March/Dometic AirCon Pump Question

I recently moved my March AC-5c-MD/Dometic PMA1000 air con raw water pump....and whether co-incidentally or not, it has now stopped pumping, though it is running. After I re-located the pump I tested it dockside and all appeared to operate normally. I've been out to sea & back since and now, the running-not-pumping problem.

I took off the wet end expecting to find a blockage, broken impeller or heat-distorted plastic...but as you can see from the first photo attached, all looks fine to my (non expert) eye. I am wondering if my new plumbing has created a water lock. The second photo shows the installation and while there is a straight short connection between pump & strainer (and the pump is kept flooded), the hose from the pump out to my air-con distribution manifold is an inverted 'U'. (Pump is under the white spray cover but you can see the wet end to the right and the manifold to the left with the hose loop between.)

What is the verdict by those familiar with these magnetic drive pumps? Is there something wrong with the wet end that I'm not seeing? or is the installation creating an airlock between pump & manifold?

The path from the out let needs to be up hill all the way. That arrangement could definitely be causing an air lock. The pump should be below the manifold.

When you say the pump is not "pumping", I take it that the pump runs, but no water is going through the system?

I have an almost identical install but no issues once primed. If I were trouble shooting this I would attempt to rotate the output to flatten that inverted U shape as maybe water is separating at the peak and allowing an air bubble to get back to the pump end. Think along the lines of "air lock" prevention. I find them a bit finicky but once satisfied, nearly maint. free. My last two installs both wanted primed from outside backwards through the pumps but once this was done where great until the next haul out. My current is in a Marinette cruised with such shallow draft I cannot get it to initially prime on it's own due to just barely being flooded from the strainer.

In your picture ...You my want to open that valve between the pump and the Distribution manifold. Just kidding ya little.

The valve is shut off.

Other than that what George said.

Good eye!
You can see there is air trapped in the discharge hose. Is it enough to prevent pumping? Maybe the impeller is not turning. Disconnect the disch hose at the pump and bump the pump with something to prevent spray all over the place. You will either have nothing or a geyser!

And I should add, as Doug noted, use a 90 fitting coming off the pump output fitting to flatten out the discharge stream and point it at the manifold intake fitting.

That valve is really not necessary that I can see by the way. And may in fact be part of the problem. As in when you close it off to work on the pump you trap water in the manifold and lines instead of the lines and manifold draining out . Now when you reinstall the pump and then open the valve the pump has to start by pushing against that standing water. Instead of just filling the empty air space. Just a thought.

Thanks guys. I wouldn't put an elbow right at the discharge because of the effect on head & pressure...I'd lift the manifold itself. Valve normally locked open. Anyone seen a problem with these pumps other than the wet end?

Looking at the specs of your pump, the max discharge head is 27 ft. More than enough to purge the air in the disch manifold unless the pump case is full of air or there is an air leak on the intake or the impeller is not free to turn. Something else is at play.

I wouldn't put an elbow right at the discharge because of the effect on head & pressure..

Click to expand...

That is no big deal or worry on this type pump. In most installations an elbow is the only way to achieve the "all uphill" plumbing.

Looking at the specs of your pump, the max discharge head is 27 ft. More than enough to purge the air in the disch manifold

Click to expand...

Doesn't work that way.

as long as there's flow (pressure really isn't the issue) and no air bubble forming right at the pump entrance...I really have never seen an issue with a/c pumps of any kind.

My last boat and this trawler both have 90 degree elbows on the pump...this boat even splits the discharge to 2 long cooling loops to my split a/cs.

Problem solved...and it WAS an air-lock! Thanks to Doug at post #3 for suggesting rotating the wet end so the discharge spigot was horizontal....I hadn't realized this was possible (& obviously, neither did the shipwright installer) but it is perfectly straightforward. Tested per Doug @ post #5 and got the geyser. Ditched the valve (agree with Bill @ #6 this valve unnecessary) and rotated the elbow on the manifold. Connection hose now is near-flat and a gentle curve: see photo.

Note to those who think an elbow at the pump is ok: MarineAir installation manual says
“Avoid…the use of 90º elbows with seawater hose…. a 90º elbow on the pump outlet is equivalent to 20' of hose.”

Thanks to all for helping.

Aquabelle said:

Problem solved...and it WAS an air-lock! Thanks to Doug at post #3 for suggesting rotating the wet end so the discharge spigot was horizontal....I hadn't realized this was possible (& obviously, neither did the shipwright installer) but it is perfectly straightforward. Tested per Doug @ post #5 and got the geyser. Ditched the valve (agree with Bill @ #6 this valve unnecessary) and rotated the elbow on the manifold. Connection hose now is near-flat and a gentle curve: see photo.

Note to those who think an elbow at the pump is ok: MarineAir installation manual says
“Avoid…the use of 90º elbows with seawater hose…. a 90º elbow on the pump outlet is equivalent to 20' of hose.”

Thanks to all for helping.

Click to expand...

Glad it worked out so easily.

I was fully aware that adding any angled fitting is the equivalent of adding X amount of hose. But it still works just fine in most cases.

While on the subject of these march pumps and pressure loss let me tell you all of a success. I had a split Air Conditioner system with the evaporator under the fly bridge console over the salon. The condenser was in the engine room. When the system crapped out I went back with a self contained unit in the place of the evaporator and got rid of the engine room condenser. I now had to lift cooling water all the way to the fly bridge. A couple of calculations said my March pump was not going to give me enough pressure. So to solve my problem I installed a second identical March pump in series with the first. Wallah, its been several years now in SE Texas living aboard and not a single issue.

obthomas: did you notice a big difference in noise level going from the split system to a self-contained unit? And is much noise transmitted from under the fly helm down into your lower helm/saloon area?

Aquabelle

I think there is a little more noise naturally since the compressor is now under the fly helm instead of in the engine room. I had to modify the box that housed the evaporator a little so I added sound insulation. As quiet as modern rotary compressors are the noise is very little more. The benefits I got from this change are: more room in the engine room and a single system that doesn't have so many places to leak from. I would do it again.

When I first got my boat, I found that the a/c's would shut down if I went much faster than idle. Long story short, the raw water intake was at the forward end of the engineroom and so close to the waterline that it would suck air into the system at anything above idle. This, in turn, caused the thermal sensors to shut down the a/c units.

The pic (taken before engine room renovations) shows the horrible brass Taiwanese strainer screwed onto the seacock. I am inclined to believe that the boat was built this way - hard to believe anyone would retrofit something like this!

The generator drew through an identical seacock on the other side. I'm sure it suffered the same air problem, but the rubber impeller in the raw water pump would have reprimed the system. Fortunately there was sufficient flow to cool the motor.

The solution was to fit two new through hulls and seacocks as low down as possible and condemn the originals. Perko strainers keep the lumpy stuff out and the old brass strainer is a now curiosity in my workshop.

BTW: I was tempted to mount the pump vertically on the bulkhead with the wet end downwards (for obvious reasons). March Pump advised against this. " . . . we advise mounting with the wet end above the motor end. If the pump is mounted vertically with the motor above the pump wet end, it may cause air to become trapped in the pump and this will damage the pump". I decided that horizontal was preferable and left the orientation as it was.