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Old 15-08-2011, 12:18   #1
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Water Heater Siphoning Through Engine

I have an Isotemp hot water heater which seems to siphon(?) the hot water thru the engine when docked. The good: The engine is always toasty warm & dry / The bad: The hot water heater needs to run a lot to keep both the water hot and the engine warm. The hoses run parallel to each other about 5' to the engine (Yanmar 3YM30).

My first thought was a leak causing the engine coolant to mix with the domestic water, but that is not the case. Oddly, it's not a constant thing - some days it does - other days it doesn't.

Any ideas?

Russ
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Old 15-08-2011, 12:30   #2
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Re: Water Heater Siphoning through Engine

just keep drinking the water and soon you will know if it is mixing.
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Old 15-08-2011, 12:36   #3
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Re: Water Heater Siphoning through Engine

Sounds like your waterheater has heating coils which go from the engine to the water heater, then cycle back to the engine cooling system. I suspect you are getting an example of heat convection--heated water in the water heater is warming the water in the heating coils, forcing it back towards the engine cooling system.

When the engine is running and the water heater is not, the hot water from the engine goes through the coils to heat water in the water heater, then circulates back to the engine. When the engine is off, but the waterheater is on, the heating water in the tank transfers heat to the cool water in the coils, drawing it back to the engine.
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Old 15-08-2011, 12:38   #4
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Re: Water Heater Siphoning through Engine

pressure test your water heat exchanger. if in doubt cut it out of the circuit.

do you have a Magnesium anode in the hot water heater drain?
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Old 16-08-2011, 06:42   #5
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Re: Water Heater Siphoning through Engine

Astrid -

Yes - heat convection sounds like exactly the problem - any idea of how to prevent it?

Russ
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Old 16-08-2011, 06:51   #6
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Re: Water Heater Siphoning through Engine

Install a ball valve in one of the coolant lines between the coolant tank on the engine and the water heater, works like a charm on mine. Keeps the hot water from becoming scalding when motoring for long periods, too!
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Old 16-08-2011, 08:57   #7
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Re: Water Heater Siphoning through Engine

I wonder if a spring loaded check valve might have just enough resistance to prevent the convection circulation?
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Old 16-08-2011, 09:12   #8
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Re: Water Heater Siphoning through Engine

Don't know if Yanmar make such a fitting, but Westerbeke uses a fitting with an line butterfly type reed. Circulating pump has enough pressure to pass the valve, but prevents back syphoning.
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Old 16-08-2011, 10:21   #9
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Re: Water Heater Siphoning through Engine

It's a thermosiphon hot water rising, cold water falling thingy. Don't use a spring loaded check valve. The water pump on the engine will not develop enough head to open the check valve with a spring.

To fix it, put a U bend about two feet long on the return line (hot from the heater to the engine) That should stop it. It's what is done for building hot water systems too.
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Old 13-09-2011, 16:14   #10
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Re: Water Heater Siphoning through Engine

SailorChic is the winner!! I added a loop to the return line up about 2' & presto! No more problem with thermosiphoning!!

Thank you all!

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