Power usage of two different HP Variable Speed Pumps at 1500 RPM

[sparcel]

Given the same pool and equipment setup I could choose different horse power variable speed pumps from the same brand such as the Hayward 1.85 or 2.7 HP Tristar. Is there any operating cost downside to the 2.7 HP pump?

I guess (based on the published performance curves) the higher HP pump (in the same line) must have a different impeller because it pushes more water at the same RPM, and therefore it must be using more watts. So, I could run the higher HP pump at a bit lower RPM. But, looking at the performance curves of the different pumps the gallons moved at a given RPM and speed don't differ by as much as the rated HP differs, so I'm not sure what that means about the power used per gallon pumped.

 

[Darin]

That is a tough call. You need a certain flow to achieve proper skimming and a certain flow for your SWG. They will be different speeds for each pump since they have different impellers.
The only real way to do it is to do a little set up with each and put an amp clamp on it capture watt usage for your required speeds. You will be running slower rpms for the larger hp motor but a larger hp motor may requires more watts and inverse would be smaller would need more rpms even though it may require more watts. Think about it this way, smaller pump running at 1800 rpms may use the same watts as larger pump at 1400 rpms but you get more flow from the 1800 rpms.
The only way to tell is from experience or setting it up on your equipment using an amp clamp. Maybe your supplier could run a little experiment that could benefit both of you.
You can get into pump curves but this is the only way to actually know how many watts are consumed.
Pump speed is linear, half speed is half flow. Energy (watts) is cubed. that is why VS is such a cost saving. Half the speed is not 50% the cost but rather 25-30% the cost.

 

[proavia]

The higher HP pump will push the same amount of water using a lower rpm and lower wattage - hence cheaper to run for the same result.

 

[Darin]

proavia, what watts did you get at the same flow?

 

[proavia]

Not sure exactly what you mean. I don't directly measure flow - I run the pump at an rpm that provides the action I need (skimming, SWG production, IFCS use, etc).
A higher horsepower pump will move more water at a lower rpm than a lower horsepower pump - with the standard impeller that comes with each pump.

At the same rpm, the higher horsepower pump will push more water. So you lower the rpm of the higher horsepower pump to push approximately the same amount of water to provide the action needed.

I run at 1100 rpm to skim and to keep my SWG flow switch happy and 2800 rpm for my IFCS to do an adequate clean. I have a 2.7hp VS pump. When I had a single speed 2hp pump, which ran at 3450rpm, the IFCS didn't do as good a job as it does with a higher horsepower pump at 2800rpm. And the watts used by the VS pump at a lower rpm are less than the watts used by the single speed pump - but the water throughput is higher.

 

[JamesW]

The same flow will be roughly equal in terms of power usage.

The difference is negligible.

The larger HP can run at a lower RPM for the same flow, which means that it is much quieter.

 

[ajw22]

Long term a 3HP or 2.7HP VS pump will save you more in electrical cost then the increased initial acquisition cost over a 1.6HP VS pump...

At 1750 rpm the 1.6 HP is putting out 0.8 HP.

At 1750 rpm the 3 HP pump is putting out 1.5 HP.

So 1.5 HP will require more electricity then 0.8 HP.

But if your pool needs 0.8 HP then you run the 3HP pump at 1,150 RPM. 1,150 RPM on the 3HP pump will use less electricity then 1750 RPM on the 1.6HP pump while both give 0.8 HP.

Your pool does not need RPMs, it needs a specific HP flow and the larger pumps are more efficient.

Note, HP is not always linear but illustrates the equivalent number.

You will find some other pump analysis at Pump DOE Regulations - Further Reading

 

[JamesW]

 

[JamesW]

Figuring out the power usage of a variable speed pump is not super straightforward unless you are measuring the power and flow at different speeds and flow rates.

You would need to know your system curve and plot it against the pump curves at different speeds and this will tell you your flow at a specific speed.

For example, the below pump would hit 116 GPM at 3,450 RPM.

If you have the power graph, you can find the power at a specific Speed (RPM) and flow.

