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Old 06-14-2013, 09:43 PM   #1
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So...have a 120v A/C unit, may add a 240v A/C as well...can they share the same pump?

So...previous owner had 3 120v A/C units on board. One was an older split unit that still works, and two newer self contained ducted style. When the boat was repossessed he took off the newer units with many other things he didn't let go with the boat.

So I have a single 12,000 BTU 120v unit in the salon & galley...no A/C for the rest of the boat. Yet. Of course budget is always a concern, and I scan whats for sale here and there.

I came across a very good deal on a new 20,000 BTU unit with a good pump. The catch is that it and the pump are 240v. I have plenty of 240v capability in my power panel, so that's not an issue. My question is about the pump.

Is there a relay available to be able to use the 120v pump with the 240v unit...or vice versa...the 240v pump with the 120v unit? Both pumps are approximately 1100 gph so unless I'm mistaken either one should have the capacity to provide water for both A/C units correct? If possible I would prefer to use one pump instead of both...
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Old 06-15-2013, 07:32 AM   #2
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Wire the AC pump to it's own switch/breaker. When you want to run the AC, turn the pump on and then you can run either #1, #2 or both units. This would eliminate the need for a trigger (relay) box that may be difficult to wire in with two different voltages.
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Old 06-15-2013, 06:23 PM   #3
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Larry, that is a pretty good suggestion...thanks , and I may end up doing that. Wondering if there is a relay that would do the job though.....curiosity more than anything.
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Old 06-15-2013, 08:11 PM   #4
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Larry, that is a pretty good suggestion...thanks , and I may end up doing that. Wondering if there is a relay that would do the job though.....curiosity more than anything.
Look at a SSR, a solid state relay. For under $20 you can get one that operates on 3-32VDC (or AC line voltage) and switches up to 280VAC at 25A.

If you have good old fashioned American 240VAC you have 120VAC as well so you can have the pump control circuit of either unit turn on either pump. Wire both pumps but include a selector switch to select one as primary and the other as secondary. Alternate pump use weekly or something like that to balance useage.

https://www.futurlec.com/Relays/SSR25A.shtml

Or even more simple, look at the wiring diagram for the pump power relays already on the units and you can most likely take 120V power off the 240 unit to drive the 120V pump and then let the 240V unit power the 240 pump as intended. All you need then is the pump A/B switch so both don't run at the same time. If the A/B switch is a DPDT you can supply 240 through one side and 120 through the other.
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Old 06-15-2013, 10:33 PM   #5
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Excellent advice Rick, thank you. I'm going to look at both options.
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Old 06-16-2013, 11:45 AM   #6
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When looking at a SSR be sure to consider the heat sink required. I built a relay box to operate one pump from two A/C units and decided on regular mechanical relay due to the cost and size of the heat sink required for the SSR.

By the way, you can buy a control box from the marine A/C companies for anywhere from about $150 - $200+.

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Old 06-16-2013, 01:23 PM   #7
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... due to the cost and size of the heat sink required for the SSR.

$7 ... seems pretty reasonable to me. Could go really cheap and just use a piece of scrap aluminum.
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Old 06-16-2013, 09:47 PM   #8
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Thanks for the info Bob. I didn't know it needs a heat sink....I have plenty of sheet aluminum though in 1/16 inch. Any way to determine what size it needs?
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Old 06-16-2013, 09:54 PM   #9
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>>>Could go really cheap and just use a piece of scrap aluminum. <<<

Could just go out and buy a triac for about a buck, screw it to the piece of aluminum and have a homebuilt SSR.

To aronhk_md, I haven't done heat sink calculations in about a hundred years. If you look up the specs for a SSR it should lead you to some curves for heatsinks. To be safe, just pick a ready made one with the right ratings.

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Old 06-16-2013, 10:02 PM   #10
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Sound like maybe the best thing Bob...to just buy one. What is a triac?
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Old 06-17-2013, 12:39 AM   #11
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Originally Posted by aronhk_md View Post
Is there a relay available to be able to use the 120v pump with the 240v unit...or vice versa...the 240v pump with the 120v unit? Both pumps are approximately 1100 gph so unless I'm mistaken either one should have the capacity to provide water for both A/C units correct? If possible I would prefer to use one pump instead of both...
Yes, its possible and easy. Relays are durable and inexpensive.

The circuit is extremely simple.

If you wanted to for example control a 240 volt pump with a 120V signal then use a double pole 120 volt relay

You could also ues a 120 volt realy to control a 120 volt pump with a 240 volt signal, just pick off one of the 120 volt legs to neutral and use it to operate the relay.

If you had two, or three or twenty AC units you could use a single relay per AC unit to turn on the pump.

This is not complicated, nor does it need to be expensive or involve a bunch of engineering.

You could use a solid state relay as Rick indicated. Thats not a bad idea, but I personally like coil type relays. They have an extremely long service life (many are 50,000 hour MTBF), and are dirt cheap. Depending on the current requirements of the pump many are socket mounted for easy replacement. Nothing against the Solid States, I'm just used to the coil types.
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Old 06-17-2013, 10:27 AM   #12
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The comment about the triac was tongue in cheek aimed at Rick. The triac is the heart of the SSR but there are many other components involved.

I'm with ksanders, go with a mechanical relay, that's what I did. If you want to go with a SSR, the data sheet will recommend a heatsink that matches the power rating and mounting surface.

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Old 06-17-2013, 06:37 PM   #13
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Quote:
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If you wanted to for example control a 240 volt pump with a 120V signal then use a double pole 120 volt relay


You could use a solid state relay as Rick indicated. Thats not a bad idea, but I personally like coil type relays. They have an extremely long service life (many are 50,000 hour MTBF), and are dirt cheap. Depending on the current requirements of the pump many are socket mounted for easy replacement. Nothing against the Solid States, I'm just used to the coil types.
The 240v pump with the 120v signal/relay is sounding like a good option. My question is...what is a double pole 120v relay?

This is what I had found in my ebay searching last night...

For Temperature Controller 25A AA Solid State Relay SSR AC 80 250V | eBay

I too am not against the idea of old school vs new school. Either is acceptable to me. I just don't know enough to do proper searches, or even to know what I'm looking at when I find something. lol

Thanks Bob & Kevin
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Old 06-17-2013, 09:11 PM   #14
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That SSR looks like it will do the job. Input and output covers 120-240 VAC. I imagine the pump only draws maybe a couple of amps so that a small piece of aluminum should be sufficient for a heatsink. Size of the heatsink of course depends on the ambient temp, so if you mount it in the engine room it will have be bigger than if you mount it in the A/C spaces.

I would be somewhat concerned about the price, seems really really low. Check DigiKey for similar items.

Bob
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Old 06-17-2013, 10:12 PM   #15
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Thanks Bob...its Chinese I believe. I was just looking to see whats there.
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