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?

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.

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.

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.

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.

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.

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.

firstbase said:

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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If the 1620 gph pump above is a constant pressure as opposed to a constant volume pump, it won't hurt to be oversized!

slowgoesit said:

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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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!

firstbase said:

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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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.

Did you tell Depco exactly what your system is, or just ask about the pump?

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.

caltexflanc said:

Did you tell Depco exactly what your system is, or just ask about the pump?

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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....

Comodave said:

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.

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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.

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.

caltexflanc said:

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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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.

firstbase said:

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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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 big advantage to multiple pumps is they all probably will not suck in jelly fish at the same time.

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.

Woodstock said:

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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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.

FF said:

The big advantage to multiple pumps is they all probably will not suck in jelly fish at the same time.

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Thanks FF. I told the manufacturer rep that I didn't really want to add 2 more through-hulls and these are "exact" replacements for what I had and they worked fine. Why would I need to change the setup? He then suggested a manifold coming off my one strainer to the three pumps rather than separate through-hulls / strainers. Another first for me as I have not read anything about doing that.

LaBomba said:

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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Allan, my understanding is the water flows through all three units when one, two or all three are turned on. Am I wrong on that? There is nothing stopping water flow from the pump to the units, no valves. Would the units themselves have a valve that opens when it is turned on? I do know that when I check the flow overboard all three are flowing no matter which is on.

firstbase said:

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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I thought this was too big a difference too. So I went back and looked up the pump specs again and it is actually rated for 900 GPH. Still a fair amount of loss. I run into a header that splits the flow to the four units. This includes valves but they are all wide open at all times.

Woodstock said:

I thought this was too big a difference too. So I went back and looked up the pump specs again and it is actually rated for 900 GPH. Still a fair amount of loss. I run into a header that splits the flow to the four units. This includes valves but they are all wide open at all times.

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From my reading, I know the loss can be significant depending on specifics.

Another question I have is whether or not throttling down the flow out of the pump hurts the pump? Is it hell bent on living up to the specs and works harder to push water out when it is restricted? For my three I need 825GPH minimum. Lets say I make sure each is good to go and adjust valves to add up to 1100GPH. It's a 1620 GPH pump which actually pumps out say 1450GPH due to plumbing, 90 degree joints, etc, 10% loss. Does the throttling down by 350 GPH hurt the pump?

If needed, tell me to stop overthinking (again) and shut up. I'm good with that.

Definitely overthinking and some "be careful of" advice you are getting.


Understanding what is going on with the needed flow of water through each unit is what is important. The "throttling" of flow by valving after each unit makes sense to me "IF" there is too much to one unit and not enough to others. Like I posted...my is naturally throttled by length of plumbing.



I have a Marinaire system I installed 3 years ago and has worked as advertised.


6000, 9000, and 16,000 BTU setup. March 1000gph pump. PLUS...a relay system that Marinaire gave to me as a sales bonus to run one pump from 3 units.


I plumbed the system with shutoff valves to isolate ACs, but are left fully open as the natural length of runs reduces the flow to the units proportionately.

My salon AC is the largest unit and also the closest to the pump. With all the valves wide open, it has significantly more flow than the other three. The forward stateroom unit is the smallest and furthest run and also noticeably less water flow. I once tried to throttle back the salon flow thinking it would increase flow to the others but it made very little difference. It just reduced the overall flow. So I keep them all wide open and, like psneeld, the length of the individual runs proportionately and properly throttles the flow as long as everything is clean.

Get rid of the valves, and if you cannot, leave them open and don't screw with them. Really, there's nothing good that can happen with that extra complication.

According to Flagship marine for the possible "reducing of flow"...which is really a very low probability....


Under normal operating conditions, the condenser (the copper tube within a tube on the unit) should be warm to the touch – if you cannot feel any heat in the condenser whatsoever when the compressor is running, you have too much cold water passing through it. This is only a problem in some areas where the water temperature is 40°F and the air temperature is 90°F. This can be remedied by installing a gate valve on the supply line at the unit to throttle down the water flow. Remember to never restrict the SUCTION side of a centrifugal pump, conversely, there is no harm restricting the DISCHARGE side of a centrifugal pump.


https://flagshipmarine.com/installation-dos-and-donts/

This is only a problem in some areas where the water temperature is 40°F and the air temperature is 90°F.

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And where is that?

If you do decide to throttle the total flow, you will get the benefit of reduced power consumption too. Power consumption is proportional to flow in those kinds of pumps.