Reduced Pump efficiency - lower variable speed Pump Flow rate

[02slowride]

I have a Pentair VSF pump. When we first opened the pool, I wrote down the flow rate at various speeds. We have a manual heater bypass valve to isolate flow away from the heater when not in use.

Other than a dirty cartridge filter (which I've already cleaned in hopes of resolving this - no luck); what would cause my pump to have reduced flow (GPM) at a given speed?

Here is some data I logged from my pool. All measurements taken reflect the heater bypassed.

Pump Speed	Initial GPM	Initial KW	Current GPM	Current KW
1050	26	86	21	81
1750	47	354	41	332
2500	73	1007	59.5	915
3450	96	2510	82	2298

Attempting to operate at the same GPM, I now require a higher pump speed and more energy consumption.

Target GPM	Current Pump speed	Current KW
26	1155	103
47	1955	456
73	3050	1616

The pump is not noticeably noisier compared with the prior operation

The only anomaly I recall, perhaps around this time, is that we put our heater into operation. Having waited out the original plaster startup wait period, plus the gas meter upgrade delay with utility. When we initially ran the heater the first time, I recall some white "pasty" substance in the water. In hindsight, I suspect it was some pipe dope on the interior of the heater's plumbing that circulated due to the initial heater operation. It has never happened again since the inaugural session.

The other factor has been the reduced water temperature, going from 80-degree temps to now 60-70 degree temps.

I welcome all advice you can offer and any potential remedies I should pursue. Thanks

 

[PoolStored]

Other *smarter* folks will chime in. Here is my go at it.

1. Why do you care? We use the pump to skim, sanitize/circulate (SWCG, chlorine stenner, mixing chemicals etc.), heat and filter the water. If your water level is correct and you have weir doors, it will skim fine at low rpms. If your SWCG (when it gets hooked up) works at the RPM you specify. If your heater is heating. If your filter is working, the flow RATE doesn't matter. I run the lowest RPM that satisfies these requirements in my pool. I don't fret the flow.
2. Ok, having said that, you have a new pool. You measured NEW cartridges. You cleaned them. They are not new cartridges, they are "clean" cartridges. This, maybe, alone could be the difference.
3. Stretch hypothesis (really need the pump guys to weigh in). Pumps tend to be MORE efficient at lower fluid temperatures. It is minor, but the higher efficiency, and the resulting back pressure from the media (as seen by the higher RPM delta at higher RPMs) could be the difference.

TFP. Trouble Free Pool. I go back to #1. We run the pump for a reason. If we can get the results (reason) at the lowest RPM, then don't fret.

 

[02slowride]

Why do you care? We use the pump to skim, sanitize/circulate (SWCG, chlorine stenner, mixing chemicals etc.), heat and filter the water. If your water level is correct and you have weir doors, it will skim fine at low rpms. If your SWCG (when it gets hooked up) works at the RPM you specify. If your heater is heating. If your filter is working, the flow RATE doesn't matter. I run the lowest RPM that satisfies these requirements in my pool. I don't fret the flow.

Thanks PoolStored for the insight. I hadn’t considered cartridge wear/deterioration.

I guess the core reason I care is - in order to get proficient skimming I need about 30 GPM flow. Based on the changes in my pump efficiency it now consumes +20 more watts / hr to perform this. This is +480 watts per day and +14.4 kW per month. On one hand it is only a few dollars a month. However I would prefer to be as efficient as realistically possible.

 

[Darin]

This could simply be your new watt number for operation due to you filter. Remember a slightly dirty filter works better than a completely clean filter. the filter works in water just like the air filter in your car.
Did you check your voltage when you got your watt number? You need to read the volt number each time to get an accurate watt number when doing the math.
Just run the pump fast enough skim and run the SWG. Don't look at gpm anymore because gpm does not matter. Skimming and chlorine matter.

 

[PoolStored]

I guess the core reason I care is - in order to get proficient skimming I need about 30 GPM flow

Do you have weir doors that function?
What is the water level in the skimmer "window"? 25% up from bottom? 50% 60% 75%?

 

[02slowride]

Do you have weir doors that function?
What is the water level in the skimmer "window"? 25% up from bottom? 50% 60% 75%?

The weir doors are operating as I believe they should. There is visible current in the skimmer And debris collection. However this is after I increased the pump speed after seeing the drop in flow, compared to the previously programmed speed. Which is what started this investigation.

Water level is auto maintained at mid of tile 50% (+ if we get some rain).

 

[02slowride]

This could simply be your new watt number for operation due to you filter. Remember a slightly dirty filter works better than a completely clean filter. the filter works in water just like the air filter in your car.
Did you check your voltage when you got your watt number? You need to read the volt number each time to get an accurate watt number when doing the math.
Just run the pump fast enough skim and run the SWG. Don't look at gpm anymore because gpm does not matter. Skimming and chlorine matter.

I am documenting the watt, speed and GPM stats from the pump’s display. Line voltage at the house might be a little higher recently given the cooler temps - fewer AC-unit demand hours on the grid. I’d say I typically observe 238-242v on the service. 238v or even 236v on a hot summer afternoon. 242v at late night, early morning (summer) or cooler months of year. Given this I should have more efficient pump operation if I have higher consistent voltage, right?

