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How effective is your A/C?
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Author:  GTBecker [ Sat Jul 10, 2010 12:02 pm ]
Post subject:  How effective is your A/C?
I had a Mermaid 6500 BTU A/C installed some time ago. The unit is at the foot of the mid-berth with vents in the mid-berth, above the breaker panel and on the front face of the galley. It has worked well, I thought, but has always had difficulty getting the cabin below 80 degrees on typically hot days in Cape Coral - although it does a great job removing humidity.

I've been reading comments by folks who say their A/Cs can freeze them out of the boat. Mine seems to be running fine and the evaporator is cold top-to-bottom. What should be typical performance of this unit in a 258 Vista cabin?

Tom
Author:  Buoy Howdy [ Sat Jul 10, 2010 2:51 pm ]
Post subject:  Re: How effective is your A/C?
If you are in an uncovered slip that can add a big load to your a/c. Another factor is the water temperature. When our lake hits 90 degrees my a/c struggles to get below 80 during the day. At night it will cool down nicely. The performance you describe sounds fairly normal for a hot southern climate.
Author:  GTBecker [ Sat Jul 10, 2010 4:50 pm ]
Post subject:  Re: How effective is your A/C?
Do you think lower water temperature is effective because it is a better coolant than warmer water or because the wetted hull is cooler?

I've modified my A/C and freshwater systems so I can use well water as coolant when in the lift. The well is a pretty-constant 74 degrees, cooler than the canal water here most of the year, but I don't see better A/C performance when using it. At the same time, of course, the hull is dry and at ambient temperature (which might be warmer than the canal), so I suppose there might be some offsetting effects.

Experimentally, I've rigged a plastic tarp on the railing to shade the forward deck surface, and the cockpit is already under camper canvas so it receives no direct sun and is actively ventilated so it rarely exceeds 100 degrees during the day in the cockpit. It'll be interesting to see if the shaded forward surface helps much.

Tom
Author:  powellcrazy [ Sun Jul 11, 2010 10:39 am ]
Post subject:  Re: How effective is your A/C?
If unit is blowing air out that is 20 deg cooler than ambient, its working as good as it ever will.

6500 btu to cool a non insulated boat in direct sun is a huge task!

We have a 5000 btu A/C in our 248, when at powell in direct sun and water temp in the upper 80's, air temp 103 F, the cabin gets down around 85, that is about all one can expect.
Granted I would like it cooler, but...... Its going to take a lot of $$$$$$$ and different boat with insulation, larger a/c, ect....
Author:  GTBecker [ Sun Jul 11, 2010 3:14 pm ]
Post subject:  Re: How effective is your A/C?
Some interesting measurements with an IR remote-reading thermometer after making three changes: the forward deck shading tarp, isolating the return air from the compartment that the A/C unit is in, and adding a small circulating fan that hits the thermostat.

I've always suspected that the A/C breathes some outside air, and unnecessarily cools the inside surfaces of the cockpit walls and line lockers, since that compartment isn't closed to them. I built a thin plywood separator that better isolates the return cabin air from that compartment volume, essentially coupling the return grille to the evaporator better.

With a clear-sky south Florida July overhead sun, and air temperature of 87F and a slight breeze, the forward deck temperature was reduced about 13 degrees F with the blue-plastic tarp suspended eight inches above it. The exposed white deck was 117F while the deck under the tarp was 104. A white tarp or fabric would probably do better, I imagine.

The underside of the Bimini top (with side camper canvas and windows in place) was 124F, while the _topside_ was oddly cooler, at 104F. The cockpit air was 102F (with a 10" ventilator exhausting through a high rear side screen). The cockpit deck was 95F. The deck under the windshield ("Sand"-colored non-skid) was 170F! Surface water under the dock was 89F, and the vertical hull surfaces were the same.

I was surprised inside the cabin where the air was 75F at the floor, 72F in the mid-berth, and 79F at the A/C thermostat, mounted on the forward face of the breaker panel enclosure; the cabin ceiling (foam-backed fabric) directly under the 170F deck was 88F, and the head ceiling under the instrument cluster, door closed, was 95F. Most other internal surfaces were between 80F and 85F. The A/C return inlet was 72F while the vents were blowing 62F air. This was significantly better than I'd measured in the past.

In addition to shading the forward deck, I also ran a small desk fan on the galley surface that blew at the thermostat to determine if improved circulation would have an effect, and it seems it did. I've never seen 79F at the thermostat in the middle of a hot day before this. And, in fact, a few minutes after entering the cabin, the compressor cycled off at 78F; usually, it runs continuously all day and is never below 80F on a hot sunny day.

So, it appears that some combination of the additional deck shading, return air coupling and additional circulation has helped. I'll try relocating the thermostat, too. High in the cabin isn't the spot of choice, so maybe I can do without the additional fan.

FWIW.

Tom
Author:  firecadet613 [ Mon Jul 12, 2010 8:01 am ]
Post subject:  Re: How effective is your A/C?
I'm going to check the pressure on my ACs freon. It doesn't cool like it should, but the water flow is there. I was with a few other boats this weekend and their AC did just fine whereas mine didn't.
Author:  Cap'n Morgan [ Mon Jul 12, 2010 8:29 am ]
Post subject:  Re: How effective is your A/C?
Your AC actually uses Freon ? I would think it is R134.......

