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  #1  
Old 08-04-2007
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Question No Hot Water

My hot water heater uses the engine exhaust from my Yanmar 3GMD to heat the water, but the water isn't getting hot. Also, the engine temp gauge is barely on scale after motoring for an hour. I suspect the engine thermostat to be the culprit, but the thermostat looked good (fully closed) when I pulled it for inspection last spring. The boat is new to me, so I have never known the hot water heater to work.

Troubleshooting suggestions will be appreciated. Duke
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Old 08-04-2007
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1. I don't think the exhaust heats the hot water tank. Its probably heated by the engine cooling water.
2. The thermostat will remain close when cold and opens when hot. What you can do is put the thermostat in boiling beaker of water and see that it opens when heated.
I'm no expert but this is what I understand. I've the 2GM20F Yanmar.
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Old 08-04-2007
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Thermostat action

Yeah, but if his thermostat is not opening wouldn't the engine overheat after a few minutes? Problem must be elsewhere in the circuit.
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You are right, it is not the exhaust that is heating the water in my HWH. It is the engine's cooling water. The Yanmar 3GMD does not have a heat exchanger (radiator type) for cooling the engine. It uses only raw water. Thus, the cooling water is cycled through the engine then discharged via the wet exhaust. My hot water heater is is plumbed into the engne cooling water circuit near the manifold, cycled through the HWH's heat exchanger, then returned to the water/exhaust mixing elbo on the engine.

I figure it is either the engine thermostat standing open allowing the engine to run cold (not good either) or the plumbing between the engine and the HWH is plugged. -- or some combination of the two.
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The problen is not that the thermostat would be stuck closed instead it would be stuck open.
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Old 08-04-2007
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The water jacket inside the HWH could be clogged up as well, so the water isn't circulating freely. That would be most likely.

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Old 08-04-2007
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I'm Korfuzed

So bear with me ald help me out a bit. Do you put antifreeze in you engine? The reason I ask is I am thinking you have raw water cooling. If you indeed have raw water cooling the water temperature (as long as everything is working to snuff) should hardly ever get over 140 degrees.
On the other hand If you have freshwater cooling and the hot water heater is plummed to the freshwater side of the engine, you will get 180 degree water. If the heater is plummed to the rawwater side the water will just get luke warm.
It will take your engine about an hour of run time to make hot water.

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Cap'n Dave
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Old 08-04-2007
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PB, may be right about the HWH. I think he has a problem with the T'stat as well because the temp guage is not showing the engine heating up. The second problem, the guage not showing the engine warming, could be a problem with the sending unit as well. The first step, I would think is to check the t'stat. Diesals are made to run hot and he will have condensation problems if he runs the engine too cool. Some hwh have a shut off valve for the coolant heating. I would check that out as well.

Last edited by ebs001; 08-04-2007 at 10:01 AM.
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Old 08-04-2007
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I have already checked the water temp. sensor. It checks good. If I had reasonable access to compressed air I would disconnect the plumbing at the HWH and check for clarity in the lilnes and inside the HWH. Trying to check water flow through the lines by disconnecting the plumbing could really cause a mess. I just cleaned up the mess from a ruptured muffler and I am not anxious to do it again.
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Old 08-04-2007
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You say your raw water cct is plumbed from the engine, to the HWH, and then to your exhaust elbow. (Odd that a 3GM is RWC) Is it in series or parallel to the engine cooling circuit?

If in series pluggage in the heater is unlikely because you would not get the flow, no wet exhaust, and a hotter running engine.

If in parallel, then it's totally possible that there is no means of ensuring that the warmer water actually goes through the heater. If the heater is above the engine (physically) there may not be enough pressure to force the water over that loop, it will much more easily all go into the exhaust. A circulating pump in the line to the HWH may help you there but there is a risk of interrupting or short circuiting the regular path of the engine coolant.

But....your cold running engine still points to poor thermostat performance, and your "cold" hot water sort of bears that out.
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