronic system once I get around to finishing all the other projects.

The main reason i was thinking about heating the engine was that once the engine was "warm", running a circ pump to keep at it a stable temp wouldn't cost tooo much energy(atleast i wouldn't think so), especially since my diesel heater runs 24/7 when i'm aboard. The other reason is that I'm fairly convinced that due to the engine being in the aft cabin, the aft cabin always struggles to stay warm(even when we had our forced air system running).

On a different note, How do you like your hydronic heater? One of the reasons i've been holding out on installing one is due to the fact that the forced air system I had was wonderful at heating but completely terrible on fuel economy and power economy. When "running" it burned roughly 6amps/hr. It did run quite a bit though(often in "low" to maintain the boats temp), but about 4-6 hours per 24hrs it'd be running in medium. What all this means is that the bastard burned around 4 gallons per 24/hours. My dickinson heater is only burning around 1 gallon a day....
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Old 22-07-2021, 19:06   #4
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Re: Using hot water heater as an engine warmer?

Well, you would need an electric pump to circulate the heated "cooling" water through the engine. Just thinking out loud, how would your closed thermostat ever open? You would need to get a lot of heat to it, and I doubt you could this way.

If you are willing to use electricity, maybe you can find an electric block heater?
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Old 22-07-2021, 19:50   #5
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Re: Using hot water heater as an engine warmer?

It’s do able but you would need to remove or bypass the thermostat ,a lot of big engines are on warmthrough the whole time that they are stationary , I e no cold starts ⛵️⚓️
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Old 22-07-2021, 20:34   #6
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Re: Using hot water heater as an engine warmer?

If it's just to keep condensation from forming you could just use a small heater like a goldn rod or something similar to warm the engine compartment. I know plenty of folks in Alaska who'll just keep a light bulb going in winter to ward off condensation.

If you want your engine to be warm you could replace one of the freeze plugs with a block heater.

Using a coil in the stove will require a circ pump to circulate the warm water and you'll have to bypass the thermostat as noted above.

Also, it will require a fair amount of time to heat up all that water in the engine, lines and . This doesn't matter if If you're living aboard running the stove 24/7 but if you're thinking that you could just fire up the stove and have a warm engine in an in an hour or so I think you'd be disappointed.

I have a Red Dot hydronic heater in my after cabin which is separate from the main saloon where my diesel stove keeps things nice and comfy even if it's 10°F outside. The heater works well enough to heat up the after cabin but it is awful noisy so I don't use it too much. I think the circ pump (Johnson magnetic type) and the hydronic heater's fan use about 4.5 amps together. Other hydronic heaters might be less noisy than the Red Dot, which does seem a bit "industrial."
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Old 23-07-2021, 00:56   #7
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Re: Using hot water heater as an engine warmer?

Ive done it for years, it works just fine. You need a circulation pump and bypass the thermostat as other says.
I have a rather massive engine (Ford 2725) and it seldoms reach above 40C with heat alone only from the 1Kw hotwater tank. In the circuit there is also a Dickinson Adriatic, Hydronic 5Kw + radiators. (Im in Norway, using my boat year round)

I have keelcooling and that made possible a bypass with motorized valve between the engine and heating circuit. A separate circulation pump in the bypass lets me circulate cooling water through engine and keelcooling after engine runs in the summertime. Drops the engine temp ~30-40C in 30mins.
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Old 23-07-2021, 09:01   #8
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Re: Using hot water heater as an engine warmer?

I do this every winter. My water heater is so close to the engine that no circulation pump is necessary. The hoses to the engine are less than 18 inches and convection keeps the engine block warm. BTW here in the PNW the water is well above freezing year round so the water keeps the boat above freezing.
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Old 27-07-2021, 00:51   #9
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Re: Using hot water heater as an engine warmer?

I have a hydronic system with a boiler that is tied into my main engines. When the boiler is hot, I can circulate the coolant thru the engines before starting. When cruising in cold weather, I use the engine heat to heat the boiler and the cabin heaters.
You don't want hot water that's been thru your engine to use for showers or coffee.
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