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Old 10-12-2017, 12:09 PM   #1
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Odd Air Conditioner Behaviour

The Cruise Air Salon AC on our Mainship 400 has been acting oddly recently.

About 30% of the time about 10 minutes after I first start up the A/C the unit shuts down and I get a high pressure reading. If I wait a few minutes, then restart it, it runs fine for as long as it is left on. The Fwd unit, which runs off of the same raw water pump doesn't do this.

I figured I had some blockage in the coil, so I flushed the system with barnacle buster but it didn't help. There is very good water flow. I also tried backflushing the system with a hose.

Maybe it's some sort of sensor going bad? If it was a flow problem wouldn't it show up all of the time?
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Old 10-12-2017, 02:01 PM   #2
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you can also get a high pressure reading when the unit freezes up - often due to poor air flow. Although it doesn't sound like that's your issue if it will work again so quickly. Usually a freeze-up takes 30 mins + to resolve.

wish I could be of more help! good luck. Let us know what you find
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Old 10-12-2017, 03:07 PM   #3
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Yeah, I literally turn the breaker off, then back on again, and it runs fine for an unlimited time after the very short built in delay for restart. It just feels electrical to me.
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Old 10-12-2017, 04:48 PM   #4
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Actually, if your unit freezes up or has poor airflow it will cause a low pressure situation. Your high pressure alarm would be caused by low/no water flow which you've already addressed. Also, a floating restriction in the refrigerant could cause these symptoms. I think you're on the right path with the sensor based on what you described. Put a set of gauges on and monitor it on the initial start up. If it's an R-22 system you should be under 250 PSI on the high side. If you get a high pressure alarm at that PSI or lower it would most likely be the sensor. I would check the wiring from the sensor for loose connection or corrosion before replacing.
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Old 10-13-2017, 03:31 PM   #5
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Hey CLChange,

Can you give me a general idea of what the sensor looks like, or where it might be located?

Thanks,
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Old 10-13-2017, 09:00 PM   #6
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As previously posted, you really need to make the diagnosis regarding the cause of the trip out. It's entirely possible the pressure switch (generally not referred to as a sensor) is tripping prematurely, but the only reliable DX is to have a set of gauges on the system and get a reading on the pressure when the switch trips. If you need instruction on how to, you should probably call a service tech. It's not terribly complicated, but there are many variables and you can guickly make a mess of things if you're inexperienced with the process. If you do call in help, resist the urge to offer a diagnosis; just tell them as accurately as possible what you've observed without drawing any conclusion.

The high pressure switch will be on the high side refrigerant line. If it's a heat pump ("reverse cycle") it will probably be located downstream of the compressor and ahead of the reversing valve. It will likely have a pair of wires coming out of the switch, and the switch itself may be about 3/8" dia. with a 1/4" flare connection to the sytem tubing. The high side tubing will be the smaller tubing, and it will be hot to the touch, maybe blister hot, depending.
Generally, the switch can be changed out by removing it from the flare connection, but it requires removal of the refrigerant from the whole system, a complication that really is more trouble than the switch is worth, because typically, the flare fittings that the pressure switches attach to don't have a schraeder valve, they're straight open, so if you crack that fitting, you'll have high pressure refrigerant blowing, and you CANNOT change them "on the fly". If the switch is in fact the culprit, there's an alternative- If the system has service ports- add a flared tee to the high side service port and install the switch on that tee, then simply abandon the faulty one. But there are some caveats to that option, so not always workable. For the purposes of this discussion, it's helpful to know that it's an option that a knowledgeable service tech should be familiar with.

Good luck with the solution.
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Old 10-14-2017, 08:09 AM   #7
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You might also check that there is not an air "lock" in the inlet water strainer. We had persistent issues with the inlet strainer apparently capturing air while underway. This would leave a small air gap at the top of the strainer that was enough to cause upset with the pump feeding the AC units. If your problem is at all similar, simply stopping and restarting might be enough to "jolt" the air through the pump. Suggest removing strainer cap and checking water level before next attempt at running AC.
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Old 10-14-2017, 08:32 AM   #8
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I had a brand new A/C start to trip on HP. Well, I cleaned the sea strainer.... back flush out to the hull intake, via an adapter that screws on the top of the strainer. Still, HP shut down. Because it was a new unit, I called the tech out. He did everything he could think of. I suggested pulling the hose between the hull valve and the strainer to see if it was clogged..... Sure'nuff, somehow the S/W pump had sucked a half of a grocery store plastic bag through the inlet grate and it lodged in the hose. He removed the bag, check the hose and presto, the A/C unit works fine.
I too have one S/W pump to supply 2 A/C units. The flow was good enough for one unit but not two. The S/W overboard visually appeared as usual, prior to removal of the plastic bag too.
One of life's mysteries.
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Old 10-14-2017, 09:29 PM   #9
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Thanks guys, I'll try those things. I'm pretty sure the water level is fine, but I will check it. I do have gauges, but it's been a long time since I used them. I may have to call in a tech.
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Old 10-14-2017, 09:46 PM   #10
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One other thought.

We have two sets of two AC units (4 in total). Each pair is supplied by a raw water pump. The port side units are for the pilothouse and salon. The starboard side for the two cabins.

Never an issue with the port side. But we did have the HiP issue with the starboard side. I found that there was air in the line between the strainer and the RW pump, meaning it would not prime. The previous owner had put bleed hoses in-line just before the pumps on both sides. I found that if I opened the bleed line until I got a steady flow of water then the AC ran fine.

We finally reset the strainer lid and the clamps from the strainer to the AC and problem resolved.

You may have air in your system before the RW pump and it is not pulling RW to cool, and shutting down the system.

Check that you are not getting an air lock in the system.
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Old 10-14-2017, 11:38 PM   #11
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Wouldn't an air lock cause a consistent failure? Not an intermittent one?
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Old 10-15-2017, 07:43 AM   #12
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Our "air lock" typically occurred after running the vessel. It seemed as though some air entered through the AC inlet to the strainer. Just an inch or two of air in the top of the strainer would cause the problem. Once air was removed by loosening strainer top, the AC would run fine again. Did not happen every time but often enough we learned how to recognize and deal with it.
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Old 10-15-2017, 06:09 PM   #13
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How hot does the outlet water feel? Should not be much more than 100F.
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Old 10-15-2017, 09:04 PM   #14
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Quote:
Originally Posted by Chrisjs View Post
You might also check that there is not an air "lock" in the inlet water strainer. We had persistent issues with the inlet strainer apparently capturing air while underway. This would leave a small air gap at the top of the strainer that was enough to cause upset with the pump feeding the AC units. If your problem is at all similar, simply stopping and restarting might be enough to "jolt" the air through the pump. Suggest removing strainer cap and checking water level before next attempt at running AC.
That's exactly what my AC unit did after I cleaned the strainer, I pulled the hose off that goes to the AC unit (after the pump) and blew thru it and then turned the unit on and while the water was discharging from the pump put the hose back on the pump and mine has worked fine since doing that. I had a air Lock.
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Old 10-15-2017, 09:56 PM   #15
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Originally Posted by Dougcole View Post
Wouldn't an air lock cause a consistent failure? Not an intermittent one?
Not if you cleared it to restart your pump and then it gradually filled in again.
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