Vertical pump A/C losing prime! Solution?

I have a self contained add on of an air conditioning unit installed in the Aft enclosed room (I call it the sun room), the one that has the slanted doors. That unit has its own pump that kicks in when the compressor starts. It is located in the starboard side of the master /aft statroom. The problem is that on rough rides and if not being used for a while, it losses prime and the compressor would shut down for (HI PS) which means over heating compressor obviously after the water stopped pouring out for couple minutes. I am thinking of adding a one way check valve in a vertical part of the hose coming from the pump up to the unit. How do I know if the pump is not too weak for this vertical elevation of water (because I think that if I had stronger pump then I would not have such problem). What do you all think? Your suggestions? Thanks

If the pump is above the water line the prime is being siphoned out. If designed to be there, I would look for an air leak first. Trouble shoot it the same way you would a fuel line that looses prime.

The pump is at the level of the inlet, well below the water line.

AC pumps must always be below water line as they are not self priming.if the pump is in the master it has to be mounted as close to the hull bottom and as low as possible since the waterline is close to the hull bottom there

I m guessing the seacock is on the starboard side behind the head near where the head seacock is/used to be?

Also make sure you have the shortest possible hose run between the seacock, strainer and pump inlet with no high point. High points on the inlet sides are usually the cause for the pump not priming

I'm. Not a fan of check valves, they often stick or restrict flow. That said they are used on watermaker just down Stream of the raw water low pressure pump so that when beck flushing the fresh water can only flow towards the hp pump and membranes, not back out the thru hull. So since these low press pumps are similar to air con pump, a check valve should work but I'd first try to locate the pump below WL and eliminate any high point on the inlet side

the water line comes from the inlet, 1 foot later it enters the pump, then there is a T valve switch, was told by the previous captain of the boat to open in order to re prime the line (this is all under water line, then the line travels 15-20 feet up to the self contained unit and from there a second line parallels the first one down for the exiting water, and now to the side of the boat above the water line (6 inches). So it is true when I loose prime opening the T valve right after the pump releases some air until a little water comes out so that the system is primed again. I measured the amount of water and it is very minimal about 1/8th of a cup. I guess it is puzzling that when you loose prime and you open the T air comes out although it is all under waterline. Could the pump be defective somehow and leaking air? But then again why would it only loose prime when driving rough and on semi planning rides?

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Originally Posted by Pascal

AC pumps must always be below water line as they are not self priming.if the pump is in the master it has to be mounted as close to the hull bottom and as low as possible since the waterline is close to the hull bottom thereI m guessing the seacock is on the starboard side behind the head near where the head seacock is/used to be?Also make sure you have the shortest possible hose run between the seacock, strainer and pump inlet with no high point. High points on the inlet sides are usually the cause for the pump not primingI'm. Not a fan of check valves, they often stick or restrict flow. That said they are used on watermaker just down Stream of the raw water low pressure pump so that when beck flushing the fresh water can only flow towards the hp pump and membranes, not back out the thru hull. So since these low press pumps are similar to air con pump, a check valve should work but I'd first try to locate the pump below WL and eliminate any high point on the inlet side

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I forgot that we are very close to the water line. You are correct. I am going down there to re-assess. I will post back.

Attachment 23123

Attachment 23124


Ok, attached are two pictures one taken from top showing the seacock, the filter, the pump and the the T valve. Theother one is from aft looking forward. Any suggestion of whats wrong?

The highest area is the 90 degrees elbow above the seacock.

Not sure if this will help with your problem...I was at DEPCO pump recently to troubleshoot a problem AC pump.
They advised to not have a 90 degree close to the outlet if possible due to excessive back pressure on the Pump.
I also learned that those silver metal tubes that run from the impeller head are cooling tubes for the motor and they can become plugged.
Good luck. Capt. Rob

Looking your photos again I noticed the pump inlet is lower than the strainer outlet.
It needs to be higher (yet still below the waterline) so a continuous uphill to avoid air locks.
Rob

Pump is lower than the strainer... thats a problem. Also having the outlet existing sideway doesn't help purging any air.

Raise the pump so it's just higher then the strainer and rotate it 90 degrees so outlet goes up. I can see how it was easier to put side ways but building a small shelf is easy

As too why you're loosing prime underway, is there a scoop on the thruhull? If not there may a vacuum created at speed.

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Originally Posted by Gusshr

Attachment 23123

Attachment 23124


Ok, attached are two pictures one taken from top showing the seacock, the filter, the pump and the the T valve. Theother one is from aft looking forward. Any suggestion of whats wrong?

The highest area is the 90 degrees elbow above the seacock.

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I have the same pump supplying cooling sea water to a eskimo ice maker. I had the same problem with the pump. I Purchased a new one same problem. I got a wild hare and took it off took it apart and cleaned it
as best as i could. It had small grains of sand in it.It is very easy to take apart, and put back together. It has a small hard plastick impalla which workes on a magnet that is not very strong . Clean it and clean the strainer on a regular basis should work ok
As stated before be sure water is supplied to it as it will not prine by itself. That is probley what the valve is all about lets the air out and the water replaces it. Just try cracking the valve untill you see water it will probley catch if clean.