Two air conditioner pumps off a single through hull and strainer.

[170122]

Perfect comment.. but always remember that free flow ball valves will not restrict water flow, normal pens do, second is Water intake pipe Barnacles inside the pipes are very common is South Florida.
I do not use strainers inside my boat for the air conditioning, small amount of sand is not going to hurt my non self priming ceramic pumps,, the strainer is outside as a wedge, is removable, every few Months I have open under my boat and remove the green lime inside the pipe to avoid barnacles to restrict the passage. Easy to open for service.

Originally Posted by SVHarmonie

I will make the opposite argument... The fewer holes you put in the bottom of the boat the better! ALWAYS!

There is no reason it will have any issues—if everything is sized correctly. To be simple about it, the cross sectional area of the line from the seacock to the pumps need to be double the area of the hoses out of the pumps. That’s a simplistic approach but it will work.

For example, if you have 3/4” hoses OUT of the AC pumps, they have an area of (3.14 * (0.75/2)^2) = 0.44 sq in. Double that= 0.88 sq in. Which is just about the cross sectional area of a 1 in diameter hose.

You will need to have check valves on the outlets of the pumps. Swing gates are best in this case. You want to be sure that when one pump is running, it can not suck air from the other system.

All the normal rules about making sure the pumps are below the waterline, and the suction line runs continuously downhill to the pumps still stand, of course.

Many people will try to say this won’t work, but a proper seachest is absolutely good practice. My boat has ONE through hull feeding the engine, the genset, the two AC pumps, and the watermaker. It has worked for 25 years, and two circumnavigations. Fully functional and safer than having 5 separate holes in the bottom of the boat.

Another option to consider: Run ONE pump, and pipe the two AC systems in series. You’ll need a relay box to turn on the pump, but this is a standard option from all AC makers.

[Davidhoy]

Originally Posted by Shrew

I honestly doubt you have two pumps to begin with. I suspect you have:

1 Through Hull
1 Seacock
1 sea strainer
1 continuous duty pump
1 120 VAC breaker for the pump
2 120 VAC breakers for the Air Conditioners (one for each unit)
2 120VAC Air Conditioning Units.

Why would you doubt that I have two pumps? I specifically noted that I have a 500gph pump *AND* a 250gph pump - was I imagining things?? Am I that clueless that I can't tell the difference between a single pump and two of them?

[mvweebles]

Originally Posted by SVHarmonie

For example, if you have 3/4” hoses OUT of the AC pumps, they have an area of (3.14 * (0.75/2)^2) = 0.44 sq in. Double that= 0.88 sq in. Which is just about the cross sectional area of a 1 in diameter hose.

You will need to have check valves on the outlets of the pumps. Swing gates are best in this case. You want to be sure that when one pump is running, it can not suck air from the other system.
.

It math works, problem is the open-flow diameter of a 1-inch barbed nipple is not 1-inch due to wall-thickness so is under 3/4-inch. You could argue it all just scales down since the pump-inlet flow cross section is smaller too, but let's face it: the symptoms are pretty clear. OP's smaller pump works fine when it's running solo. Problems only occur when the other pump is turned on and creates contention for water. The system is not sized properly.

BTW - to the OP: is the affected pump have a longer hose-run? If you can reduce friction in the intake, might make some marginal improvement.

Peter

[Shrew]

Originally Posted by Davidhoy

Why would you doubt that I have two pumps? I specifically noted that I have a 500gph pump *AND* a 250gph pump - was I imagining things?? Am I that clueless that I can't tell the difference between a single pump and two of them?

Because you referred to one as GPH and the other as GPM............and I assumed you were confused.

Because I've seen many, many 2 AC systems with a single intake and a single pump on a relay. I've seen several 2 AC systems on 2 pumps with 2 intakes and dedicated pump breakers. I have yet to see a 2 AC, 2 pump system on a single intake.

Because I mistakenly made an assumption. I'm human. My apologies.

