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  #1  
Old 07-07-2007
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Water Heating Questions

I have read through several posts on this forum about heating water, but I still have a few unanswered questions.

I agree with the idea of having the diesel engine heat up the water heater through a heat exchanger while it is running. But like many other boats my engine is currently set up to use raw water to cool it. As such it should be using low temperature thermostat to prevent salt build-up in the engine. There have been several mentions of conversion to a fresh water system (kit)? What would that entail? Is that a closed fresh water cooling system? Is that even an option if I am sailing in salt water?

If I also install a hot water line cabin heater like the Hurricane, how would I join these systems up? How would they heat up the same hot water tank?

Thanks in advance for your help.
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Old 07-07-2007
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A raw water cooled system will still have a thermostat set about 140˚ F. This is still hot enough to run a water heater off of...

Most of the in-line water heaters are on-demand water heaters, and don't require the use of a tank as such.
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The hot water heater does not use raw water. It is included in a loop that holds only a fresh water/antifreeze mix. This mix leaves the engine, exchanges heat with fresh water in the hot water heater, then goes into the heat exchanger for cooling with raw water before returning to the engine. The only part of the circuit that sees outside water is one side of the heat exchanger on the engine.
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Exactly. You need to think out the diesel cooling provision before contemplating an engine-heated hot water tank. The heater uses water tank water heated by the "radiator" of the closed coolant loop, which under normal conditions gets a lot hotter than the 145 F. of the raw water circuit, because it isn't salty. I get to about 175-185 F on my Westerbeke W52, and this promotes better combustion.

The 145 F figure is a compromise of running in salt water and avoiding the somewhat more complex plumbing of a heat exchanger circuit. It's a false economy, in my opinion, for salt-water engines. In fresh water, you can run at 180-200 F, and just flush the passages out (and use a post-raw water pump, pre-block basket strainer) during winterization.
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Old 07-07-2007
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I believe Mithril has a raw water cooled engine, not a fresh water cooled engine with a sea water cooled heat exchanger. A number of the smallest diesels are made this way. It is possible to buy a "kit" to convert it to fresh water cooling. The kit basically consists of the heat exchanger and a second water pump for the salt water. The existing pump is used as the fresh water pump.
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We have an after market fresh water cooled Yanmar 2GM. What happens is the fresh side is circulated with the existing pump. This pumps the fresh through the engine and to the hot water heater, then back through the heat exchanger, and finaly back through the engine block. An additional pump, belt, and pulley are mounted for sea water. This pump brings sea water in, and is plumbed directly back to the heat exchanger and then to the exhaust mixing elbow. In my system the water heater will keep water hot all night. If I motor for about an hour in the evening to top up my batteries, that will be enough running time to have hot water the next morning. My water is really hot too, I have to use the mixer so I don't scald myself. So, you now have hot water, and as a bonus you no longer are introducing salt water to the engines cooling passes.

Our set up is small, but I would think the principals apply to larger systems too.
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I think I get it, is the heat exchanger part of the water heater or a separate unit?

So how would you integrate, for example, a Hurricane cabin heater to this system?

Thanks again all for your help.
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The heat exchanger and hot water heater are two different parts of the system. They both are quite similar though. The engines heat exchanger is used to cool the freshwater (water and antifreeze mix) before returning to the engine block. This happens by a heat transfer process where the hot freshwater passes through tubes in the heat exchange vessel, around the freshwater tubes is the cold seawater. The cold seawater takes the heat away from the hot freshwater side and then goes out to the exhaust mixing elbow.
The hot water heater works by the same principal. The "hot" freshwater tubes pass through a vessel that is your hot water tank. As the hot fresh water passes through these tubes, the cooler water in the tank draws the heat away. After enough time you have a tank full of hot water.
The two fluids exchanging heat never meet, they transfer heat from the tubes they pass through, into the fluid in the vessel that the tubes are passing through.
I suppose if you wanted to plumb in a cabin heating device you would just plumb the freshwater into that equipment before running back to the engines heat exchanger. You can plumb a bunch of heat exchanging devices in a row, but there will be less and less heat to use as you go down the line.

Here is a link to a pic of a two pass heat exchanger, there are many different types though.

Image:Heat exc 2-1.png - Wikipedia, the free encyclopedia

and the original article:
Heat exchanger - Wikipedia, the free encyclopedia
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Old 07-09-2007
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Hi Mithril

I had a similar decision to make because of a raw water cooled Yanmar. The cost, hassle and likely ineffective result of adding a heat exchanger was out of all proportion to the benefit - especially since the cooling water temperature was very low. If you have the room for a 9 kg gas cylinder where it can be secured and drain to the sea, plus a (separate) place for the heater unit and it's exhaust vent, why not consider an "on-demand" gas water heater? The cylinder and units on my boat are located in two separate seat-high lockers aft of the cockpit and require the pilot to be lit only when in use. If you confine the hot water usage to "shower-time" it needs to be on for only an hour or so a day. This is highly efficient, cheap and safe.

Cheers from Oz

Alan
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