Are two small pumps equal to one big pump?

For a fountain I am planning, I was recommended to use one 5HP pump. I could special order it, but all my local suppliers have 3HP pumps and everyone knows how to service them and has the parts at hand. Special ordering one might mean down time in the future waiting for parts.

Can I use two 3HP instead? How do you hook up two pumps to work in concert? An expert plumber will do the actual installation, btw. I just need to have a rough idea of what can be done and how so I am not taken for a fool by such plumber.

For an alternative setup I was recommended two 5HP. Would that then be four 3HP pumps? Is there a practical limit to how many pumps you can hook up? Thanks in advance.

You need to look at the gallons per hour rating, not the horsepower.

You realize that a 5HP motor is going to cost almost $9/day or $260/month to run, if electricity costs 10¢/KWH, don’t you?

That was my original impression from my first internet research but everyone I have contacted, both suppliers over the internet and local vendors speak of HP all the time.

In any case, can you hook up two smaller pumps to do the job of a large one? Practical limits and considerations to keep in mind if it is indeed possible.

That’s about right for my location and although I hadn’t done the math, it is not too far from what I had guesstimated. This is not for my backyard, though (and if it were it wouldn’t be on 24/7). This is for a commercial setup and the city will be paying for the electric bill (woot).

Centrifugal pumps are limited in the amount of backpressure they’ll work against.
For that reason, connecting two small pumps to a single pipe will not give twice the flowrate of a single large pump, unless it’s a big pipe.

There is going to be a way to use two pumps, but I’m not going to be able to tell you how. It would certainly require a y junction before and after the pumps to run them in parallel and a back flow valve for each pump.

So, yes, there is a way. I presume that it is too complicated to explain to the layman over the internet but my plumber should know how to do. But it comes at a cost in efficiency and the smaller pumps won’t just add up to the sum of their powers. Right?

You are not my plumber and all that. Considering that the 5HP pump costs about 6 times what the 3HP pump costs (and factoring in the issue of not being the daily bread of my local suppliers), is this something I should consider or is it so much of a hassle that I am always better off buying the big pump?

You need to look at the pump curves, it’s not just a matter of horsepower. A pump’s power requirements can be calculated by multiplying the flow in gpm by the head in feet by the specific gravity of the liquid then dividing by (3960*efficiency). So a pump pumping 80 gpm of water with 100’ total developed head (TDH) at 70% efficiency needs around 2.9 horsepower.

The same pump with a different impeller might pump 40 gpm with 200’ TDH and at the same efficiency take the same horsepower to pump half the volume but at twice the pressure. The head requirements can be estimated by adding together the static head plus frictional losses in pipes and fittings. The efficiency comes from the pump curve and will vary with the flow rate. A well designed pump will usually be in the 70 to 80% efficiency range most of the time.

So what you’ll need are two pumps, each designed to pump half the volume of the 5 HP pump, but at the same TDH. Pumps usually don’t do well on a shared pump suction, which gets into net positive suction head (NPSH) issues, but if there’s enough static head between the fountain and the pumps to provide NPSH you should be all right.

You’re really going to have to figure out what flowrate(GPM) and pressure is required for your fountain. Those are your two main factors you should be looking at first. Not all 5hp pumps are created equal.

Yes, you can configure two pumps in series (doubling the pressure), and parallel (doubling the flowrate), if your system is properly designed. But you really need more information first.

How about splitting your grid of nozzles into two halves and running each half on its own pump? This would eliminate all of the mysteries of how to plumb two pumps together, while still letting you enjoy the benefits of cheaper and readily serviced smaller pumps.

You’ll have two runs of pipe (four pipes total - suction and output times two) between the fountain and the pumphouse, but that would be about the only complexity vs one large pump.

I am a professional in this field so far their is not enough information to answer your questions. Moving water is and can be much more complicated then your trying to simplify it down to. A motor and a pump are two different things. A motor is what powers a pump. A pump moves the water. Just in the realm of 5HP motors I can pick up books from several manufacturers and each will have 5 pages of related flow charts for the different pumps that can be powered by a 5HP motor. To determine the correct pump/motor combination you need a lot of data.

submersible pump or an above ground variery
Available gpm from your source
Desired gpm at the point of use
friction loss calculations-size and type of piping
elevation changes
available power- amperage voltage and phases available

If you can describe what this project is I’m sure we can help. Likely your going to have to talk to the ‘plumber’ anyhow to get the information so he might be the best person to start this line of questioning on.

You’ve already got enough data to point you in the right direction, but without trying to hurt your feelings, it’s like you said, “I’m thinking about sailing solo across the Atlantic, the pointy end of the boat is the front, right?” You probably want to practice in some shallow water first.

That being said, if you want to try, buy this book; Cameron Hydraulic Data. It’s detailed enough to design your system and still accessible to someone without a PhD in fluid dynamics. For the size of your project, $90 is pretty cheap.

Point well taken. It is a bit more like “I’m thinking about sailing solo across the Atlantic and I hired this captain to sail the boat. Help me not look like a total idiot when I talk to him”. (or maybe “… and I hired this dude with an anchor tattooed on his arm. How do I know if he knows what the pointy end of the boat is?”). Special note of the fact that I call him a “plumber” because that’s what he is, not a hydraulic engineer or anything of the sort. He has done pools and fountains before, just maybe not at the scale we are thinking now.
I did consider the idea of just splitting the system into smaller parts that can be managed by each smaller pump. Any caveats at this other than the multiple pipe sets running to and from the installation?
The project is a Fountain Maze. It will be a zero grade fountain (aka pop up jets. Jets that come out from the floor with no standing water) with a grid of water jets that will define the maze “walls”. We are talking about a 100’x100’ grid with a 6’x6’ grid. I started a thread about it a while ago that didn’t go too far.

Thanks for all the responses so far. I appreciate the fact that I am asking for stuff well over my head and that I am not providing enough detail. I will be contracting professionals for all the work, I just want some general info on what I am dealing with so I can talk to these professionals and not be bullshitted too badly.

When active, are all of the maze “walls” constantly running, or are you talking about popping up individual walls as a user goes through?

Constantly running. The maze is configurable by turning walls on and off but that happens off line, not when people is in there. Doing that would be almost trivial from a technical point of view but well out of our budget right now. v2.0 maybe.

Yes you can run two pumps in plaace of one larger pump. But the system will have to be engineered. And you will need to get two pumps that are designed right. It is done all the time. when I worked in SF the buldings central plant had three chilled water pumps and three tower water pumps. Depending on the cooling load the plant was designed to run on one, two, or three pumps.

Nothing to add except: while one 5HP pump might require longer downtime, two 3HP pumps double the odds of a failure.

Consider 3 pumps. No down time.

This is where I was going.