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Old 13-08-2014, 14:34   #1
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Water throughput for an A/C unit

I am planning on installing a Webasto 9000 BTU A/C unit, but I cannot see in their spec how many gallons per minute of cooling water this will require.
Has anyone else installed one of these? Some boats in the marina seem to have a ridiculous amount of water going through.
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Old 13-08-2014, 16:45   #2
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Re: Water throughput for an A/C unit

Is that an old unit. I don't see a 9,000 btu unit listed in their data sheet. However, download the data sheet for the S units and it has flow requirements for each size of AC unit.
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Old 16-08-2014, 07:52   #3
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Re: Water throughput for an A/C unit

To be on the safe side I would install a 500gph pump. I have two 16k units with a 1000 gph pump.
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Old 16-08-2014, 08:20   #4
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Re: Water throughput for an A/C unit

There is an FCF 9000, I put in a 5000 in and agonized on whether or not to go with a 9000, in retrospect I should have as it's not much more electrical draw.
The pump that comes with the factory kit is a Chinese made pump, I believe it's actually intended to be a fountain pump, My thru hulls for the AC's are right next together and from the appearance it looks like it's water output is about half of the bigger AC's which has a 500 GPH March pump on it.
When the Chinese pump goes out I'm going to replace it with a 500 GPH March pump partially to keep the same pump on both AC's and because I think the March pump is a good unit.
I believe pump size is water temp and how much you need max output form your AC dependant, in other words if your running AC wide open and it barely keeps up and your water temp is mid 80's or above you need a whole lot more pump than the person on the Great Lakes
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Old 23-08-2014, 22:55   #5
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Re: Water throughput for an A/C unit

From a engineering standpoint, the minimum flow rate required for a 9000 btu a/c with a 10 degree delta T (conservative) is 2 GPM or 120 GPH.

A march 500 GPH pump probably only puts out 300 ish GPH at the pressure head needed. You have to plot in on the March Pump curve available here:Magnetic Drive Pumps | Mag Drive Pumps | March Pump Select the DC pump curve.

A 16,000 BTU unit would need a minimum of 200 GPH or 3.2 GPM.
A 24,000 BTU unit would need a minimum of 300 GPH or 4.8 GPM.

For small marine A/C, assume 5 to 8 feet total system head. You find the pump that is correct where the pump curve is to the right of the intersection of head / GPM and pressure. Do not select a pump with a curve to the left of the system design point.

If your in 90 degree water a lot, double the flow rate. The 10 degree Delta assumes 85 degree F max condenser supply temperature.

If you need a real pressure drop calc on a system, drop me a PM
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