Pump and Motor Replacment Planning

[Oly]

Planning a pump and motor replacement this winter and have been playing with some of the online calculators without much confidence.
As I live in CA I will be going with a 2-speed motor and am trying to determine the ideal motor size. Not sold on variable speed pumps due to reliability and cost.
Currently I have a 2hp single speed motor with a Pentair Wisper Flow pump and run the pump now 4 hours per day utilizing only the skimmer, no bottom drain, and keep the pool clean with a Maytronics robot. No water features or heater or SWG or bottom drain or suction side cleaner, only one skimmer and two outlets.
I am unsure of the pipe size underground but the 2" inlets and outlets at the pump are stepped down to 1.5" PVC. I was told that there are copper lines underground but have not done any excavation to confirm this as it is under brick decking.
I know I can replace with a 2hp 2 speed with the idea of using the lower speed most of the time and having the 2hp option. Can I downsize to a 1.5 hp 2 speed? This is what I am attempting to calculate.
My Pentair DE filter is 48 sq ft and it lists the max operating pressure at 50psi and I currently am operating at 25psi when clean using the 2hp motor.
The label on the filter lists the GPM at 120 and I'm not sure if that is max flow or the recommended flow rate. Any ideas?

My plan is to tap the inlet and exit lines and insert a vacuum and pressure gauge to measure head, than target my GPM to determine motor size. The head calculators I have explored involve a lot of assumptions that leaves me guessing.

Any thoughts or ideas would be welcome, thanks.

 

[JamesW]

Pentair might have a rebate available now for its variable speed pump.

IntelliFlo VSF - Efficient Pool and Spa Pumps - Pentair

Your utility might also have a rebate available.

A variable speed pump is a good choice.

There are variable speed motors available.

You really don't need much hp.

You can drop the hp way down. To go to a lower hp motor, you need to replace the impeller.

You really shouldn't need more than 1.0 to 1.25 total hp.

Motors are sized by hp x sf = thp. Horsepower x service factor equals total horsepower.

Impeller.......total hp
073126.........0.95
073127.........1.25
073128.........1.65

If you go 2-speed, the high speed total hp has to match the impeller.

2-speed motors are single voltage, either 115 or 230. So, make sure that you know what the supply voltage is.

 

[MyAZPool]

There is a $100 rebate available for the IntelliFlo VSF (011056) until September 30.
I just got approved for it.

 

[JamesW]

The Pentair SuperFlo VS has a $50 rebate.

By the time you buy a two-speed motor, impeller, seals etc. your cost is close to a new pump.

And, that assumes that the pump housing is still good. You might get everything apart and find that the housing is bad.

Then, you have basically bought a new pump, piece by piece.

A VS pump will use a lot less total energy. If your electricity cost is high, a variable speed pump is the way to go.

In my opinion, your best option is to get a new variable speed pump.

 

[MyAZPool]

There is a $100 rebate available for the IntelliFlo VSF (011056) until September 30.
I just got approved for it.

CORRECTION!!

I just saw where the rebate for the IntelliFlo VSF has been extended until 31 December 2018

 

[Oly]

Tapped in a vacuum and pressure gauge today to calculate my total dynamic feet of water from psi. Total psi is 30.3 (-2.3 vac & 28 press.)
So I have 70 feet of head pressure. It is a long run to the pump from the discharges.

I looked at a flow chart graph for the 3/4 hp Pentair WisperFlo (motor and pump) at that head and it would take 10 hours to pump the pool volume.

It seems everyone is sold on the VSF pumps but I am not seeing the advantage(s). A single speed motor and pump is 1/5 the $$. What am I missing? Thanks.

 

[MyAZPool]

Tapped in a vacuum and pressure gauge today to calculate my total dynamic feet of water from psi. Total psi is 30.3 (-2.3 vac & 28 press.)
So I have 70 feet of head pressure. It is a long run to the pump from the discharges.

I looked at a flow chart graph for the 3/4 hp Pentair WisperFlo (motor and pump) at that head and it would take 10 hours to pump the pool volume.

It seems everyone is sold on the VSF pumps but I am not seeing the advantage(s). A single speed motor and pump is 1/5 the $$. What am I missing? Thanks.

I think you may be using the wrong term (VSF) for what you are actually asking about. I think you mean a VS (Variable Speed) pump. There are many advantages to a VS pump.

You can run your pump longer at lower speeds which will provide quite a bit of energy savings $, rather than running your pump at higher speeds for shorter periods of time.
The new VS pumps are much quieter and they also run cooler, vibrate less, have advanced diagnostics and are digitally controlled.

If you are talking about VF or VSF pumps, those are a different animal and are not usually required in most normal situations. The VF is variable flow and the VSF is Variable Speed AND Variable Flow.

