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Old 07-17-2018, 11:18 AM   #1
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AC and DC fresh water pumps?

I have a killer AC pump for the fresh water system that pressurizes a large accumulator. I get great volume and pressure from it. But because it's such a great pump, it draws a lot of juice, and when the batteries start getting close to the inverter's Low Battery Cut Off point, it's almost always the pump that cuts it off.

Lately, I've been thinking about taking the AC pump off the inverter and installing a DC pump that will run only when there's no source of AC power (generator or shore power).

Do any of you have this on your boat? Any thoughts? Thanks!
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Old 07-17-2018, 11:36 AM   #2
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That would be more efficient, since you're losing energy with every step in the conversion.
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Old 07-17-2018, 11:45 AM   #3
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I used just a 12vdc pump. As the OP has stated great pressure and volume on the ac pump which is translated to using A LOT of water......
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Old 07-17-2018, 11:48 AM   #4
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You may not like this idea but install a smaller capacity pump. It will run longer but sometimes where high power is not available it makes sense to lower the power needs but run longer.
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Old 07-17-2018, 12:16 PM   #5
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Quote:
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I used just a 12vdc pump. As the OP has stated great pressure and volume on the ac pump which is translated to using A LOT of water......
I make about 60 gph every time I run the gennie for almost any reason, so I'm not worried about using water. I'm far more interested in using electricity efficiently, and as was noted above, the AC pump is not super efficient when it's running off the inverter (which I knew).

I'm mostly interested in hearing from someone who has this setup on their boat, or who has had it in the past and now doesn't, or who was going to do it and decided not to for some reason. The plumbing could be as simple as a T-connection joining the outputs of both pumps - but maybe not. (Is back pressure a problem for a pump?) The electrics should be pretty simple - a relay that cuts off the 12V to the DC pump any time there's 110V going to the AC pump. (But maybe there's more to that, too?)

Thanks.
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Old 07-17-2018, 01:13 PM   #6
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This may help:
I'm getting ready to redo my freshwater pump and pressure tank. I installed a carbon block whole house water filter (it's great!) and need just a little more pressure (low flow) before the pump kicks on. Decided to go with the same 12 volt 7 GPM 70 PSI pump I use for saltwater wash down. While it does have a pressure switch, I'm going to install a standard Square D adjustable well pump switch. I'm only looking for 40 to 50 PSI water pressure. Will change to a 4 gallon expansion for longer cycling periods.

Regarding 12 volt DC versus 120AC, when practical, I try to use 12 volt whenever possible, as it's always available. There are certainly times when 12 volt isn't practical because of large wattage requirements (such as air conditioning), but most of the time wattage consumption is less when not converting from 12 volt to 120 volts.

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Old 07-17-2018, 01:15 PM   #7
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We had both on our old Hatteras and it was a great combination. WE didn't have the AC pump on the inverter so it would only kick in when on shore power or using the generator. I thought about hooking it to the inverter, our system would have handled it, but never got around to it as it wasn't urgent.


We lived aboard full time, most of that on moorings or at anchor so the fresh water system was important.



We liked having the redundancy too.



It was rigged so both pumps used the same accumulator. The whole set up worked seamlessly.



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Old 07-17-2018, 01:25 PM   #8
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Leave your big AC pump alone and add a DC pump in parallel. Pick which one you want to run based on your electrical situation.
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Old 07-17-2018, 01:35 PM   #9
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Glad you agree with our set-up Ski !



Brian see my picture as to how we "Teed" them. Yes, it's nice to have a check valve on the pump so there is no back pressure; the big centrifugal shallow well pump didn't really need that, but the diaphragm GallyMaid pump did. It's in the manual...
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Old 07-17-2018, 04:25 PM   #10
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cmon... Admit it.



Thats not an fresh water pump, thats your jacuzzi's pump.
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Old 07-17-2018, 04:55 PM   #11
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cmon... Admit it.



