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Old 11-26-2013, 05:33 PM   #1
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AC Air Lock

I have a Marine Air 7000 BTU reverse cycle AC unit, with the March PML500L raw water pump. It has performed great in the two years that I've owned the boat, except for the last two times I ran it. Both times the pump refused to prime. Each time I've pulled the output hose off the pump, which broke the air lock, and it's worked great. I should mention that I trailer the boat, hence the air...

So my question is, should I suspect that the pump is getting weak, and if so can it be rebuilt or does it need to be replaced? Clearly I could close the seacock before pulling it out of the water, but I never needed to do this before and it always worked fine.

Thanks for your advice.

BD
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Old 11-26-2013, 05:44 PM   #2
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You probably always got lucky before or you might have changed the angle of the boat on the trailer and all the water didn't drain out.
The pumps generally are not self priming. When in the water, open the seacock valve, loosen up the nuts or whatever that hold the top on the sea strainer. let the water fill the sea strainer and overflow a little so you are sure the strainer is full. Tighten up the top nuts again to make sure there is no leak and you are good to go.
This is pretty much the same procedure when removing the filter from the sea strainer for cleaning. Except you start that procedure with the valve closed. when the filter has been cleaned, put it back in place, loosely put the top back on and then open the valve and let the water back in to fill the strainer bowl and tighten up.
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Old 11-26-2013, 05:48 PM   #3
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I don't know about that model, but many March pumps require a flooded inlet. In other words it needs to be mounted below the water line.

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Old 11-26-2013, 05:52 PM   #4
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Thanks. Yes my first check has always been the strainer, and it's always been full, so pulling the output hose was really just to see if the pump itself was pumping. Probably the strainer is right at the water line, so it fills itself but not any higher?

Anyway, thanks for the help. I guess I'll just have to train myself to close that seacock before I put the boat on the trailer.
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Old 11-26-2013, 05:53 PM   #5
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Quote:
Originally Posted by BobH View Post
I don't know about that model, but many March pumps require a flooded inlet. In other words it needs to be mounted below the water line.

Bob
BobH,

It is not below the waterline. That explains a lot. Thanks!
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Old 11-26-2013, 06:06 PM   #6
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Check valve?

Would an inline check valve make sense here?
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Old 11-26-2013, 08:01 PM   #7
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Probably not. It don't take but a small piece of trash to affect the check valve. Unless you are dealing with really clean water, check valves are a PITA.
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Old 11-26-2013, 10:11 PM   #8
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Probably not. It don't take but a small piece of trash to affect the check valve. Unless you are dealing with really clean water, check valves are a PITA.
Great point. Lake Superior is very clean but few would say the same about the Mississippi River. Accessing the seacock is also a PITA in this boat but I think I'll need to get used to it. Thanks.
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Old 11-27-2013, 06:38 AM   #9
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My new to me boat has one sea cock for the AC intake in a very difficult position. Unfortunately, I have to get to it at least once a month so I can shut it while cleaning/changing sea strainer filters. Some year I will relocate it to a more convenient place and then wonder why I didnt do it sooner. So for now, get used to it.
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Old 11-27-2013, 08:59 AM   #10
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I had the same issue. I inserted a tee with a valve to bleed the air so I don't need to remove the hose. I have a plastic threaded plug to keep in the open end of the valve, an empty plastic coffee can to catch the initial startup water if I am slow reacting. I also have a plastic tie wrap to hold the valve in the closed position (overkill likely). I don't need it often, but it does come in handy.
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Old 11-27-2013, 09:15 AM   #11
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I had the same issue. I inserted a tee with a valve to bleed the air so I don't need to remove the hose. I have a plastic threaded plug to keep in the open end of the valve, an empty plastic coffee can to catch the initial startup water if I am slow reacting. I also have a plastic tie wrap to hold the valve in the closed position (overkill likely). I don't need it often, but it does come in handy.
That seems like a really good solution. Taking the hose off is clearly going to break the fitting eventually, but a valve could be better mounted and be more convenient than the seacock.

Sadly it will be months before I see her again to do the work. Lots of lakes are already frozen here.

Thanks,
BD
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Old 12-01-2013, 01:23 PM   #12
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I don't trailer mine, but had the same problem after a day of running. I put a check valve first thing out of the seacock and solved that problem.
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