AC raw water pump prime

[Dallasron]

So after I installed the new AC unit (MarinAir 16,000btu) It is always a pain to get the pump (Little Giant) primed to push the water through the unit.

If I leave it overnight the next morning it is all good to go and as soon as i turn it on the water is pushed out the hull.

Should I maybe be looking to install a check valve?

Thanks in advance
Ron

[rslifkin]

Those pumps are generally not self priming. Is the pump installed below the waterline with no high spots in the plumbing between the thru hull and the pump? If not, the plumbing layout will need to be corrected for it to work reliably.

[SailingHarmonie]

Originally Posted by rslifkin

Those pumps are generally not self priming. Is the pump installed below the waterline with no high spots in the plumbing between the thru hull and the pump? If not, the plumbing layout will need to be corrected for it to work reliably.

This! The only change I would meke to this comment is to change

Those pumps are generally not self priming.

To:

Those pumps are never self priming.

[CaptTom]

I just mentioned in another thread, on my last boat I put a "T" fitting at the pump inlet, with a hose going straight up above the water line, with a valve on it. Opening the valve primed the pump. This assumes the pump is below the waterline.

[rslifkin]

Originally Posted by Dallasron

pump is below the waterline however the top coil on the unit may be above it a hair

It's ok if the A/C unit is above waterline as the pump can push upward from the output. It just has to be below waterline with no high spots to trap air between the thru hull and the pump inlet.

[SailingHarmonie]

Originally Posted by Dallasron

pump is below the waterline however the top coil on the unit may be above it a hair

There are additional requirements for reliable operation. All of these requirments are there to be sure the pump can fill itself with water and purge air simply by gravity. Pretty much every time you move the boat, air is going to get into the system, so it needs a way to rise through the plumbing and let the pump fill.

The pump must be below water level. The hose from the thruhull to the pump should be continuously rising, no high spots. So the pump should be above the thruhull and below the water level.

The discharge of the system must be above the static waterline.

The hose from the pump to the AC should be continuously rising, no high spots.

If the High point of the AC system is above the discharge, then the line to the discharge should slope downward to drain when stopped. If the AC is below the discharge, the hose should slope continuously UP to the discharge.

Heree is sombody else's explaination of the issues...

https://www.youtube.com/watch?v=A2JzRMOdAc0

In some systems you can cheat on some of these requirements a bit, but if these rules are followed, the pump WILL self-prime every time.

[Madehn]

Capt Tom had the right idea. Jose going up can be small. It is just to let air out.

[Bowdrie]

An easy first check is to make sure the water flow is correct as to the instructions.
Most units use a "counter flow" condenser, the water flow being opposite to that of the refrigerant.

[Dallasron]

I want to thank everybody and have ordered a 1/2" t stop valve to put on the pump out flow.

Should fix it and will update with a pic later

[CaptTom]

Originally Posted by Dallasron

I want to thank everybody and have ordered a 1/2" t stop valve to put on the pump out flow.

Should fix it and will update with a pic later

Good plan!

One quibble, and maybe it's no big deal. But will the valve be below the waterline? On mine I used a hose going directly up from a "T" at the intake, with the valve at the top of the hose, above the waterline. So if the valve failed or corroded, no harm done. Of course the "T" could fail too, but that's a lot less likely. And if you're the type who closes thru-hulls when not in use, it's a moot point anyway.

[Dallasron]

it will be below the waterline and will open it to a drip then close. Pump is mounted to the side of the bilge.

[boat-bidder]

I am having a similar problem with a 14yr old system. My A/C pump suddenly loses prime. When it runs it is perfect, but suddenly loses prime and shuts down the AC with a “high pressure fault”. This happens sitting at the dock with no movement whatsoever. It all worked well for 10 yrs I’ve owned it, and suddenly this happens. AC techs are all scratching their heads!
Any ideas?

[rwaldrop13]

Originally Posted by Dallasron

So after I installed the new AC unit (MarinAir 16,000btu) It is always a pain to get the pump (Little Giant) primed to push the water through the unit.

If I leave it overnight the next morning it is all good to go and as soon as i turn it on the water is pushed out the hull.

Should I maybe be looking to install a check valve?

Thanks in advance
Ron

First, I used Marsh AC pumps from BestBuyPumps.com (great service good pricing fast shipping) and never had to prime. The Marsh AC5 pushed water over 80 feet from the midships generator space to the aft stateroom AC and back to the discharge in the generator space without a problem. That being said I would say you're already in a bad way with a MarinAire unit. The 16K unit I bought from them was dead out of the box, and they tried to charge me for the repair part and shipping. Then when the pump control relay was also bad, they told me they were voiding my warranty and if I didn't shut up and go away they'd sue me- Which they did when I filed with my credit card because of the continuing unit failures. The unit I bought from what was Mermaid Manufacturing here in Ft Myers outperformed their unit, with no issues and outstanding support. They're now https://fmmsusa.com/ for whan that MarinAire unit quits on you.

[Jetx]

Another possibility is air-lock. To explain....

We have 2 aircon systems on board. On both systems, the pump is below the waterline, and the flow path -- from through hull -> hose -> strainer -> pump -- is continously uphill with no place to trap air. Both pumps are always self-prime. One system never has a problem, while the other one refuses to pump water every now and then... generally if it hasn't been run in a month or more. After futzing with this several times, I finally realized that the problematic A/C unit is installed higher in the boat, and occasionally gets air-locked.

Basically, even though the pump has prime, there is air between it and the A/C unit, and the condenser coils (wound vertically, with inlet on bottom and outlet on top) are only partially filled with water. With multiple coils, and multiple "slugs" of water separated by slugs of air, the effective head that the pump has to overcome increases dramatically. The centrifugal pump just doesn't produce enough head to compress all of the air and push the multiple trapped slugs of water over the top of each coil. (This probably all sounds like BS, but sometimes happens in cross country pipelines after maintenance work.) It doesn't happen in normal operation because most of the condenser coils stay full of water between run cycles.

Anyway, the fix in our case is to close the inlet through hull, and use a shop vac to blow the water out of the condenser coils. The unit then starts normally, and will continue running without hickups until I shut it down for an extended period (months typically).

I have one of these (https://defender.com/en_us/trac-ecol...ushcap-adapter) that I screw into the top of the strainer so I can shop-vac-purge without having to remove any hoses. Also useful for blowing out clogged through-hulls with air or with a garden hose. Could of course make your own, or plumb in a permanent fitting to achieve the same thing.

[GordMay]

There are several things you can do, to find what might be causing your pump to lose its prime:

Check for leaks: 90% of all pump problems occur on the suction (intake) side of the pump. Leaks on the pump’s intake line, as well as around the shaft seal [of the pump housing, itself], can cause your pump to lose its prime. Ensure that the pipe connections, threaded into the housing, are tight, and that the intake line is not cracked, or has any loose or poorly glued fittings.

Valves: Most pumps are equipped with foot valves, to keep water in the intake line, and casing, when the pump is not running. If your foot valve is leaking. your pump may lose its prime between starts. Replacing a faulty valve may solve the problem.

Obstructions: One possible reason your pump may be losing its prime is an obstruction, or blockage, in a line. Debris, blocking the suction strainer, or foot valve, is the most common cause. A blockage, in the intake line, can cause the water [in the pump casing] to overheat, and literally boil out of the casing, causing the pump to lose prime.