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A/C Seawater Pump Size Question

JRacer

.
Aug 9, 2011
1,239
Beneteau 310 Cheney KS (Wichita)
My First 310 has an older 12,000 BTU Marine Air AC Unit (circa 1997). The seawater pump it uses is a 10 GPM March Pump. I have no idea whether this pump is the one installed when the AC unit was put in or if it is a later replacement. I am not impressed with the amount of water I see coming out of the side of the boat (relative to others I see on the dock). As yet, I have not attempted to try to see how much actual GPM it is producing.

Question One is: Is a 10 GPM pump a "proper" size pump for that sized AC unit?

Question Two is: What is the proper sized pump for a 16,000 BTU AC unit?

My installation only has about a maximum of a foot of "lift" from the through hull to the pump so the pumps should operate at max GPM.

What I am thinking is that if I need to replace the pump now, I might as well go with what would be a proper size for the next larger AC unit as that is probably what I would replace the aged 12,000 BTU unit with when it is time.

What say the knowledgeable Sailboat Owners out there.
 
Sep 25, 2008
6,315
Alden 50 Sarasota, Florida
Having just done this research in prep for replacing one of our A/C units, I can offer the following from our local A/C expert:
for a 12K BTU unit - 180 gph pump with a min 1/2 inch hose
for a 16K BTU unit - 360 gph pump with 3/4 in inlet hose


Your 600 gph pump should be 'over-kill' and if operating properly, should sound like a waterfall
 

JRacer

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Aug 9, 2011
1,239
Beneteau 310 Cheney KS (Wichita)
Thanks, Don. My current pump is 3/4 inlet and 1/2 outlet. Good to know it should be sufficient when the time comes to replace the AC unit. Now, guess I need to measure what it is actually putting out - maybe a "wet end" rebuild kit (impeller, etc) is needed. I suppose the old AC unit could be clogged up in it's piping and restricting the flow. Another thing to check, I guess.
 
May 17, 2004
3,483
Beneteau Oceanis 37 LE Havre de Grace
Our boat came with a 16K BTU AC and a Dometic PML 500 pump. The specs on the Dometic pump show it as 510 GPH and adequate for 24K BTU AC units. Those specs sound consistent with Don's research. Not sure why Beneteau uses it rather than something smaller given the AC size.
 

Gunni

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Mar 16, 2010
5,937
Beneteau 411 Oceanis Annapolis
That is roughly the March pump sizing for my 12,000/8,000 dual system A/C and it has two discharge circuits. You need to consult the A/C manufacturer flow specifications because too much flow can cause problems with the designed heat transfer, just as too little flow can cause problems.
 

capta

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Jun 4, 2009
4,309
Pearson 530 Admiralty Bay, Bequia SVG
You must take into account "head" when sizing a pump for any job. A pump rated at 600 gph pump might only pump half that with a few feet of head.
The March pumps are the common OEM installed AC pumps. Little Giant makes suitable replacement for a lot less.
When you see AC water streaming out of a hull like a fire hose, it's most often because they have used too small an exhaust thruhull or water lines. The biggest mistake folks make is to assume the more water flow the better. Not true. Read the manual on your AC unit, figure your 'head' if any, then pick up the appropriate pump.
If your AC is cooling well, then you probably do not have a water flow problem. The most common AC problem is growth in the cooling water lines. A wash with Mary Kate ON & OFF will do wonders. Put intake and exhaust hoses in a bucket filled with a little water and ON & OFF and flush for a few minutes. Muriatic acid works too, but it is very dangerous to work with, or even have around on a boat.
 
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JRacer

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Aug 9, 2011
1,239
Beneteau 310 Cheney KS (Wichita)
Thanks, all.

AC works but IMO doesn't cool as well as I think it should. Of course it's 95-100+ here these days so there is that side of the equation. Don't know off the top how cold it is coming out of the ductwork, probably need to stick a thermometer in it and see.

Capta, yes, lift is the term I was using above to refer to "head". We have at most a foot between the bottom of the through hull and the level of the pump which, when looking at the charts on the pump, results in a negligible decrease in volume. From there to the exit through hull, there is probably another foot. Good idea on the Mary Kates circulation - will do that. Maybe ought to do that before I head down the road of rebuilding the pump with a Wet end kit. Only real concern with the pump is that it is probably the same age as the AC unit so about 20 and I have no idea how it was treated for the 15 years before I got the boat. Just wondering if the impeller is due for replace and maybe has been run dry at some times over the years (who knows). The boat is a 92 and the AC unit was added in 97. The manual I have on the AC unit is not very helpful. Tells you how to install it but not very expansive on "specs", like silence on the recommended pump size/flow volume.
 

