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  #1  
Old 07-05-2012
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AC questions about ratings (BTU)

I was talking to my neighbor the other day about AC on our boats. I am wondering about the fact that a 6.5k btu window shaker will cool off a 34 ft boat (stuck in companion way), which is what he does. I have a 12k btu AC unit that struggles when it is hot to get the temp down to 80. I've checked the output temp, output is 59-60 which I've been told is right. I've got covers on the topside hatches to reduce solar heating and I've even hosed down the top of the boat.

The web sites of makers of boat AC units all seem to indicate I need 16k to properly cool off the boat.

I've seen the same comment about window units before on other web forums.....

How is that right? I would think that BTU output is a simple calculation.
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Old 07-05-2012
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Re: AC questions about ratings (BTU)

and your neighbor has a better engine.. slip... and a better boat then you... I'd not fall into that type of discussion, I mean really...

Checking the output temp means nothing.. to know if the unit is running right you need to check the temps right at the coil. if air going is 80. the air coming off should be about 15-20 colder. on the other side (the condenser) just the reverse. air going in at 100 = air coming off about 120 +- Eventually if the area being cooled does drop in temp.. this "temperature split" remains the same across the coils, even though the temps are lower. or higher depending on ambient conditions.

newer units using 410 A take a little longer to start cooling as the refrigerant gets to operating pressures.
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Last edited by deniseO30; 07-05-2012 at 08:45 AM.
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Old 07-05-2012
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Re: AC questions about ratings (BTU)

To the OP, my perception is that window air conditioners tend to be more "efficient" at cooling the boat than build in systems.

My suspicion is that the window air conditioner has a great advantage of cooler air to work with sitting in the companion way vs. being stuck in a locker, settee, berth ... with limited airflow.

Would the efficiency that is perceived as "cooling effectiveness" be largely influenced by the amount of ventilation the AC has to work with?
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Old 07-05-2012
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Re: AC questions about ratings (BTU)

Quote:
Originally Posted by HeartsContent View Post
To the OP, my perception is that window air conditioners tend to be more "efficient" at cooling the boat than build in systems.

My suspicion is that the window air conditioner has a great advantage of cooler air to work with sitting in the companion way vs. being stuck in a locker, settee, berth ... with limited airflow.

Would the efficiency that is perceived as "cooling effectiveness" be largely influenced by the amount of ventilation the AC has to work with?
In some cases, yes. With a window unit the air is not being forced through ductwork, which, on many boats, is uninsulated. So even if the AC unit is providing the right temperature cold air off the coil, by the time the air gets to the "room" it is supposed to cool, that cold air is warmer, and thus cools less.

A BTU is a BTU. Really. 1 "ton of cooling," a common unit in North American refrigeration and air conditioning applications, is 12,000 Btu/h.

The "supply air temperature" which is the temperature of the air introduced into the space to be cooled is critical to ability to cool, as well as the amount of air.

So a "free flow" wind AC unit could be more efficient than a ducted (uninsulated ducting) unit.

The supply air temperature of 59-60 is also a tad high. Most air conditioning systems, in buildings for example, introduce air at 55 degrees. Sometimes in moderate weather this temperature is raised to conserve energy when full cooling is not required. when it's not as hot outside as it possibly can get on the hottest days of the year.

So, it's not so much where the unit is located, but rather the temperature and amount of air that is applied.
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Old 07-05-2012
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Re: AC questions about ratings (BTU)

I duuno guys. I think the OP asks a real good question. If his marine A/C is double the output of the window shaker, to my way of thinking it should work better. Not worse.

Many marine A/C units are water cooled. Could a strainer be clogged? Is your sea water temperature in the 90's? If air cooled is the space the condenser in ventilated?

I think there's more to the story.
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Re: AC questions about ratings (BTU)

OK,
1st - output temp is being measured at the coil.
2nd - input air temp starts at 95 output is 60 (checked after running for 1/4 hour)
3rd - The system on my boat is approximately 25 years old
4th - Water cooled system, water is running strong. Water temp upper 70's I believe (mid Chesapeake bay) .
5th - four hours of running drops the temp inside to 80 output still at 60 and that's as low as it will get and the unit is running constantly.

Now a window unit vents hotter air to the outside. Using a water cooled system. I'm assuming a heat exchanger and thus venting heat via the water. So is my cooling limited by water temp? Strange if so since water temp is lower than air temps.

Denise - Are you saying I should check output temp of the water vs input temp of the water to seem if unit is running correctly?
BTW Denise my boat is better, his is a marina queen, mine gets used

I'm not too concerned about the system on my boat. It seems to be undersized, my calculations indicate a 16k unit is about right and that is what a survey of listings of my boat (hunter 34) on yachtworld indicated. I think it's on its last legs. I'm just curious about why a window unit rated for less BTU seems to work better. My question really is what TropicCat said 'If his marine A/C is double the output of the window shaker, to my way of thinking it should work better. Not worse.'

Thank you all for your input.
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Re: AC questions about ratings (BTU)

All due respect.. but really, it's a "my neighbor said" thread by the OP. There are just too many variables to know what's what. Sure, someone can stand in the main saloon where a window box is cooling and say.. wow.. how nice. And in a given amount of time it could even cool entire boat. What we don't know; is the OP turning his on and thinking it will cool the boat in an hour? where the "neighbor" may have his running close to 24/7? OP also doesn't say what size boat he has, if he does have a water cooled unit. Water cooled condensers have only 10 +- degree temp rise in cooling mode BTW. Variables can be, water flow, air filters, solar gain on boat compared to neighbor, air distribution.. It would take a ducted system longer to cool the whole boat then it would for a window box to cool one area. 12,000 will do 4- 700 sq ft of living space generally, but.. with the demand a boat has it's really difficult to come up with a size that will cool on demand rather then time. (demand, meaning "now" when I just turned it on")
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Re: AC questions about ratings (BTU)

Looking at the recent temps we are having in this area Sailife.. I'm surprised anyone can get below 80 on a near, 100 degree day.. does it "catch up" when the sun goes down?
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