So, the power at 3,450 RPM would be about 2,900 watts at 116 GPM.

Note that the pump curve is not for the IntelliFlo, so it’s not exactly accurate, but just used for example.

In any case, none of this is super important as the pumps will all use roughly the same power at the same flow.

The bigger pump is generally preferred so that it can run slower and it is much quieter for most applications.

 

[Darin]

Not sure i worded my reply above very good so i will try again.

Two different sizes of VS motors on two different sizes of impellers in the pumps.
The larger motor will have a larger impeller, smaller motor will have smaller impeller. Flow is different at the same rpm.
Since smaller motors draw less watts than a larger motor at the same RMP, then the only way to really know what the difference needed to work on a pool at the minimum flow needed to adequately operate the SWG and the skimmers would to put an amp clamp on the two and compare.
For instance, a 1.6 hp running at 1750 rpm will not draw 0.8 hp as power (watts) is not linear but rather cubed. A 2.7 hp motor will not draw 1.35 hp of power at half speed either as it is also cubed. Pump Affinity Law. Flow is linear, half speed = half flow, pump Affinity Law.
So the question is on your particular system which is cheaper to run? A 2.7 hp VS at XX% speed to achieve what you need or the 1.6 hp VS at a higher speed (smaller impeller) to achieve what you need. The only way to know is by running a watts test.
The canned answer on this site tends to be to always go bigger but I have never seen actual watts collected at the same pool on the different sizes i.e. 1.6 vs 2.7 . Pump curves overlaid do not give the whole answer as every pool is laid out differently. Pump curves are a good place to start but don't give the whole answer. I wish i have the money to run this little experiment to share will all. A pump installer could easily do this.

 

[JamesW]

The canned answer on this site tends to be to always go bigger but I have never seen actual watts collected at the same pool on the different sizes i.e. 1.6 vs 2.7 .

I think that they are close but the larger is believed to be slightly better.

The larger pump is definitely quieter, which is a good reason to choose the larger pump even if the power usage is the same.

Even if the larger pump was 10% more costly to run, you're only talking about less than $5.00 difference per month because you're running at low speeds.

 

[sparcel]

Discussing this pump efficiency, I started wondering whether at RPMs down around 1000 even workable for almost anyone. It seems not. After looking at performance curves for speeds of 1000 rpm, and seeing that TDH at and above 8 appears quickly shut down all flow, it made me curious what my own TDH is at these speeds. So I just did an estimate of my TDH at 30 gpm by adding up all the contributing components (given my broken pump I cannot measure it).

I come up with a TDH of ~9.3 at 30GPM which suggests that I would get near 0 gpm for this pump. Anything other than a squeaky clean filter would quickly push TDH even higher. My broken Ecostar with similar curve was often run at 1000 RPM but I never knew the GPM at that speed - it was hard to measure. It was certainly only a very gentle flow, barely enough to disturb the water at the outlets.

Maybe my calculations are flawed, but if not I don't see how hardly anyone is going to get down below the 8 TDH needed to get a flow of 20-30 GPM - at least not if they have a multiport or push pull valve.

(My TDH calcs: 3.35 ft for pipe and fittings, 1.5 ft averaged for skimmer + main drain, 1.5 ft for 2" multiport, 1 ft for elevation rise, 1.75 ft for clean DE filter, .5 ft for 2 eyeball inlets).

 

[JamesW]

You're way overthinking this.

All you have to do is install the pump and adjust the speed until it gives you the flow that accomplishes the purpose you are running the pump for.

 

[Jimrahbe]

Sparcel,

As the manager of our Repair Center, I used to interface with our engineers on a daily basis. It often made me realize that in some cases it is a blessing to be dumb, like me..

As JamesW points out above, you just adjust the pump to fit your pool's needs. I run my IntelliFlo at 1200 RPM and that is plenty of water to turn on my SWCG and skim the pool well. If I had a heater, I might have to crank it up a bit.

When my pump was new, at 1200 RPMs it is so quiet that I literally had to touch the pump just to make sure it was still running.