GPM is my proxy for what speed to run the pump. Initially I got good skimming at 1050 RPM. Now I am having to run the pump at 1250 RPM to get similar skimming results.

So my core concern is why my plumbing/pump got less efficient.

I guess this is a long term test - I’ll be interested to see if the improvement is realized first time I replace the cartridge filters. We cleaned the filter about a month of pool fill. Removing any startup plaster debris. During this whole time, I didn’t see any pump efficiency degradation. However, given the marked drop in such a narrow window of time (heater startup) I’ll be surprised if the aged filter (3 month old) was the culprit.

Any other thoughts? All ideas welcome.

 

[PoolStored]

Water level is auto maintained at mid of tile 50% (+ if we get some rain).

Is it at 50% now, or did you get some rain?

Water level makes a huge difference. Even a 1/4" will change my skimming dynamics.

 

[Darin]

Your plumbing did not change but as PoolStored states that the height of water in your pool can and does change. (height in skimmer)
Your pump is not going to be less efficient because your impeller will not change size for a very looooong time since we are just pumping water, but your pressure will change due to your filter.

 

[Newdude]

I have found my new hayward wide mouth skimmers want 2/3 full or they don't skim well.

There is visible current in the skimmer

This stood out to me when catching up in the thread. I don't see any water movement in mine. The door flaps every 10 seconds or so and the baskets stay secured. (As does any leaves / debris).

1) your idea of skimming well may be more flow than you need.

2) 9 times out of 10, if we see a flow issue, it's algae. Perform an Overnight Chlorine Loss Test to rule it out.

 

[02slowride]

Is it at 50% now, or did you get some rain?

Water level makes a huge difference. Even a 1/4" will change my skimming dynamics.

Yes, 50% I just checked. We are right at the mid grout line (3 tiles above, 3 below). I have not observed much skimmer correlation either before or after regarding pump efficiency. I do see your point on skimming efficiency.

Worth noting this reduced pump efficiency has been sustained for more than the past month. The “initial”reads were sustained over the first 2.5 months of pool operation.

 

[mas985]

The pump should also report pump PSI and the filter gauge PSI reading is also important. Likely those have changed as well and the amount they have changed can give you a clue to where the issue might be.

For example, an increase in filter pressure at a given RPM generally means there is higher head loss from the filter to the pool. If the filter pressure is less, then it generally means that there is more head loss on the suction side of the pump but it could also mean there there is an obstruction in the impeller or the impeller inlet. Have you checked both?

Also, taking measurements off the Intelliflo can introduce errors as the numbers are not always stable. Before taking a reading from the display, make sure it is stable over several minutes.

 

[02slowride]

The pump should also report pump PSI and the filter gauge PSI reading is also important. Likely those have changed as well and the amount they have changed can give you a clue to where the issue might be.

For example, an increase in filter pressure at a given RPM generally means there is higher head loss from the filter to the pool. If the filter pressure is less, then it generally means that there is more head loss on the suction side of the pump but it could also mean there there is an obstruction in the impeller or the impeller inlet. Have you checked both?

Also, taking measurements off the Intelliflo can introduce errors as the numbers are not always stable. Before taking a reading from the display, make sure it is stable over several minutes.

Thanks mas985 for the considerations.
When running at 1050RPM (before or current), I don’t register any pressure on the filter gauge, but I do have flow confirmed via the returns. Where do I see the pump PSI reading? I have not seen that in the display screens (primarily from the IntelliCenter2 app)

I have used my fingers to search for debris when the filter basket is removed searching in the area as it enters the impeller inlet. What is the best way to check for obstruction on the impeller?

100% agree on the intelliflo readings, I typically monitor for 1-2 mins after a speed change or re-read the data points a few hrs/days later to confirm the data I have recorded.

 

[02slowride]

9 times out of 10, if we see a flow issue, it's algae. Perform an Overnight Chlorine Loss Test to rule it out.

Thanks for the advice, I performed an OCLT and it was steady at 3.5 ppm free chlorine, 0 combined.

 

[mas985]

Sorry forgot to mention that filter pressure needs to be taken at higher RPMs close to full speed to get a valid reading. Like nearly all measurements including the Intelliflo internal measurements, error increase with decreasing RPM so you really can't make any conclusions based upon low RPM measurements only.

Pump pressure should be displayed on the pump display.

But for both the pump pressure and filter pressures, you would have needed a "before" measurement as well for a proper assessment.

To inspect the impeller, you would need to remove the motor from the wet end. The impeller is on the end of the motor shaft.

 

[02slowride]

To inspect the impeller, you would need to remove the motor from the wet end. The impeller is on the end of the motor shaft.

Got it. Thanks.

Does this require any new seals when reassembling? Pump was installed 5 months ago, been in operation 4 months. Any concerns Pentair would make a future warranty fuss, if they conclude the pump was previously disassembled?

 

[mas985]

Pumps are made to easily separate at the wet end so that you can remove any debris stuck within the volute.

Note that #8, pump wet end, in the picture will come off in one piece, everything else remains mounted together. The diffuser (#30) sits over the impeller (#27) but easily slides off.

A new shaft seal should not be necessary as long as you don't remove the impeller from the motor shaft.