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Decreased AC Performance in Warmer Water
August 24, 2009
7:20 am
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Steve Pooler
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November 12, 2008
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What you are asking may seem a simple question however it really is not…There are many variables…

Even the new systems will experience a performance drop in warmer climate but still maintain 15-18 degree TD.

  • Some with newer style high Efficiency Blowers may move more air in restricted duct situations…but TD would increase if restricted.
  • Sea Ray has not been known to do the best/proper job of A/C installation in the past…Air flow could be your issue…
  • Again your systems could have scaled up condenser coils…What are the system pressures running ?
  • Has a qualified service man had a look at your installation and it's performance ?
  • Do you leave doors/hatches open ?

What I might suggest is, Air Conditioning is exactly what the phrase implies and is not meant/designed to be refrigeration…

If you are looking for cabin temps lower than about 68 degrees A/C systems will ice up running in those temps…A system that will get the cabin to 68 degrees in a Florida summer day is likely oversized and not removing as much humidity before cycling off, than one that may only provide 74 degree cabin temp…

The longer it runs…The more humidity is removed…The lower the humidity…The more comfortable the cabin…

Oversized systems tend to cool & shut off on the t-stat before they have had sufficient run time to remove that humidity…So, many times a smaller BTU system may provide a more comfortable living space…But take more time doing so.

Again…Installation & Air Flow is a big part of A/C performance…The newer boats you mention that have like sized systems may have a better installation where air is distributed better…thus feeling colder….Or it could be worse and their TD is above 18 degrees with ice up problems.

The newer boats may be better insulated too...

All of this is hard to say without seeing both examples, or having much more detailed info on both…

Steve~

August 24, 2009
6:10 am
wingless
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Steve Pooler said:

Post edited 10:08 am – August 23, 2009 by Steve Pooler


Direct Expansion A/C systems are engineered for a 15 to 18 degree Temperature Differential from the Air entering the coil to the Air exiting the grill…Even in water up to 90 degrees F.

If your TD is either More or Less than that 15 to 18 degrees…Something is not right.


That is the performance I've seen on my system.

My hope was that by providing a greater volume through the system more of the heat exchanger would be presented w/ lower temperature water, permitting better response.

The factory system has the expected runs / placement / fitting size and hose size.

From my perspective, I could change the hoses / fittings / pump / seacock / strainer into and out of the heat exchanger, to try to pass a greater volume of this warmer water, to get better cooling in warmer water. It sounds like nobody has done this before. That sounds like most of my projects so far...

So, do the newer boats, with colder cabins under the same conditions just have larger systems? Or, have the systems been re-designed to permit better cooling w/ the same size?

Thank you,

wingless

August 23, 2009
9:54 am
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Steve Pooler
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With water flow up to spec (250 gph per ton) and clean seawater condensers….(no scale buildup inside) 

Direct Expansion A/C systems are engineered for a 15 to 18 degree Temperature Differential from the Air entering the coil to the Air exiting the grill…Even in water up to 90 degrees F…They just work harder and draw more amperage to do it.

If your TD is either More or Less than that 15 to 18 degrees…Something is not right.

Increasing seawater flow rate is not usually the answer unless you don't have rated flow at present…It can also cause more problems than it helps…

  • Faster moving water may not be there long enough to absorb the heat it could when staying longer.
  • Fast moving water (with abrasives) can cause erosion in fittings that have a bend or elbow.

Larger diameter hoses do not increase flow unless the fittings etc. are increased in size to match…You cannot increase the size of your condenser, so it will tend to limit any increased flow you provide. 

I will say that supplying the proper sized pump with as much water as possible can help that pump attain, and maintain it's rated output…

Make sure you have the largest fittings, hose, strainer & thru-hull that the pump's suction side pipe size fittings will allow…Don't ever downsize on the suction side of a centrifugal pump….The Bigger the Better on the suction side.

It can also help a centrifugal pump to increase the output hose size to the manifold or tee that splits the flow to the different A/C units.

Another consideration with centrifugal pumps, is distance & how high they need to pump…Most are rated for 3 ft of head, and that rated flow will dramatically reduce when the spec is exceeded…See the flow chart for your model pump.

Last but not least is…I know you said your system is clean but has it been acid flushed recently ?

It is often a forgotten thought as folks clean & replace hoses to get proper flow, but over time the A/C unit condensers themselves will build up a hard water scale inside that acts as an insulating blanket preventing the designed heat transfer to the water…Like a dirty radiator in a car that needs to be flushed…So do A/C condensers… 

It is usually done with a fairly weak acid solution that is recirulated thru the coil and back into the bucket with a pump for 20-30 minutes, and can make a world of difference in performance bringing the inside of the condenser back to like new condition…  Cool

Steve~

August 23, 2009
5:03 am
wingless
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The two factory HVAC systems on my 2000 Searay 380 Sundancer don't perform cooling as well in southern Florida as in New England.

The Florida water temperature is ~85F versus 70F max in New England.

The raw water system is maintained very very clean and I have great flow.

The raw water exiting the boat is much warmer than the sea water.

Can I improve cooling my bumping up the raw water pump one or two sizes? Would replacing the raw water hoses w/ larger diameter help?

My hope is to attain much faster cooling than I currently have.

My hope is to improve the system w/ changes I can effect, ie not changing the refrigeration system.

It seems reasonable / possible that more flow would transfer more heat to the sea water, giving more cooling.