Most prefer the Pentair IntelliFlo VS pump (011018). Extremely reliable and highly recommended by most. Surge protectors are recommended by many (myself included), due to the advanced electronics of the newer pumps.

 

[JamesW]

The head loss is dependent on the flow rate. It's not a single number.

The total hp of a single speed filter pump in California has to be less than 1.0 hp.

The 1/2 hp Whisperflo has a total of 0.95 hp, so it just barely slips under the wire. The cost is about $460.

A SuperFlo VS is about $645 before any available rebates from Pentair or your local utility.

So, the price isn't that big of a difference.

The Superflo will use about 200 to 300 watts vs about 1,000 watts for the WhisperFlo.

The Superflo vs is also a lot quieter.

 

[Oly]

Thanks for the replies but I need some clarification.

I had a look at the performance curve for the SuperFlow VS and only the top speed of 3450 rpm is pushing any water above 70 TDH (total dynamic feet of head in water). The highest speed will flow 60 gpm at 70 TDH, connected to 230v. The speeds, flow rates and TDH for the SuperFlow VS are as follows:

3450 rpm = 0-10gpm at 88 TDH and 60gpm at 70 TDH.
3000 rpm = 0-10gpm at 65 TDH
2200 rpm = 0-10gpm at 35 TDH
1400 rpm = 0-10gpm at 13 TDH

Am I correct in reading these performance curves that only the high speed on the SuperFlow VS pump will circulate water above 70 TDH?

Reading the performance curve of the WisperFlow 3/4hp shows:
3450rpm = 0-10gpm at 85 TDH and 45gpm at 70 TDH.

So what is the comparison on the power usage between these two units figuring my rate at 0.285 cents/kWh (tier 2)?

Also are you saying I cannot purchase a 3/4hp WisperFlow in CA?

 

[KDpoolguy]

I will add one more consideration for a Variable Speed Pump. Check your electric utility. We have Edison and received a $200 rebate on our VS pump, which brought it down to within $100 of a new single speed pump+motor. Most utilities are offering sizable rebates only for VS, not for 2 speeds.
In the 3 years I’ve had my VS pump, had no issues, and saved at least $1700 already in electrical savings. Maybe more actually. We stay out of Tier 2 rates ($0.25kwh) some months and that was never possible with single speed usage. It pays for itself. I keep my single speed in storage in case of failure.
I run at 2500rpm for my in floor cleaner for 4 hours and 1000rpm for 7 hours. I use approx .80 to just over 1 kWh per hour of use at 2500 rpm, and .08 to .20 kWh for each hour At lower rpm.

 

[JamesW]

Your TDH is only 70 at one flow rate.

The TDH is different for every flow rate.

General example:

Flow (gpm)..........TDH (feet of head loss)
10.............................1
20.............................5
40...........................20
80...........................80

If you graph the information on a chart, you get a "system curve". If you plot the system curve on the same chart as the pump curve, you can see where the lines intersect. The intersection of the system curve and the pump curve is called the operating point.

https://aquamagazine.com/images/old_site/articles/images/Pumps_multispeed_0115.jpg

How to Read Pump System Curves - AQUA Magazine

The basic idea is that it takes more force to move the water at 80 gpm than it does at 40 gpm.

If you double the flow rate, your TDH increases by about 4 times.

Every system has its own curve depending on what equipment and plumbing you have.

For example, a system with 1.5" plumbing will have more TDH than a system with 2" plumbing at the same flow rate. Pushing 80 gpm through 1.5" pvc might create 80 feet TDH but only 24 feet TDH if the pipe is 2".

In California, the rule is that a filter pump has to be less than 1.0 hp total or 2 speed or variable speed.

The total hp is the rated hp x the service factor. A 1/2 hp WhisperFlo has a service factor of 1.9 and a total hp of 0.95.

The power usage depends on the flow rate. For example:

Gpm........watts
20..............40
40............320
80..........2560

It's a cube factor. So, doubling the speed increases the power usage by about 8 times. Pushing 80 gpm uses about 64 times as much power as pushing 20 gpm.

So, even if you factor in that you have to run 4 times as long, your total energy usage for the same total gallons moved is 1/16.

Running for 6 hours at 80 gpm uses 16 times as much energy as running for 24 hours at 20 gpm.

[Edit]See below for a more accurate power calculation.[End edit]

Affinity laws - Wikipedia

 

[mas985]

Pump affinity laws only apply to the pump head and not the actual energy use. So while the power delivered to the impeller is a cubed factor, the energy use is actually much different depending on the motor efficiency. For example, the Intelliflo VS on plumbing Curve-C would have the following wattage at each of those flow rates:

20 GPM -> 83 Watts
40 GPM -> 233 Watts (~3x)
80 GPM -> 1436 Watts (~17x)

 

[JamesW]

If the power delivered to the impeller is a cubed factor, why isn't the power usage a cubed factor?

