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

[Davidhoy]

Looking for some advice. My boat has two AC systems - a 16k BTU unit for the main salon and forward cabin, and an 8k BTU unit for the aft cabin. My wife and I live aboard in South Florida, so the AC systems run pretty much 24/7 in the summer (which is most of the year in Florida). Both AC pumps draw from a common 1" through hull and strainer, one pump being 500gph (fwd AC unit) and the other 250gpm (aft AC unit). The flow coming out of the aft discharge is noticeably less when the fwd pump is running, and we sometimes get the dreaded "HPF" error on the aft unit.

So, my question is, does it make sense to have two differently rated pumps drawing off the common inlet and strainer? Would it be advisable to upgrade the 250gph unit to 500gpm to get better flow when both pumps are running? Or would I end up starving the fwd unit of it's necessary flow?

Regards,
David

[slug]

Originally Posted by Davidhoy

Looking for some advice. My boat has two AC systems - a 16k BTU unit for the main salon and forward cabin, and an 8k BTU unit for the aft cabin. My wife and I live aboard in South Florida, so the AC systems run pretty much 24/7 in the summer (which is most of the year in Florida). Both AC pumps draw from a common 1" through hull and strainer, one pump being 500gph (fwd AC unit) and the other 250gpm (aft AC unit). The flow coming out of the aft discharge is noticeably less when the fwd pump is running, and we sometimes get the dreaded "HPF" error on the aft unit.

So, my question is, does it make sense to have two differently rated pumps drawing off the common inlet and strainer? Would it be advisable to upgrade the 250gph unit to 500gpm to get better flow when both pumps are running? Or would I end up starving the fwd unit of it's necessary flow?

Regards,
David

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

[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 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.

I'm not talking about adding another through hull, nor adding a sea chest, both options are well beyond the scope of this discussion. Instead I'm trying to figure out the best option I have with my existing 1" through hull and strainer. It seems that the current two-pump arrangement may not be ideal, which doesn't surprise me. Switching to a single pump looks to be a good option, in line with the recommendations I found on Dometic's website. What would be the ideal size of pump for two AC units, one 16k BTU and the other 8k BTU?

Your "other" option was exactly what the previous responder suggested, so I'm confused why you say you're taking the "opposite argument".

Regards,
David

[SVHarmonie]

Originally Posted by Davidhoy

I'm not talking about adding another through hull, nor adding a sea chest, both options are well beyond the scope of this discussion. Instead I'm trying to figure out the best option I have with my existing 1" through hull and strainer. It seems that the current two-pump arrangement may not be ideal, which doesn't surprise me. Switching to a single pump looks to be a good option, in line with the recommendations I found on Dometic's website. What would be the ideal size of pump for two AC units, one 16k BTU and the other 8k BTU?

Your "other" option was exactly what the previous responder suggested, so I'm confused why you say you're taking the "opposite argument".

Regards,
David

Without knowing the water temperature specification,and flow rate recommendations from the AC vendor and the details of your piping arrangement, I don't have the information I'd need to reliably recommend a pump needed to cool both units in a parallel setup.

May I suggest you call Dometic? They get paid to make those calculations and will support you if they get them wrong...

[a64pilot]

Originally Posted by slug

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

I disagree, the one pump breaks and your whole AC system is down, if you chose or go this route, best to have a spare.
I don’t like single pints of failure.
I don’t believe you need a check valve either. My boat came from Island Packet with both heads and the engine plumbed into one thru hull, and they have worked fine that way for over thirty years.

[Coolerking]

We do this for a living.
3.5 to 4 gallons a minute per 12k of ac.
PM500 is perfect size for both units, 500 divided by 60 is 8.3 gpm
One pump, with a pump relay box to allow each unit to call for the pump.
1 inch strainer is perfect size for the amount of ac you have.
If you feel you need a spare, then get one, but a new pump properly cared for and installed correctly will last many years.
Too much flow is as bad as not enough.
Good luck!

[slug]

Originally Posted by Coolerking

We do this for a living.
3.5 to 4 gallons a minute per 12k of ac.
PM500 is perfect size for both units, 500 divided by 60 is 8.3 gpm
One pump, with a pump relay box to allow each unit to call for the pump.
1 inch strainer is perfect size for the amount of ac you have.
If you feel you need a spare, then get one, but a new pump properly cared for and installed correctly will last many years.
Too much flow is as bad as not enough.
Good luck!

Flow rate is reduced by plumbing runs , fittings , valves

Always oversized the pump and plumbing ID

[flyingfin]

GPM or GPH?? check your pumps and make sure you understand the difference. Also, which pump pulls straight off the thru hull/strainer and which one is off the side (tee)

[Coolerking]

4 gallons a minute per 12k of ac.

Please, everyone, read this and you will find everything you need to get it right.
https://www.marine-j.com/pdf/222000650.pdf

[mvmojo]

A 1" inlet (thru-hull) should supply enough water for a 16 BTU & an 8 BTU air conditioner. But, it's not a good idea to have two pumps competing for the same water supply - the pump with the greater suction can starve the lesser pump. I suggest you simply buy a larger pump and run both a/c's from the same pump. I'm running three a/c's (2-16 BTU, 1-24 BTU) from a single 20 gpm pump on a 1-1/2" inlet supply. All water goes thru all three units any time one unit is running. The pump is controlled by a relay box that turns it on any time any one of the a/c units comes on. Been working great for 20 years!

[170122]

Must add a pump relay, this way if the high pressure switch the pump off due to water flow restrictions you will not damage your pump, look for 950/ 1000 pump brushless, they are very quiet and last a long time, the power consumption is about 1/2 of the regular March pump,

Originally Posted by slug

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

[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.

[Shrew]

In order to run a dedicated pump for each Air Conditioner, I believe you will need a dedicated through hull, seacock and seastrainer for each. Otherwise, both pumps are fighting for flow from the same water intake. One might essentially be creating a water bottleneck for the other.

I could be wrong on this.

[mvweebles]

You do not have enough flow for two pumps. Each pump wants 3/4" and you have a 1-inch thru-hull.

You have two choices:

1. Larger intake (MVMojo's suggestion). Single 1-1/4" thru-hull to a sea strainer, then tee-d to 3/4" for your 500gpm pump and your 250gpm pump (also likely 3/4").

2. Single larger pump to replace the two smaller pumps. A 1000+ gpm pump has a 1-inch inlet, then tee-off that to each AC. You will also need a relay that turns-on the pump regardless of which AC is turned-on. As A64 states, this gives you a single-point-of-failure. I think that disadvantage is partially offset by having a single larger sea strainer which is less likely to clog than two smaller strainers, plus easier to service, but A64's point is well taken. Plus a single larger pump is pretty dang expensive.

Pick your poison - #1 means keep your pumps but install a larger thru-hull + strainer. #2 means keep your thru-hull and strainer but install a new pump +relay. I lean towards #2 as I like all the thru-hulls in a single, easy-to-reach location and having a single sea strainer is easier to place than two. But there is a single point of failure in the pump. Notably missing from my list of benefits is fewer holes in the hull. As long as proper seacocks are installed, I don't see a problem with multiple holes in the hull. I worry more that sea-strainers are consolidated and accessible.

Good luck!

Peter