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Old 09-24-2018, 04:17 PM   #1
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A/C Pump Loosing Pressure

I have a AC-5CP-MD March pump running 3 units. Pump will run for a few hours with great pressure, then will start loosing pressure, the longer it runs the less pressure until it is just a drip. This a new pump, bought one about a year old ago and they both do the same. Was told that it a flooded pump, I put values in each line to restrict the flow. Not any better.
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Old 09-24-2018, 04:46 PM   #2
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I have a AC-5CP-MD March pump running 3 units. Pump will run for a few hours with great pressure, then will start loosing pressure, the longer it runs the less pressure until it is just a drip. This a new pump, bought one about a year old ago and they both do the same. Was told that it a flooded pump, I put values in each line to restrict the flow. Not any better.
Valves? What kind? Where? and what for? The only valve should be at the seacock. Sounds like you have a plumbing issue inducing an airlock.
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Old 09-24-2018, 05:22 PM   #3
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Ball valves to slow down the flow in the discharge lines. Was told that if the discharge lines are not correct that the pump will pump to much water and pump will lose its prime, it is a flooded pump not a suction pump. Had a A/C Service change all water lines, intake and discharge, still losing pressure.
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Old 09-24-2018, 05:52 PM   #4
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Ball valves to slow down the flow in the discharge lines. Was told that if the discharge lines are not correct that the pump will pump to much water and pump will lose its prime, it is a flooded pump not a suction pump. Had a A/C Service change all water lines, intake and discharge, still losing pressure.
Find a new AC tech.
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Old 09-24-2018, 06:18 PM   #5
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Typically A/C pumps need to be below the waterline since they will not self prime. Also the hoses need to be on a constant uphill route from the through hull to the pump. If there are low spots, air can be trapped and the pump can loose prime. What size is the through hull? Maybe it isnít large enough to supply the pump.
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Old 09-24-2018, 06:32 PM   #6
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I had exactly the same problem running 2 AC units.
After a whole big runaround, turned out to be air was getting in at the strainer.

Check all your connections from pump to thru hull. I only found the problem when I bypassed the strainer. It was an old hose on the pump side of the strainer had become too rigid and let just a little air in, so that after a few hours, the flow would decrease and eventaully stop as air accumulated in the top of the system.
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Old 09-24-2018, 06:34 PM   #7
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Find a new AC tech.

Yes, total BS. But do make sure that you have no restrictions on the suction side of the pump: the through hull, the strainer, tubing running upward from the strainer to the pump.



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Old 09-24-2018, 07:25 PM   #8
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Originally Posted by kartracer View Post
Ball valves to slow down the flow in the discharge lines. Was told that if the discharge lines are not correct that the pump will pump to much water and pump will lose its prime, it is a flooded pump not a suction pump. Had a A/C Service change all water lines, intake and discharge, still losing pressure.
OK, that's a new wrinkle. I'm with the find a new tech group. It's amazing the unvarnished CRAP a marginally experienced tech will spout to divert attention from their lack of knowledge. In this case, your tech is in dire need of some schooling.

Ditto all the previous posts on the suction piping. There can be air ingress with no symptom of a leak. Even if the line is pressurized, it can sometimes not show a leak yet allow air into the head of the pump. If that happens, the air can trap and accumulate with the eventual result that the pump starts to cavitate and at that point, it stops pumping. That's likely what's going on. You should be able to hear if the pump is cavitating. A couple of things...
  • Your pump has a 1" suction. The piping from thru hull to the head of the pump should minimally be FULL SIZE. Fittings should be full size with no reduction of ID. I'm a fan of piping the suction a size larger than the pump inlet, and use "full flow" adapters (look at Groco FF fittings).
  • It is helpful to pipe the discharge from the pump head vertically at least 4-5 pipe diameters to assist with air purging. There should be a steady slope up from thru hull to suction connection of the pump.
  • The strainer is the most likely source of air ingress, but barb (or any) fittings can also leak air in, and show absolutely no leakage otherwise. Keeps it interesting.
  • There should be no other fittings or tees to other pumps on the line from thru hull to pump head. Yeah, it can work, but it's an invitation for trouble!
  • Unless there is a serious piping imbalance in the discharge piping, get rid of the ball valves, particularly if they aren't full port valves. That's not the cause of your problem.
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Old 09-24-2018, 08:03 PM   #9
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This was the third tech that looked at the system, all had a different opinion and solution but none worked.
I think Wxx3 may be on to something.
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Old 11-01-2018, 11:32 AM   #10
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You are getting air into the suction side. Had this same experience, My strainer lid gasket was letting air in.
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