After a week and a half of 85 to 90 degree weather on our boat, AC had no problem maintaining 69 to 72 degrees.
It sure is nice to have on these hot humid days we have had this summer. Glad we now have AC.
Author:  firecadet613 [ Mon Jul 12, 2010 8:41 am ]
Post subject:  Re: How effective is your A/C?
R22. The other boats I was with over the weekend had air shooting out of the vents 10-15 degrees cooler than mine. Has to be a charge issue as water flow was fine.
Author:  GTBecker [ Mon Jul 12, 2010 10:12 pm ]
Post subject:  Re: How effective is your A/C?
I learned today that the thermostat indicates three degrees high, so what I thought was 80F has been actually 77F. The controller can not be recalibrated, says Mermaid Marine.

Also, I removed the locker under the shift control today to pull a cable and felt cool air blowing out of the hole - confirming that I've been cooling the cockpit walls. I went below to find how that happens and realized an oversight. The mid-berth is trimmed with a 3"-wide Velcro'd strip of fabric to close a two-inch gap between the ceiling and the starboard wall. That gap was not well closed, but it is now after reattaching the strip.

I also put a Kill-a-Watt power consumption meter on the shore power connector to see just what the A/C uses in a day of operation, and to provide a benchmark for further improvements. Overall, though, I am more pleased with the A/C performance - although today was mostly overcast, so was not a good day for comparison.

Tom
Author:  wkearney99 [ Tue Jul 13, 2010 6:25 pm ]
Post subject:  Re: How effective is your A/C?
GTBecker wrote:
confirming that I've been cooling the cockpit walls.


I mentioned something along these lines in another message thread here. The duct hoses used in most boats has a tendency to work loose. It's always a good idea to re-check how well the band strap it attached where the hose meets the air distribution manifold. This is not necessarily an easy thing to reach, but worth checking if your AC isn't working effectively.

It's also handy to have a roll of aluminized duct tape. NOT CLOTH DUCK TAPE. The metal kind has much better adhesive on it and deforms to the surfaces providing a good seal. The cloth crap rots or dries out. That and it never makes a decent seal in the first place. Much like WD-40, lots of people use it for the wrong purposes.
Author:  298VISTA2000 [ Tue Jul 13, 2010 6:36 pm ]
Post subject:  Re: How effective is your A/C?
firecadet613 wrote:
R22.


Yes, R22 is what dick-head Al Gore put in place and one of the BS "green" crap that is making him rich. I bet that asshole is behind ethanol as well. May his masseuse nail him to the wall.
Author:  298VISTA2000 [ Tue Jul 13, 2010 6:46 pm ]
Post subject:  Re: How effective is your A/C?
Cap'n Morgan wrote:
Your AC actually uses Freon ? I would think it is R134.......



I think Al makes us use R22 now. I probably have the same size unit as you and this is about what I run (69 to 72 degrees) in 90 degree Georgia heat/humidity so I am happy even with R22. We have been living on the boat for about 2 1/2 weeks now (at a covered slip) and have had to clean the filter a couple of times. I find that when the filter gets dirty, the unit starts to freeze up and puts out less.
Author:  GTBecker [ Tue Jul 13, 2010 9:21 pm ]
Post subject:  Re: How effective is your A/C?
The Mermaid installation was reasonably well done, thankfully, except that they used uninsulated duct, and it is noisy. I found the duct couplings in good condition when I slipped insulation sleeves on. Aluminum tape certainly is better than cloth, I agree. In my case, the starboard cockpit wall cooling was due to the mid-berth ceiling leak, not a duct leak.

BTW, the boat used 12.7KWh in 24 hours; about 2KWh of that is used by the charger/powersupplies (I develop nav software and continuously run a WiFi'd helm computer and a number of instruments), so the A/C used about 11KWh. We currently pay $0.116/KWh, so it cost about $1.28 to run the A/C today.

I found, incidentally, that thermostat placement is a large variable. Unless an additional fan mixes the cabin air (I used a small desk fan), the temperature at the thermostat, which is mounted portside near the top of the forward face of the breaker panel/stereo enclosure (not far from the companionway hatch and maybe a foot below the ceiling), is significantly warmer than lower in the cabin - by as much as 10 degrees. It was 73F in the mid-berth when it was 83F at the thermostat this afternoon. With the desk fan running, though, the temperatures were much more uniform and the compressor cycled when set at 78F. I think the thermostat will do better if it is mounted on the starboard wall above the foot of the V-berth; I'll try that soon.

On the list, too, is silencing the thing. I've lived with the compressor vibration for long enough. Has anyone tackled that?

Tom
Author:  wkearney99 [ Tue Jul 13, 2010 10:05 pm ]
Post subject:  Re: How effective is your A/C?
GTBecker wrote:
On the list, too, is silencing the thing. I've lived with the compressor vibration for long enough. Has anyone tackled that?

I've been considering changing how the AC unit it mounted. There are various ways equipment can be insulated against vibration. Pads under the whole thing, rubber suspension fittings, etc. But given the cramped quarters in which my AC unit is located (on a 348) I decided we could live with the noise. If the unit ever needs service that would require removal then I'm definitely going to look into ways to isolate its mounts.
Author:  GTBecker [ Thu Jul 15, 2010 7:14 pm ]
Post subject:  Re: How effective is your A/C?
wkearney99 wrote:
... Pads under the whole thing, rubber suspension fittings...


Yup. When I was last there, I picked up a second sheet of the resilient foam pad that Mermaid used in its installation. My intent is to make a ply/foam/ply/foam/ply sandwich that will allow mounting the bottom sheet to the boat and the top sheet to the A/C chassis, without common hardware. I think the plywoods should probably have differing masses, too.

Tom
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