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Old 16-12-2009, 18:25   #1
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Reverse Cycle A/C in Cold Water?

So I'm docked in NYC, and having good success so far with heat. Using a combination of reverse cycle A/C, Espar diesel heat, and a portable oil-filled electric radiator, the interior easily stays at a comfortable 68 degrees F without overly taxing any of the heating systems. I just measured air coming out the A/C at 121 degrees in 45 degree water. But the water temperature is dropping fast - 5 degrees in the last two weeks. What can I expect from the A/C as the temperature continues to drop?

Thanks in advance for any insight from those who have been there, done that.
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Old 16-12-2009, 18:35   #2
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Your A/C is going to become more and more innefficent until it won't heat at all as the water gets cooler. Reverse Cycle heat needs heat in the water to transfer into the boat, you'll hit a point where the freon and the water are at the same temperature and no heat transfer can take place.
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Old 16-12-2009, 19:11   #3
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Thanks, Sailmonkey..but I understand the principles involved, I'm looking for practical experience. At some lower temperature, will the raw water circuit freeze up due to the temperature drop resulting from said heat transfer? Or is it likely to continue working, albeit at a lower efficiency, down into 40 degree water, or colder? What have others seen?
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Old 16-12-2009, 19:49   #4
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My only experience with reverse cycle in cold water has been with the cruiseair smxII controls (electronic). With a water temp of 40 (I seem to remember this number) the Hi/Lo pressure switches shut the system off and gave the warning loPs.
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Old 16-12-2009, 19:59   #5
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I've had my Cruiseair's working down to 39 measured. Last year at one point we had a thin layer of ice on the water and they were still ok, though they didn't cycle off very much and the air coming out was not hot though it was warm. The instructions say don't run it below 39 or there is a chance that the water could freeze in the heat exchanger.
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Old 16-12-2009, 21:43   #6
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40F is the practical limit for the injection temperature of the raw water although I seem to remember CruiseAire recommending 42F. Think through the process. The reverse cycle principle extracts heat from the raw water, thus dropping the temperature of the raw water as it goes through the system. If the temperature of the raw water drops enough so that it starts to freeze, then you can damage your condenser because the annular space where the water circulates is dimensionally small and water expands as it converts to ice.
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Old 16-12-2009, 21:55   #7
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I know people who have put ice eaters unde their boats to provide an upwelling of warmer water and others that have put inline hot tub heaters on the pump side.

I suppose they both worked to a point.

I also know of people who plumbed shore water to their AC so they don't have to run the pump while at the dock.
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Old 17-12-2009, 06:22   #8
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I seem to recall that the temp differential you can expect is, for most reverse cycle systems, going to be ~40 degrees. That would suggest you'll find it useable at temps ~40F or perhaps a wee bit less...but with increased inefficiency and long run times. (Are you on your own electric meter?) We found our Mermaid Marine system to produce bountiful heat and great efficiency over two London winters...but the water temps at St. Catharine's Docks never dropped below mid-40's.

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Old 17-12-2009, 07:03   #9
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Thanks everyone for the help. Sounds like I can continue to pull some heat out of the Hudson River well into the winter. I wondered if the Marine Air system has some sort of shutoff to prevent freeze-up? One of my concerns is leaving the system unattended and filling the bilge with seawater if the condenser freezes.

Has anyone seen a condenser freeze in saltware (really it's the evaporator when in heat mode right?). Seems to me that between the salt lowering the freezing point, and the rapid motion of the water through the system, and the latent heat that has to be removed for the water to change to solid, you'd have to have some pretty extreme conditions to freeze one up.
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Old 12-01-2010, 05:54   #10
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Hi RunningFish
You may have already found this out but once the water temp gets down to about 38 degrees (approximate refrigerant temp) you will experience icing on the water exchange coils. If this is left to continue the water will actually freeze and inhibit/stop water flow. When I lived aboard in DC I set my reverse cycle down to about 66 and then augmented with a couple ceramic block heaters. The 66 degree setting was high enough to get some work but low enough that it shut off for long enough to de-ice coils and prevent freezing. Now I am down in Norfolk and am hoping the water temp doesn't stay as cold down here as it did in DC. Stay Warm
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