[Davidhoy]

Originally Posted by mvweebles

BTW - to the OP: is the affected pump have a longer hose-run? If you can reduce friction in the intake, might make some marginal improvement.

Yes, the aft AC unit is located in the lazarette, and the through hull, strainer, and both pumps are about midships. The forward AC unit is maybe 6 feet away from its pump.

I flushed all the lines with Barnacle Buster this weekend, and the flow, while better, is definitely restricted to the aft unit, particular when the forward unit is running. The point made about the stronger pump depriving the weaker unit is spot on.

Based on the responses here, and what I've read on Dometic's website, I think the best solution is to replace the dual pumps with a single larger pump, along with the requisite relay board and piping manifold (or just a tee). The question now becomes, what size should the new pump be?

Longer term I can look at upgrading the through hull, strainer, and getting larger piping, but that's for another day.

[mvweebles]

BTW - here's a nice picture of a raw water intake manifold, courtesy of Steve D'Antonio (I snagged it from one of his articles a while back). Appears to be a 1-1/4" thru-hull and sea-strainer reduced to 1-inch Tee's with three 1/2" valves off, one of which is nicely marked for watermaker intake - essentially a seachest.

For the OP - a couple more ideas:

1. If the pumps each have a valve on their intake, try closing the larger one a wee bit to reduce water flow and provide more to the smaller pump. A work-around to be sure, but might work. Plus will absolutely identify the problem.

2. Groco makes a way-cool raw water manifold. If you are only using AC at the dock, you could tee-off your engine intake. Now, this has inherent risks that will require manual attention, but will very likely make your life at dock happy with minimal cost. Manifold or tee off your sea strainer would be best - not uncommon for thru-hulls and strainers to be slightly over-sized with a reducing bushing. But please be careful - there are risks to your engine, and if your engine is new, this will void your warranty. But it will get you going. Just make sure you always (ALWAYS!!) put your ignition key on the thru-hull as a reminder to switch valves. Not a good idea to run AC with engine running in this config, but as a liveaboard, this will get you going until your next haul.

3. I should have included an Option 3 on my previous post: Install a second 1-inch thru-hull and sea strainer and run your second pump to it (I'd run at least 1-inch and reduce to 3/4 after the strainer).

Good luck!

Peter

[mvweebles]

Originally Posted by Davidhoy

Based on the responses here, and what I've read on Dometic's website, I think the best solution is to replace the dual pumps with a single larger pump, along with the requisite relay board and piping manifold (or just a tee). The question now becomes, what size should the new pump be?

I have a similar setup. It's not installed yet, but I went with a March 1000 gph pump. https://www.defender.com/product.jsp?id=2205719

I bought it over a year ago. Had I thought of it, I probably would have pinged a forum line this for recommendations on best 120VAC AC pump. TrawlerForum.com (sister site to this CF is also a good source of knowledgeable group-think on stuff like this). I'd also ask about a relay - the one I had from a previous install is in a gigantic box and takes a ton of room. There must be a better way....

Peter

[Coolerking]

The new relay box is much smaller than the old ones with the individual triggers in it, as the triac and the triggers are now integrated to the board itself.
If you do get a relay box, pay the extra few bucks for a 6 station, its about $300, that way you will have spares for the future.
The pump you want is the PML/A 500 to drive 24k total ac.
PML is submersible, the PMA is not.
You can also use a simple definite purpose contactor to actuate the pump, each units' pump output simply wires to the coil, which in turn starts the pump.

[Flatswing]

I have 3 split units off 1 strainer and 1 pump delivered via a manifold so all three have water circulating even if only 1 compressor running. The unit with the longest hose run(s) always has the least flow. Have you flushed your AC sea water system with Barnacle Buster or Ridlyme? The longest run will have the slowest flow and the greatest buildup, so when flushing with your favored solution, be sure to flush the aft unit (presumably the long run) separately to ensure it's really clean. I doubt you need to use separate pumps or through hulls. At most, consider adding PVC valves with pressure gauges between the strainer and the individual lines, to allow you to balance the flow and make it easier to flush the individual lines annually. The supply lines are low pressure/high flow.

