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Old 02-21-2020, 09:48 AM   #1
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RW pump for A/C units

I am replacing my old A/C with 3 MarinAir units, 6K, 11K and 16K BTU's. Water flow from specs totals 825 gph (150, 275, 400 gph). Going to replace my 1050 GPH "Little Giant" pump with a new March pump. I am wondering about the GPH for the new one. I have one pump for all three A/C's.

The manufacturer says I need 1300-1500 GPH for these 3 units together. March makes a 1500 GPH pump but is that TOO big for specs stating 825GPH? I know 1500 GPH isn't exactly 1500 GPH and I will lose some more with the plumbing, 90 degree turns, etc. Is it possible to oversize and hurt the units in some way? Any other options you would suggest for a smaller pump from another high-quality brand? What exactly do I look for i.e. magnetic, ceramic, seal-less, etc?
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Old 02-21-2020, 10:03 AM   #2
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It would appear that your little giant pump has the flow capacity for all three and unless there are other reasons you want to change out this pump, I would give it a try first. Do you run all 3 at the same time? The worst that can happen is one may go out on high temp.
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Old 02-21-2020, 10:05 AM   #3
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For the most part, more flow is better. I seriously doubt you'll have any issue with the bigger pump. Real world flow will be less than the nameplate rating due to plumbing restrictions and that pump won't push enough pressure to hurt anything in the system.
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Old 02-21-2020, 10:08 AM   #4
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Thanks for the response. My replacment idea was based on the manufacturers suggestion that I needed 1300 gph minimum as well as not knowing the age of my current pump. Not new by any means so I figured I am replacing everything else replacing it would be a good idea.
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Old 02-21-2020, 10:41 AM   #5
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As witnessed in another recent post, why not just call Depco over in Clearwater and they will tell you exactly what you need. They're extremely knowledgeable and helpful. Fair prices too.
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Old 02-21-2020, 12:57 PM   #6
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For what it's worth, our 430 Mainship has three A/C units (don't know what size they are, sorry) and they are all supplied by a Cruisair PMA1000C pump. It's rated at 1000 GPH. I can run all three at the same time with no problems.
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Old 02-21-2020, 04:53 PM   #7
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Thanks for everyone's responses. I did call DEPCO and they said that the March model number I was looking at was 1800 GPH not 1500 GPH. March does make a 1620 GPH. Too big? How to control the water to each unit. I know I really need to install separate pumps but now into changing plumbing off the strainer to separate pumps and finding space for the additional pumps. Never ends. I think I will hook the new ones up to my current pump and see what happens...as was suggested above.
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Old 02-21-2020, 05:02 PM   #8
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Thanks for everyone's responses. I did call DEPCO and they said that the March model number I was looking at was 1800 GPH not 1500 GPH. March does make a 1620 GPH. Too big? How to control the water to each unit. I know I really need to install separate pumps but now into changing plumbing off the strainer to separate pumps and finding space for the additional pumps. Never ends. I think I will hook the new ones up to my current pump and see what happens...as was suggested above.
If the 1620 gph pump above is a constant pressure as opposed to a constant volume pump, it won't hurt to be oversized!
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Old 02-21-2020, 06:46 PM   #9
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If the 1620 gph pump above is a constant pressure as opposed to a constant volume pump, it won't hurt to be oversized!
I sure wish I knew the difference. It is a March centrifigul, magnetic pump. It says that it can withstand 50psi internal pressure. Thats all I know!
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Old 02-21-2020, 07:03 PM   #10
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I sure wish I knew the difference. It is a March centrifigul, magnetic pump. It says that it can withstand 50psi internal pressure. Thats all I know!

It's a constant pressure. Constant volume pumps are normally gear or gerotor pumps. centrifugal pump is almost always constant pressure. You should be fine.
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Old 02-21-2020, 07:40 PM   #11
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Did you tell Depco exactly what your system is, or just ask about the pump?
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Old 02-21-2020, 07:59 PM   #12
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I think you will be fine with the one pump since it is rated for more than what you need. You may need to have some type of manifold with valves in order to force water to the further units if one of the A/Cs has a low resistance path so too much water doesnít flow out of that A/C.
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Old 02-21-2020, 08:17 PM   #13
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Did you tell Depco exactly what your system is, or just ask about the pump?
I went through my application and specifics on what i was told by Marinaire. The guy with DEPCO looked up several pumps trying to sort it out and came back with the 1620 GPH suggestion telling me it was what I needed.