Thats not an fresh water pump, thats your jacuzzi's pump.
That's what's called a shallow well pump. Best showers ever. Plus if water supply was not an issue, made for a great fresh water wash down pump.
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Old 07-17-2018, 07:40 PM   #12
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Sure, I like centrifugal pumps a lot.



I intend on using a couple 3-4 hp ones to make thrusters. Whats the hp rating on that one?
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Old 07-17-2018, 08:15 PM   #13
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Around 3/4 or 1 IIRC
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Old 07-19-2018, 05:25 AM   #14
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IF the boats accumulator is of a good size , the slower capacity of a DC pump may not be notices.

The question is how much water do you need of big volume over what period of time.
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Old 07-19-2018, 08:26 AM   #15
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Did you use that big AC pump as a thruster as well?
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Old 07-19-2018, 09:18 AM   #16
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Did you use that big AC pump as a thruster as well?
No, but I am curious as to how someone is going to use one for that purpose; that's a new one on me.

We loved that pump. We could be running the dishwasher, laundry, and taking a couple of showers simultaneously and it wouldn't let up. Ahhh, pleasure boating!
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Old 07-19-2018, 10:25 AM   #17
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I use a shallow well pump and on shore power, generator or the inverter, it runs the same. I have a Magnum MS-PAE inverter that supplies 240v or 2 legs of 120v and the pump motor is wired for 240v.
The AC/DC pump setup pic would be more efficient if it had a larger intake pipe. The pumps are usually are plumbed for 1.25" to 1.5". Right now it's like sucking soda thru a straw. It has to work harder to draw water and run longer to make cut off pressure. My boat had the same problem when I bought it. Changing to a larger intake cut the running time 75% and greatly reduced battery loss. Boat had 3 showers, but only one at a time could be used.
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Old 07-24-2018, 11:33 AM   #18
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Sorry for the slow reply, everyone - been in the Exumas Land and Sea Park the past 3 days w/o cell coverage. (It didn't suck.) Thanks for all the input.

OC Diver - what brand is that 7gpm pump you've selected, and why did you decide to use an external pressure switch? Simply so you can fine tune it to what you want/need?

caltexflanc - love the picture of your setup. I see that the AC pump is the Craftsman shallow well pump, but the DC pump doesn't look like anything I've seen from Jabsco or the other common marine pumps. Do you know what it was / who made it / capacities, etc?
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Old 07-24-2018, 12:09 PM   #19
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Quote:
Originally Posted by BrianSmith View Post
Sorry for the slow reply, everyone - been in the Exumas Land and Sea Park the past 3 days w/o cell coverage. (It didn't suck.) Thanks for all the input.

OC Diver - what brand is that 7gpm pump you've selected, and why did you decide to use an external pressure switch? Simply so you can fine tune it to what you want/need?

caltexflanc - love the picture of your setup. I see that the AC pump is the Craftsman shallow well pump, but the DC pump doesn't look like anything I've seen from Jabsco or the other common marine pumps. Do you know what it was / who made it / capacities, etc?
The one I've decided to go with is actually 6 GPM with a 70 PSI rating. Will likely run the pump between 35 and 50 PSI. Using a 70 PSI pump at the lower pressure should improve life expectancy. The separate pressure switch allows me to choose the on off set points and use a smaller differential between them. The larger expansion tank will keep the pump from short cycling.

https://www.hodgesmarine.com/Jabsco-...82605-0092.htm

Ted
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Old 07-24-2018, 12:09 PM   #20
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Started looking at pumps online. Anyone have any experience with the variable flow pumps, like the Jabsco V-Flo? Since I have an accumulator tank already, and I won't remove it (since the AC pump is not variable flow), does it even make sense to use a variable flow pump? Seems like it could be a little more energy efficient, but with added complexity.

Brands that you like, or have had trouble with? Jabsco, Johnson, FloJet, SeaFlo, others?
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