[GreenWave]

Originally Posted by slug

Best to have one pump , one sea strainer ....servicing multiple ac units

THIS! I have this on my Catalina 470. Works fine.

[Davidhoy]

Originally Posted by GreenWave

THIS! I have this on my Catalina 470. Works fine.

What size AC units do you have on your C470? And what size pump is driving them?

Regards,
David

[GreenWave]

Originally Posted by Davidhoy

What size AC units do you have on your C470? And what size pump is driving them?

Regards,
David

I should know this answer by heart but I don't. Won't be back on boat for over a week to look that up. I have the Main salon and the Forward cabin on the shared pump. I want to say that the combined BTU is 24K. Will have to look up thru hull and pump size.

[a64pilot]

Majority of multiple AC units runoff of one pump. This is done because it’s easier and cheaper.
How deep the thru hull is in the water will determine how much flow it will have.
Properly sized no raw water pump should ever pull any vacuum, there should always be a positive pressure even if slight to the suck side of the pump.
Now let the strainer clog and of course this isn’t true.
My boat came with both heads plumbed into the engines thru hull, so one thru hull had three lines connected.
Anyone with a Super Maramu want to show a pic of their raw water manifold? I believe it has several connections, hence the name manifold.
I have two AC’s and have two thru hulls, two strainers and two pumps. Not long after getting the boat the March pump quit, thankfully West Marine had one in stock and was open so we only did without AC for a few hours. In a Marina in July or Aug in the Panhandle of Fl, AC is essential. You won’t die most likely, but it’s miserable and who wants to end miserable.

Here is a link Amel manifold, apparently the engine, both AC’s the water maker and the generator all are connected to one thru hull. Amel’s are not known to be poorly Engineered.
https://fetchinketch.net/boat-projec...ld-inspection/

[mvweebles]

Originally Posted by a64pilot

Here is a link Amel manifold, apparently the engine, both AC’s the water maker and the generator all are connected to one thru hull. Amel’s are not known to be poorly Engineered.
https://fetchinketch.net/boat-projec...ld-inspection/

The Amel intake thru-hull manifold seems like it might be subject to contention. Personally, I would go with a 2-inch strainer/thru-hull vs 1.5-inch, especially if one serves the A/C as these pump a lot of water. The reason I say this is the cross-sectional sum of the branches are slightly above the intake thru-hull (1.5"), meaning the max draw is 103% of the intake capacity. With 2-inch thru-hull, draw is 58% of the supply thru-hull. Maybe factoring wall-thickness of fittings improves the calculation, but since 2-inch thru-hull/strainer is not appreciably more expensive, would certainly recommend the added margin of error for a clogged strainer or hi-demand use.

Here is a diagram with part numbers for a raw water intake manifold for my Willard 36 with Perkins 4.236 75hp diesel engine. Note, the Jabsco RW pump for the perkins is actually a 3/4" intake but mine has a 1-1/4" adapter on it to give full-flow to pump and reduce frictional loss. I went with 2-inch thru-hull for a few reasons: Not a lot more expensive than a 1-1/2", sea strainer is considerably larger, but most importantly, the oil cooler for my stabilizers has 2-inch fittings. I could reduce them with no harm as the cooler is designed for larger applications, but decided more flow was better than less flow. I also have a similar manifold with 1-1/2 thru-hull serving 6kw NL generator (3/4") and watermaker (nominal 1/2").

I mentioned it before but will again, this setup with void a Yamnar warranty (perhaps others) so if you have a new engine, please be cautious in considering this guidance.

Peter

[OS2Dude]

You could also share the pump if you don't want another hole. When I was thinking about adding a second A/C for our v-berth, the manufacture said it was OK to have a single pump and strainer (Again, if sized correctly for simultaneous usage) but you will have to use a set of relays so either/both A/Cs can turn the pump on. Not sure why, but have had other people confirm that.