Then the engineer from Marinaire, having been briefed by the person I spoke with when I picked up the units, called me. He told me that I needed 3 separate pumps, a manifold off my strainer, and told me all the reasons this is better. You probably already know what they are. He said that if I had to use just one pump I need to add ball valves to control the flow in each unit. He said to add a valve between the unit discharge and the overflow through-hull. Not between the pump and the unit, after each A/C. Then I should measure the water flow that goes overboard and adjust the valve to get the correct GPH.

I told him that I purchased pretty much exact replacements for what I have.
He questioned me on what pump they were running on (Little Giant 5-MD-SC @ 1050 GPH). He then told me to hook up the new units to this pump and check the pressure on the gauges that come on the units. "It may be fine."

So, I have made great headway today. At least I got down to Miami and bought the new ones....
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Old 02-21-2020, 08:51 PM   #14
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I think you will be fine with the one pump since it is rated for more than what you need. You may need to have some type of manifold with valves in order to force water to the further units if one of the A/Cs has a low resistance path so too much water doesn’t flow out of that A/C.
The pump is in the engine room about midship both bow/stern and port/starboard. The 16K BTU and the 11K BTU units are located maybe 10' water line run form the pump. One in salon and one in aft stateroom. Same height above the pump. The longer run is the 6K BTU unit in the v-berth which is probably twice the distance of the others.

I don't understand the reasoning behind ball valves after the water discharges. Seems that valves at the manifold I have omcing out of the pump to direct water three ways would be the solution. However, as you can tell, I know little about pumps, hydrodynamics, etc.
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Old 02-21-2020, 09:21 PM   #15
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That all sounds weird to me. We had five units that ran very happily fed through a valveless manifold from one Oberdorfer pump. Yes, we did carry a spare. I just don't understand what that Marinaire guy is thinking or smoking. You may want to talk to a good marine AC shop, though I would be inclined to take the Depco guy at his word.
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Old 02-21-2020, 09:58 PM   #16
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That all sounds weird to me. We had five units that ran very happily fed through a valveless manifold from one Oberdorfer pump. Yes, we did carry a spare. I just don't understand what that Marinaire guy is thinking or smoking. You may want to talk to a good marine AC shop, though I would be inclined to take the Depco guy at his word.
I did take the NDEPCO advice and ordered the pump he recommended. The MArch 1620 GPH. I ordered it at 4:30pm and just got UPS notice that it will be delivered tomorrow. Gotta love a company that ships to you on a Friday afternoon at 4:30p.

What I will do is install the new units with the old pump just to see what sort of pressure/flow I get on the gauges. Hopefully curiosity doesn't kill the air conditioner. Or something like that.
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Old 02-21-2020, 10:26 PM   #17
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I did take the NDEPCO advice and ordered the pump he recommended. The MArch 1620 GPH. I ordered it at 4:30pm and just got UPS notice that it will be delivered tomorrow. Gotta love a company that ships to you on a Friday afternoon at 4:30p.

What I will do is install the new units with the old pump just to see what sort of pressure/flow I get on the gauges. Hopefully curiosity doesn't kill the air conditioner. Or something like that.
What kind gauges are you using to measure pressure/flow? I'm curious to hear what you are or will be getting for actual flow.
We have 4 AC units and recently installed a flow meter on the single main centrifugal pump that is rated for 1500 GPH. I'm actually getting right at 500 GPH total actual flow but that seems to be enough to do the job. System was recently descaled so I'm assuming this is typical.
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Old 02-22-2020, 07:55 AM   #18
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The big advantage to multiple pumps is they all probably will not suck in jelly fish at the same time.
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Old 02-22-2020, 08:42 AM   #19
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Multiple pumps, multiple through hulls (intake and, typically output), multiple hoses and fittings, multiple wiring.... all taking up more space, some of it likely in hard to access locations A sea chest helps a bit.

Advantages and disadvantages to either approach, no question.

I noted that Hatteras, in a variety of very well ergonomically designed boats, Hargrave architected, used the single pump / manifold approach. On the bigger boats like ours, there was certainly plenty of room for all the extra stuff required for the multiple pump approach. They made access to the pump and the intake extremely easy.
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Old 02-22-2020, 10:59 AM   #20
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What kind gauges are you using to measure pressure/flow? I'm curious to hear what you are or will be getting for actual flow.
We have 4 AC units and recently installed a flow meter on the single main centrifugal pump that is rated for 1500 GPH. I'm actually getting right at 500 GPH total actual flow but that seems to be enough to do the job. System was recently descaled so I'm assuming this is typical.
The gauges are on the yet to be installed A/C units themselves, not a separate guage. The 16K and 11K have them. The 6K BTU for the v-berth does not.

500GPH out of 1500GPH pump seems very low. I know there is loss due to head, plumbing etc. but from what I read that is more along the lines of 10% or so depending on specifics.
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