Gunni

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Mar 16, 2010
5,937
Beneteau 411 Oceanis Annapolis
With a 20 y/o marine A/C unit you should go ahead break out the bucks and have a marine AC guy inspect and gauge the pressures, seawater flow and give you a report. You are very close to EOL (end of life) on that thing. A total systems replacement would be better than parting it.
 

JRacer

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Aug 9, 2011
1,239
Beneteau 310 Cheney KS (Wichita)
With a 20 y/o marine A/C unit you should go ahead break out the bucks and have a marine AC guy inspect and gauge the pressures, seawater flow and give you a report. You are very close to EOL (end of life) on that thing. A total systems replacement would be better than parting it.
Yes, I know that Gunni. Much easier said than done inland. But, I am in search of someone who can pressure test it. But, don't want to spend 2K replacing the whole thing at the moment if I have a 100-400 problem. Eats into my go fast equipment budget. ;-)
 

Gunni

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Mar 16, 2010
5,937
Beneteau 411 Oceanis Annapolis
Ha, I saw your location after I posted that! I hear there is a "marine A/C guy" in every Kansas seaport! Good luck, stay cool.
 

JRacer

.
Aug 9, 2011
1,239
Beneteau 310 Cheney KS (Wichita)
Ha, I saw your location after I posted that! I hear there is a "marine A/C guy" in every Kansas seaport! Good luck, stay cool.
Yeah, it's not like Annapolis where you can't swing a dead cat without hitting one! ;-) We are surrounded though - Grand Lake in OK and Lake of the Ozarks in MO each only 4 hours away.
 

capta

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Jun 4, 2009
4,309
Pearson 530 Admiralty Bay, Bequia SVG
Most AC pumps are magnetic drive centrifugal pumps. I do not believe you will find a rebuild kit or an impeller for that type of pump. Possibly a seal kit is available, though I've never seen one.
One other factor, I believe you are on a lake? If the water is too warm these units will not cool well, no matter how much circulation water it has. Same with cold, but that's not our focus here.
After a haul out many AC pumps will require bleeding to get them flowing again. Don't think your pump has gone bad.
PS. Air conditioning and refrigeration are the same principle, no matter whether ashore or on your boat. The important information a qualified and capable tech needs should be available from the manual, online or the manufacturer. If a guy knows AC, he could probably fix the AC's on the space station.
This ain't rocket science. As it was explained to me by a refer tech on my ship in 1965, "We light a fire to make cold." Got that? If so, you are half way to becoming a refer tech, yourself. lol
 
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Likes: JRacer
Nov 6, 2006
9,226
Hunter 34 Mandeville Louisiana
J Racer.. I had to replace my compressor a couple of years ago .. I took the unit out and took it to a local shop so that they could do the silver solder and pressure test and evacuate and charge.. I'd dropped by and talked to the service guys before taking it there.. Compressor failed because standing condensate water corroded a pin hole through the case.. The base is now shimmed to prevent standing water. I bought the compressor on line and brought it in with the unit.
 

JRacer

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Aug 9, 2011
1,239
Beneteau 310 Cheney KS (Wichita)
Capta:

Good thought on the warm water - hadn't considered that. There is a "wet end kit" available for my AC-3C-MD pump that includes the impeller and all the O rings, etc to refurbish the guts of the pump end of things.

http://www.marchpump.com/site/files...h-series-3-pump-manual-0130-0032-1000-r30.pdf

Think your "clean the pipes" idea is a good place to start as I'm sure its never been done. For sure has not been done by me and doubtful its ever been done from what I know of the PO. Then I'll measure the flow out of the pump and see how much it's producing and decide on the refurb of that - will want to know that is in good working order in any case. Same time try to locate a refrigeration guy that can measure the pressures on the box and see if it's up to snuff or if she's just old and tired like the rest of us.
 

JRacer

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Aug 9, 2011
1,239
Beneteau 310 Cheney KS (Wichita)
Anybody have a clue where to find tech specs on an old Marine Air Systems "Cool Mate" CLM12KC(Z). I've got the Install/Operation/Maintenance Manual but it doesn't give any information re: pressures, etc. Marine Air is now owned by Dometic.
 
Oct 29, 2016
1,821
Hunter 41 DS Port Huron
Vertical lift is not the only consideration for a head pressure calculation, you also have to consider the relative roughness of the hosing and number of bends and the degrees (ie 90*, 45*) and the radius of the bend. The hosing will be stated in PSI / 100'. Good luck with your adventures.
 
May 17, 2004
3,483
Beneteau Oceanis 37 LE Havre de Grace
Capta makes a good point about priming the pump. Without that the flow could be very intermittent and reduced. We have a bleed valve just after the pump that we need to open and let the pump run for about 30 seconds to bleed all the air after a haul out.