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post #1 of 6 Old 11-10-2009 Thread Starter
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Raw water cooled issues and hot water

I am looking at a boat with a 2QM20H. I have heard that they do not heat the hot water very well. Is this true? What can be expected? How is it different from say a fresh water univeral 25 that I am used to?
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post #2 of 6 Old 11-11-2009
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A raw water cooled engine has a thermostat set at 140 f because if it ran any hotter there would be salt build-up in the passages. A fresh water cooled heat exchanger engine like the universal you had in the past runs at 160-170f, so will make hotter water.
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post #3 of 6 Old 11-11-2009
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Not only is the thermostat set to 130 - 140, instead of 160+ - remember that all raw water coming from the pump (even the cold water that bypasses the thermostat, when it is closed) will go through the water heater, before going out the exhaust - not just the "hot" 140 degree water coming from the thermostat. this will result in the water to the heater being considerably less than 140, unless you are running the engine hard, so that the thermostat stays open much longer, and more frequently.

I have a Yanmar 2GM (raw water cooled). Eeven aftre motoring for an hour or more, the water from the tap is only luke warm. Wasn't pwrth the effort to connect to the water heater. And, I am pumping salt water through the heater, so it will rust out much quicker, I would imagine.
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post #4 of 6 Old 11-11-2009 Thread Starter
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Thank you gentleman.
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post #5 of 6 Old 11-11-2009
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Quote:
Originally Posted by Northeaster View Post
. . . remember that all raw water coming from the pump (even the cold water that bypasses the thermostat, when it is closed) will go through the water heater, before going out the exhaust - not just the "hot" 140 degree water coming from the thermostat.
I can't recall seeing a hot water system connected that way.

If the by-pass water that cools the exhaust is directed through the hot water heat exchanger, change it so that only the thermostat-governed water does. That way your hot water temperature should be adequate.

Only engine-temperature water comes from the top of the thermostat cover and if that continues to flow through the heat exchanger in your hot water tank it will too reach 140F. OK so it may only be 140F but you can't shower in 140F anyway (too hot) so where's the problem?

Yes, I think that salt water into the heat exchanger could be a problem but surely the heat exchanger is built accordingly?


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post #6 of 6 Old 11-12-2009
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Andre - At least on my Yanmar, there is no way, that i am aware of, that would allow you to route "thermostat open, hot" water through the water heater, while routing the cold bypass (closed thermostat) water straight out the exhaust.
All raw water comes from the hose at the top of the thermostat, be it hot water that circulated inside the block and came out the open thermostat, or bypass water, that is constantly pushed through the small bypass hole in the thermostat, and out the same exit hose.

If I am mistaken, feel free to sho me a way!
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