I always recommend getting a larger VS pump vs. the smaller VS pumps, as they move more water at a lower a RPM. My cost to run 24/7, most of the time at 1200 RPM, is less than $20 bucks a month. My pump is about 7 or 8 years old, and the newer IntelliFlo pumps are even more energy efficient.

Thanks,

Jim R.

 

[Bill1974]

To do the same amount of work they will use a similar amount of energy. I doubt it will be a significant amount difference, unless the pump curves are drastically different. Both of these pumps were design to do a similar job, not like one is meant for high head and one is a low head pump, then you would see one be more economical as the other would not be running in the range it was designed to run.

Things to consider are will the larger pump ever be able to run at full speed (what is the max flow of the rest of your equipment) or are them times when the smaller pump will not able to perform as desired at full speed (do you have a spa with many jets and need to close off some to get the desired flow?

 

[sparcel]

So what I think I'm hearing (and otherwise concluding) is that yes, it might be hard to get much gpm at 1000 RPM (such as with a Tristar 2.7), but don't worry about it, just crank it up a little to 1200 or 1500 until you get reasonable flow. Those bottom 1000 RPM curves may not apply to many people.

 

[Darin]

The cost to move the amount of water you need is the unknown difference between the two. Since running your pump is results based, just enough flow is what you are looking for.
No one has ever posted data comparing the cost difference on the same system, its always assumed that bigger is better. I don't assume. I instead use actual data, and if there is none i make a SWAG.
I really doubt there is much decibel difference between a 2.7 hp at 33% speed and larger impeller vs. a 1.6 hp at 43% speed and smaller impeller.

 

[JamesW]

I really doubt there is much decibel difference between a 2.7 hp at 33% speed and larger impeller vs. a 1.6 hp at 43% speed and smaller impeller.

If you can get the necessary flow below half speed, the smaller pump should be relatively quiet.

However, once you need to go above 50%, the noise increases significantly.

The back mounted fan gets really annoying above 50%.

It also depends on where the equipment is relative to where people are.

If the equipment will be where people can hear it, then the less noise it makes the better.

If the equipment is 1,000 feet away in a soundproof room, then the noise issue isn't very important.

 

[sparcel]

Given my (now dead) Ecostar pump is only 15 feet from where we regularly sit outside (behind a fence), I could notice a bit of difference even between 1000 rpm and 1400. It was tollerable, but I would not want to go much higher for the low speed mode. Maybe 1500 could be ok, but I’d prefer 1200 or lower. This is why I was concerned about TDH and how to get down toward 1000.

 

[Mike1000]

Given the same pool and equipment setup I could choose different horse power variable speed pumps from the same brand such as the Hayward 1.85 or 2.7 HP Tristar. Is there any operating cost downside to the 2.7 HP pump?

I guess (based on the published performance curves) the higher HP pump (in the same line) must have a different impeller because it pushes more water at the same RPM, and therefore it must be using more watts. So, I could run the higher HP pump at a bit lower RPM. But, looking at the performance curves of the different pumps the gallons moved at a given RPM and speed don't differ by as much as the rated HP differs, so I'm not sure what that means about the power used per gallon pumped.

Hi, based on my recent experiences trying to balance the HP of a pump with power consumed and GPM, I'd suggest you get a flow meter (Amazon around $80) to measure the actual flow as GPM is not directly proportional to RPM, impeller size and HP. Two factors, pipe diameter and the state of the filter directly affect the flow or GPM. Right now, we're getting yellow pollen dumped in the pool from trees that inside 2 days has reduced the low speed flow from 20 GPM to less than 5 GPM. This last for 1-2 weeks in March and we'll be back to normal after that. A more powerful pump than my 2 speed 1 HP pump, say 2 or 3 HP will obviously cope better with the filter being clogged but the power consumed increases exponentially to maintain an adequate flow even at a lower speed. Its all a trade off trying to balance size of pool, pump size, GPM, RPM and running costs whether its a VS pump or 2 speed pump. Single speed pumps should not even be considered.