March Pump already on A/C circuit for use on watermaker?

[S/V Alchemy]

A chance remark on a recent thread made me think that I could use a simple valve on my existing A/C seawater line to supply my watermaker (as yet unpurchased). It's a standard March pump feeding a Marine Air 12,000 BTU unit and is properly below the WL and is therefore self-priming off a standpipe. Fact is, I don't even use the A/C much, but I will be using the watermaker as our tanks on a full-keeler contribute to the ballasting. My pump is similar to this model:

Any objections to this approach? I have enough inverter power and battery capacity to run the watermaker on DC and the March pump via inverter. It would just simplify the plumbing in some respects.

[Nicholson58]

1. does the WM require a boost pump? It may be unnecessary. You may be able to locate the WM so that no boost is needed.

2. Will the 115 VAC pump supply the flow volume required? Not enough? Too much? If a boost is needed, most WM’s specify a centrifugal pump of specific capacity. A positive or fixed displacement pump may cause trouble.

3. Certainly no need to punch another hole in the boat.


Our Spectra 200T runs on 24 DC from solar. No boost pump. Intake about at the waterline but can run up to about a foot above. Pump is lower right.

[a64pilot]

It will easily supply enough water, question I have is will it supply enough pressure? Your looking for 5 or more PSI after both filters, usually.
March pumps are I believe high volume, low pressure pumps.
I think it will work, but not sure.

[S/V Alchemy]

The watermaker I'm looking at has an appropriate pump. Perhaps the tee-off I should use should be post-filter and pre-March pump, so I can make use of the one intake. Because if I'm making water, I don't need to have A/C, as I am likely underway and charging with the alternators.


So I may be, once again, overthinking something.

[S/V Alchemy]

Thanks for all the replies...

[Dartanyon]

Alchemy, we just removed our forward AirCon (keeping the aft one) and are using the thru-hull that supplies our AC March pump. As mentioned, our March is high volume / low pressure and would not be ideal for our Cruise RO Watermaker boost pump, although it was my first thought to use that pump. We made the split just after the thruhull with 2 valves (so that there is no possibility that the Watermaker pumps could pull air back through the AirCon system). We've only had it running a couple of months but everything seems to be happy.

[mvmojo]

I use my air conditioning pump as a boost pump for my 33 gph watermaker. It's a 33 gpm Little Giant pump, valved to supply 3 air conditioners and the watermaker. The only thing you need to determine is whether your March pump will deliver (through your pre-filters) 10-15 times the water that your watermaker will make. Most watermakers only produce a max of 10% product water and ideally you'd like to keep it down to maybe 6% or so. Starving the high pressure pump is a recipe for quickly ruining the pump.