Advice on using 1 pump to service two devices

[GaryLosAngeles]

On my boat, a thru-hull and seacock control raw salt-water that is pumped to an A/C unit. It's a 120V circuit. When I’m not using the A/C I want to use that same pump for my desalinization watermaker. That way I won’t need to put another hole in the bottom of my boat - the fewer of those, the better, right? I tried putting in a second pump but it didn’t deliver enough water to the watermaker because the water was going through the lower A/C unit and being discharged overboard because the A/C unit is lower. Would it work if I put a "Y" connector and two valves after the pump to divert the output to either the A/C or the watermaker? Also, I need advice on how to wire the pump when I’m using the watermaker. The A/C unit automatically turns the pump on or off to control the temperature of the unit. The pump itself has the standard three wires, black is power, white is neutral and green is neutral. Can I put in a separate 120V electrical switch to turn on the pump when I use it for the watermaker or could this create a problem with the A/C unit? As I said, these two devices will never be used at the same time. Any help or suggestions is appreciated. Gary

[Coolerking]

You could buy a pump relay box from dometic and wire the pump outputs on the watermaker and AC to a relay of the correct voltage.
This allows one pump to drive cooling to anything that calls for it via the relay.

What are the specs of the AC?

What pump is in there? Im guessing its a March 500gph if you only have one ac on it.

[Gary S]

I wouldn't advise trying to use the same pump, but if you wanted to drive the same pump from two devices, you would use a relay like this: https://citimarinestore.com/en/micro..._stations_9800

If either device calls for the pump to turn on, the board will give power to the pump. This is commonly done for boats with multiple A/C units that use a single seawater pump to feed them all.

Is the watermaker a 120V system? What is the required flow rate and pressure for this boost pump? What is the air conditioning pump you are using? A/C pumps like the small March ones are cooled by the water flowing through them. If you restrict the flow too much by trying to run it to your watermaker, you could cause a problem. Adding a y and valves would certainly allow you to control the flow, but do you really want more fittings and valves in a raw water circuit with the potential to flood/sink your boat?

Depending on the watermaker requirements, I would instead advise splitting off the intake to the a/c pump strainer. This avoids having the put in another thru-hull and seacock. Add an appropriate boost pump (and strainer) for your watermaker and wire it so it comes on with the watermaker.

[Cheechako]

You could install a 3 way valve, 1- both - 2

[GordMay]

Greetings and welcome aboard the CF, Gary.

[a64pilot]

An air conditioner pump doesn’t supply enough pressure for a water maker.
Be nice if it did though, cause those pumps run forever and don’t fail.

You need higher pressure because the feed water has to be filtered usually to 5 microns and still have plenty of pressure left after going thru the filter to ensure the HP pump doesn’t cavitate.
Usually that’s at least 10 PSI to the HP pump after all losses.

[GaryLosAngeles]

In the system I have, the pump I'm concerned about only supplies raw water to the high-pressure device.
Gary

[a64pilot]

Originally Posted by GaryLosAngeles

In the system I have, the pump I'm concerned about only supplies raw water to the high-pressure device.

Gary

I’m confused, you said you want to use one pump to supply two devices?
If you use a watermaker type LP pump to run the AC, your going to go through a lot of pumps.
If you try to use a March type pump, it’s not enough pressure to supply the high pressure pump, March pumps give good flow but little pressure.
You need more pressure than one of those can make to push enough quantity of water through a filter, plus you need pressure to the suck side of the HP pump to prevent it from cavitating.
It’s a piston type pump so it pumps in surges, during those surges if you don’t have good pressure, there will be a negative pressure and the pump may cavitate, if it cavitates, it’s going to wear quickly.

You need about 10 PSI of pressure at the inlet side of the HP pump to prevent problems, you can go higher with no issues and I’m sure a little lower, but I’d try to maintain about 10 PSI min.

[Seighlor]

My boat is set up just as you describe: One pump provides water to both the A/C unit and the water maker. The pump has its own switch on the breaker panel as does the air conditioner and the water maker. All that is necessary is to turn on the pump and verify the water flow prior to switching on the A/C or water maker.

[Coolerking]

Originally Posted by Seighlor

My boat is set up just as you describe: One pump provides water to both the A/C unit and the water maker. The pump has its own switch on the breaker panel as does the air conditioner and the water maker. All that is necessary is to turn on the pump and verify the water flow prior to switching on the A/C or water maker.

Is the pump breaker you describe powering the relay box I pointed out?

Or does the water flow regardless of whether cooling is being called for/ RO in operation?

I have seen this done lots of times.

As a general rule, most seawater cooled ac systems need about 3.0 gpm per ton (12,00btu/hr) capacity. Most seawater pumps are spec'ed to deliver about 4.0 gpm under ideal conditions, before plumbing and all of that.

Look at the Btu rating, do the math for the amount of water flow you need per hour/or minute and your set.

The Cu-Ni condensers like clean water flow, no cavitation, just enough water to do the job in all conditions, then they last a loooong time.
I've also seen them washed out and leaking in a few years due to way too much water flow.

[Seighlor]

Originally Posted by Coolerking

Is the pump breaker you describe powering the relay box I pointed out?

Or does the water flow regardless of whether cooling is being called for/ RO in operation?

I have seen this done lots of times.

As a general rule, most seawater cooled ac systems need about 3.0 gpm per ton (12,00btu/hr) capacity. Most seawater pumps are spec'ed to deliver about 4.0 gpm under ideal conditions, before plumbing and all of that.

Look at the Btu rating, do the math for the amount of water flow you need per hour/or minute and your set.

The Cu-Ni condensers like clean water flow, no cavitation, just enough water to do the job in all conditions, then they last a loooong time.
I've also seen them washed out and leaking in a few years due to way too much water flow.

There is no relay. As noted, the pump is on its own breaker panel switch. When on, the water flow is constant without regard to operation of the RO or A/C.

[Coolerking]

Totally wrong setup, The seawater pump should NEVER run unless the AC unit is calling for cooling.
What make of vessel are we talking about?
What make AC?
If you are only using it for the one AC unit, then it should be wired to the units pump outputs in the main control box, if you have more than one AC unit, and only one pump, then you have a relay box somewhere (which would explain the pump breaker being required) and the triac in it is shorted closed, which would make the pump run on power up.