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Old 03-13-2019, 01:14 AM   #1
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Onan MDKD 8kW generator

While rebuilding my Oberdorfer N202M-11 pump, I was considering just how much more I run the generator than my main engines. So, the question is does anyone else use their generator coolant to heat domestic water rather than one of the main engines? This Kubota D950 engine does have pipe plugs in both the cylinder head and water outlet housing. These are likely intended for cab heat on whatever tractor/equipment these engines were built for? Not worried about under loading the generator as I have electric stove and use the AC a lot.


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Old 03-13-2019, 06:38 AM   #2
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I looked at a boat that was plumbed that way. It actually had valving to allow you to either run the water heater loop off the main engine or the generator. It would seem to me that you could throttle the water heater loop so that it doesn't cool the generator engine during low load periods.


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Old 03-13-2019, 07:43 AM   #3
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You can do it, but it's not the greatest idea in my mind. That engine under light load makes very little heat, so it would have to run quite a while to heat a tank. Also a good chance of undercooling the engine unless you throttle the flow, again due to it making little heat.

And since it is running, just let the electric element do the heating. The incremental increase in fuel burn is very small.

It's a pretty big advantage to have one tied to a main engine as it removes the need to run the gennie at all.

Another is that an air pocket in the heater loop creates a real risk of air loading the circ pump and creating an overheat. Personally been involved with two ruined Yanmar prop engines that cooked and root cause was air in heater loop introduced when coolant was changed.

On my personal boat I tied neither gennie or main to the WH, just to keep things simple and as low risk as possible. I find in the summer there is very little need for hot water as the main tank gets warm enough (engine room, my fault there) and if I do want hot water, gives the gennie a chance to run under a decent load for a half hour.

But it certainly can be done. Depends on how you use the boat.
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Old 03-13-2019, 01:18 PM   #4
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Originally Posted by Ski in NC View Post

And since it is running, just let the electric element do the heating. The incremental increase in fuel burn is very small.
To me, this is the biggest reason. Why bother, when it has the capacity to heat the water when it is running by default. However, I could theoretically run all day and arrive at the anchorage without hotwater. Now I have to fire up the generator (which I wouldn't normally need to do after running all day) just to heat the water.
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Old 03-13-2019, 09:53 PM   #5
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I have my Detroit mains and a Perkins 4108 generator plumbed to heat my boiler. All have valves that either isolate engine system or allow the coolant into the boiler and hydronic system. I also have a Onan MDL-3 that doesn't produce enough extra heat to be usable unless under a full load.

Cruising in cold weather I use the heat of one main, I don't need to run with a generator because of my inverter system. At anchor I use the generator for boiler heat if it's running, but I only run about an hour a day. Otherwise at anchor either the diesel stove heat is enough or I let the boiler oil burner do the heating.
I haven't had any plumbing issues. The engine coolant and boiler circulation pumps seem to move the water efficiently. The coolant leaves the engines before the heat exchanger and returns after.
A side benefit, if the boiler is hot before starting in cold weather, I can circulate that heat into the engine for faster, easier starting.
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