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Old 04-01-2021, 19:35   #1
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Using Generator Cooling to Heat Water

One of my primary use of my generator is to heat water. This is done electrically and draws a significant portion of my generator electrical capacity. In addition to an electric resistance, my hot water accumulator has two coils inside. One coil is connected to my main engine cooling system and is used to heat the water when my engine is running. The other coil is left un used. It makes no sense to run my generator and dump dump all that cooling heat down the ocean while using electricity to heat water in my accumulator. The efficiency of the operation must be terrible. What I would like to do is connect the primary side of my generator cooling system (fresh water side) to the hot water accumulator 2nd coil just like what is done with the main engine. This would not only be a much more efficient use of my generator but it would also produce hot water much more quickly. My generator is a 7Kw ONAN with a heat rejection of 197Kcal/min or 780 BTU/Min. This would bet me hot water in a matter of minutes. The electrical power from the generator would be left to charge the batteries quicker and run my water maker. Does anyone have experience in doing this. ONAN does not appear to be selling kits in order to accomplish this. Your suggestions and experience on the topic is appreciated.
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Old 05-01-2021, 04:15   #2
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Re: Using Generator Cooling to Heat Water

I've no experience operating such an arrangement, but it seems like a good idea, to me.
Your rational seems valid.
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Old 05-01-2021, 04:47   #3
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Re: Using Generator Cooling to Heat Water

What engine powers your 7kw onan? Thatís where Iíd start when determining where to pull the water from.
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Old 05-01-2021, 05:28   #4
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Re: Using Generator Cooling to Heat Water

The generator is powered by a 1.123L (68.53 in 3) 3 Cylinder Kubota engine. It would not be difficult to cut its primary cooling circuit open before the heat exchanger and create a loop that goes through the calorifier. I do not know if I can divert the entire cooling flow into the water heater or only part of it. My preference would be to divert all of it for maximum benefit. I debate wether the Kubota water pump is capable of dealing with the additional load while maintaining a proper cooling flow. I would need to add about 5 to 6 m of pipe plus some 90 deg fittings to the cooling circuit (2.5 to 3m each way) to get the job done. I could add an electrical pump in the circuit to assist but this adds complication and cost.
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Old 05-01-2021, 05:38   #5
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Re: Using Generator Cooling to Heat Water

You could install a thermostat in the departure of the waterheater loop to keep the generator coolwater at the correct temperature. Otherwise I think your idea is excellent.
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Old 05-01-2021, 05:45   #6
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Re: Using Generator Cooling to Heat Water

It's no different from using main engine heat for the same purpose, which is common. Take it from just before the heat exchanger, and consider insulating the "in" hose. Keep the hose diameters large, and where possible use curves rather than elbows, to minimize load on the pump. It would be a good time to clean out the heat exchanger and any other sources of resistance.
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Old 05-01-2021, 06:31   #7
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Re: Using Generator Cooling to Heat Water

All good input, thank you. The thermostat of the Kubota won't open until the engine block reaches proper operating temperature and therefore I am not worried about it running too cold. Indeed, nothing spectacular in the concept, this is what is being done with the main engine on every boat out there. What beats me is that the yard went through the trouble of installing a double coil calorifier but did not bother to connect it to the generator??? Further, this makes so much sense that I don't understand why Onan does not offer an optional kit to do what I am talking about. They offer all sorts of optional kits such as dry exhaust and keel coolers for these generators but I haven't seen anything for this purpose. Is there an issue I haven't thought of?
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Old 05-01-2021, 06:52   #8
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Re: Using Generator Cooling to Heat Water

Well I can think of one reason not to do this...the potential for a leak in the coil inside your water heater. Doesnít take a lot of antifreeze to kill somebody. Nasty chemical in the body even at sub lethal levels.
If you run pure water in both engines you would not have a problem with the toxicity of antifreeze but Iím not sure how happy the engines would be.
I have no idea if you can run diesels on non toxic antifreeze.
An antifreeze leak into the water you wash your dishes with would be very difficult to detect. In a shower, you breathe in mist and the air in your lungs might contain very fine droplets of antifreeze. Iím not sure there are even any studies on the toxicity of anti corrosive boiler additives. Iíve been thinking about adding them into my hydronic system which runs on distilled water as it flows through a coil in my hot water tank. Just havenít had time to research this. The coil and the tank are 316 stainless but pin holes could push antifreeze through a coil or plate heat exchanger. Once antifreeze starts to accumulate in your tissues, it does some interesting damage. Your lungs are more delicate than you think. People exchange something like 10% of their lung volume while manatees exchange 90 % in a breath. Letís them sleep on the bottom for 15 minutes at a time. They have great lungs...just bad breath.
Iíll do some more research on anti corrosive additives but Iím a bit concerned about the toxicity issue of antifreeze in this configuration.
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Old 05-01-2021, 06:59   #9
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Re: Using Generator Cooling to Heat Water

The engine sounds like a kubota 1105. If thatís the case, the water heater is typically attached to the thermostat bypass hose. No weird plumbing required. The only concern I would have is the 5-6M round trip
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Old 05-01-2021, 07:04   #10
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Re: Using Generator Cooling to Heat Water

You could do it, but if you're not going to have enough electrical load on the generator without the water heater powered electrically, I'd skip it. With less load, you may have trouble getting the engine warmed up with heat being siphoned off to the water heater. And running a generator with too little load isn't good for it either.



Basically, it would make sense to free up electrical capacity if you find the water heater often limits what else you can power from the generator while you have it running. But if it's already fairly lightly loaded, the small theoretical fuel savings from this aren't worth it.
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Old 05-01-2021, 07:48   #11
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Re: Using Generator Cooling to Heat Water

rslifkin has a good point, but there is a simple solution. Put a valve in a hose that bypasses the water heater and leave the electrical connection as is. Then choose engine heat, genset heat, or electricity as your source for hot water depending on circumstances. Really, the most difficult problem in this entire project is likely to be running the hoses. It's barely science, much less rocket science.
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Old 05-01-2021, 09:41   #12
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Re: Using Generator Cooling to Heat Water

I think it is good idea to use heat from the engine to heat up water. Many (?most) propulsion engines have it plumbed that way with a calorifier already.
No different for a genset, principle is the same; would even make more sense as you have a calorifier with 2 coils.

I have a similar setup, but I have throw a few valves, as I have only one heating coil. My genset has a small Yanmar diesel.

On the downside: indeed if cooling water mixes with drinking water: not a good idea as Manatee mentioned. I wonder how often that has happened, do not know and haven't heard of any cases.
In theory it might take longer for the genset to heat up, but I would say, it would be marginal, not a reason not to go ahead with it..... just my opinion.
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Old 05-01-2021, 09:47   #13
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Re: Using Generator Cooling to Heat Water

Hello,

You spoke of the Onan units having keel cooler adaption so instead of hooking up the keel cooler connect it to the second loop. If the antifreeze was really a concern then everyone who has a hot water heater powered by their cooling loop would have a problem.
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Old 05-01-2021, 09:51   #14
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Re: Using Generator Cooling to Heat Water

It seems simple but, does the Onan have a hose in the cooling system? You just need to tap off those fittings for the loop. But if it has no hose... then what? Is that your issue?
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Old 05-01-2021, 10:06   #15
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Re: Using Generator Cooling to Heat Water

Calorifiers are specifically made for this job for small main engines and would most likely would suit your Onan.

See

https://www.asap-supplies.com/domest...gs/calorifiers
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