Intelliflo VS (011018) RPM / watts to GPM

2,800 rpm = 1,280 watts.
1,400 rpm = 200 watts.
At double speed, the power usage is 6.4 times.

2,800 rpm = 1,180 watts.
1,400 rpm = 155 watts.
At double speed, the power usage is 7.6 times.

SuperFlo VS RPM vs Watts

3,450 rpm = 1,950 watts.
1,750 rpm = 336 watts.
At about double speed, the power usage is about 5.8 times. Probably about 6 x at 1,725 rpm.

1,000 rpm = 143 watts.
2,000 rpm = 450 watts.
About 3.14 x at double speed.

So, about 3 to 6 times difference when going double speed depending on start and end speed.

20 GPM -> 83 Watts
40 GPM -> 233 Watts (~3x)
80 GPM -> 1436 Watts (~17x)

Doubling at low speed is about 3x and doubling at higher speed is about 6x, which matches the reported real usage.

 

[mas985]

Because motor efficiency is not a constant with RPM for any pump/motor combination. A VS motor+drive can be modeled (assuming a constant plumbing system) as

Wattage (RPM) = C1 + C2*RPM^3

It is the C1 factor (~60 watts for the Intelliflo) that modifies the cubed relationship. That C1 factor for the Intelliflo is related to the drive control electronics which draws a constant power no matter what the RPM. The motor efficiency itself (excluding the drive) is fairly constant although there is a minor reduction (~2% per Pentair) at lower speeds.

For a two speed motor, C1 is much higher and due to the efficiency change of the winding resistance (i.e. thinner wire).

Also C2 changes with movement along the head curve. The full power curve for any pump on any plumbing can usually be fit to:

Wattage (RPM,GPM) = C1 + C2'*RPM^3 + C3*RPM^2*GPM

 

[JamesW]

Ok, thanks for the help.

 

[Oly]

Great, a lot of good information here, thank you.
Yes utilizing the rebates will close the initial cost gap.
I have a better understanding now of the TDH, flow rate and pressure relationship thanks to the links provided.

Now I realize my piping runs are long and my discharge sizes small so assume I have a fair bit of flow resistance or friction loss already in my system.
The pump manufacturers provide the pump curves, do they have system curves as I do need both correct?

Everyone agree not to consider a 2 speed unit but better with a variable speed?
If I do go with a variable speed pump what is the difference between a VS and a VSF pump and pros and cons?

Thanks.

 

[JamesW]

You don't really need a system curve with a variable speed pump. It's nice to have, but not necessary.

You just adjust the flow rate until it meets your system requirements.

The main things are skimmer skimming action and flow for special equipment like heaters, solar or in-floor cleaners.

If all you need is skimming action, 20 gpm per skimmer should be sufficient.

A VS is a variable speed pump that you can adjust the speed.

A VF or VSF allows you to set the actual gpm because the pump measures TDH by measuring the difference between the suction and discharge pressure.

It then uses software to calculate the gpm based on the speed and TDH.

You can generate a system curve with a VSF pump by setting a speed and noting the gpm and TDH as reported by the pump display.

You do that for about 10 points and draw the system curve.

You can do it with a VS pump but you have to measure the suction and discharge pressure and the flow rate with a flow meter.

The VS is usually sufficient unless you really want the VSF.

 

[Oly]

So how do you estimate flow rate accurately without a flow meter?

If you calculate say a head loss of 35' simply due to friction loss in the plumbing and filter system do the low speeds on VS pumps produce any flow at all?

 

[mas985]

So how do you estimate flow rate accurately without a flow meter?

Use the spreadsheet in my signature. It can get just as close as most flow meters.

If you calculate say a head loss of 35' simply due to friction loss in the plumbing and filter system do the low speeds on VS pumps produce any flow at all?

Again, head loss is dependent on flow rate and flow rate is dependent upon RPM AND the plumbing fixtures. It is a multi-variate problem with the solution being the crossing of the plumbing curve with the head curve. But you need to know both to find a solution. The spreadsheet does this calculation for you using one of several different methodologies.

But for most pools, knowledge of flow rate is really not necessary. There are very few reasons that you would need to know flow rate.

Here is a VS pump with several different plumbing curves plotted over the head curve which has several different RPMs. The crossing points of the head curve with the plumbing curve is the operating point. Note how the operating point changes with both RPM and the plumbing curve.

 

[Oly]

Thanks mas985. The information you link is great and helping me understand my system and how to approach this upgrade. I have also gleaned a lot of good information from the older threads. I realize now that VS pumps provide pump/filter system performance and efficiency options not available with single and 2 speed units and the cost difference appears to be a wise investment.