Single speed pump vs. Variable speed pump

[mas985]

You are assuming that the 13’ of head the pump is pumping out is sufficient to overcome the head loss in the piping to achieve the flow rate. In some cases it may be. However, in cases where it is not, the user may not be able to lower the RPMs enough for a cost savings because they cannot achieve the flow they need.

The 13' of head is the head loss for typical for a 2" plumbing system @ 40 GPM including a filter, heater, valves, fittings, etc.. Energy Star, California Energy Commission and APSP assumptions for plumbing Curve-C. The plumbing curve defines the relationship between head and GPM:

Head (ft) = PC * GPM^2

Here is analysis for 1.5" plumbing, Curve-A:

Intelliflo: Plumbing Curve 0.0167 @ 1865 RPM = 40 GPM, 26.7' of Head, 469 Watts
Whisperflo: Plumbing Curve 0.053 @ 3450 RPM = 40 GPM, 84' of Head, 1686 Watts

Pump cost: $1130
Savings per month: $44
Break even: 27 months
Total savings for 120 months: $4150

Not as good as the 2" case but still far better than throttling.

You can download the spreadsheets in my signature and run the analysis anyway you want.

 

[Randrx2]

Also, we pay

The 13' of head is the head loss for typical for a 2" plumbing system @ 40 GPM including a filter, heater, valves, fittings, etc.. Energy Star, California Energy Commission and APSP assumptions for plumbing Curve-C. The plumbing curve defines the relationship between head and GPM:

Head (ft) = PC * GPM^2

Here is analysis for 1.5" plumbing, Curve-A:

Intelliflo: Plumbing Curve 0.0167 @ 1865 RPM = 40 GPM, 26.7' of Head, 469 Watts
Whisperflo: Plumbing Curve 0.053 @ 3450 RPM = 40 GPM, 84' of Head, 1686 Watts

Pump cost: $1130
Savings per month: $44
Break even: 27 months
Total savings for 120 months: $4150

Not as good as the 2" case but still far better than throttling.

You can download the spreadsheets in my signature and run the analysis anyway you want.

Yep I agree this is true in high utility cost situations (Sorry California!).

In your situation, your breakeven is 27 months. Others have a much lower utility rate. Our cost per kW is $0.11. So our break even would be more like 70-75 months.

 

[PoolNewb2020]

And if you get with the flow restriction valve and cut the power consumption in half, as I was able to do with my system, you get.
Ball valve:$12.65
Savings per month $30
Break even: 13 days
Total saving over 120 months: $3600

Savings are there for those that, for whatever reason, have a single speed pump.

 

[JamesW]

And if you get with the flow restriction valve and cut the power consumption in half, as I was able to do with my system, you get.
Ball valve:$12.65
Savings per month $30
Break even: 13 days
Total saving over 120 months: $3600

Savings are there for those that, for whatever reason, have a single speed pump.

Flow restriction is not a good choice. You're not getting the value that you think you are. The flow rate is decreased. So, your number of gallons pumped vs. energy used is not favorable.

 

[mas985]

Yep I agree this is true in high utility cost situations (Sorry California!).

The analysis I showed was for $0.15 kw/h which is half of what we pay in CA. At $0.11/kwh, break even is 36% longer.

And if you get with the flow restriction valve and cut the power consumption in half, as I was able to do with my system, you get.
Ball valve:$12.65
Savings per month $30
Break even: 13 days
Total saving over 120 months: $3600

Savings are there for those that, for whatever reason, have a single speed pump.

Ok, but the savings I showed was on top of your savings (i.e relative to a throttled pump).

But what pump can you throttle to 50% power?

 

[PoolNewb2020]

Flow restriction is not a good choice. You're not getting the value that you think you are. The flow rate is decreased. So, your number of gallons pumped vs. energy used is not favorable.

I never claimed it was a better choice than VS, but it is still a choice.
VS pumps are great because they allow the flexibility of having big HP to power a spa or water features, but when these extras are no being used, they can be dialed way down.

But if you have a pool that needs flow for filtration only, to me it makes little sense to get a say 1.5hp VS pump, only to run it at 50%, or lower all the time. Why not get a 3/4hp pump instead, which could be further dialed down with a simple flow valve?

Problem with many industries catered to regular people is that they heavily oversell their equipment. Bigger and more powerful is always better. Nobody seems to be interested in sizing their stuff properly.

 

[mknauss]

In July 2021 this discussion becomes moot in the United States. Any pool pump motor replacement of 1 THP and higher must be a VS motor. In California, it is required now.

 

[setsailsoon]

Marty, I've heard this a lot. Does it apply to booster pumps for spa's also or just the filter pump? Seems unreasonable for a pump that only runs 1 speed.

 

[mknauss]

All pumps. Except waterfall pumps that already pump at 1750 rpm.

 

[mas985]

But if you have a pool that needs flow for filtration only, to me it makes little sense to get a say 1.5hp VS pump, only to run it at 50%, or lower all the time. Why not get a 3/4hp pump instead, which could be further dialed down with a simple flow valve?

Because it would still be more expensive. Even a two speed pump will end up more cost effective.

 

[PoolNewb2020]

Marty, I've heard this a lot. Does it apply to booster pumps for spa's also or just the filter pump? Seems unreasonable for a pump that only runs 1 speed.

Unreasonable is putting it mildly. A VS pump that has to run at max speed is actually less efficient than an equivalent single speed one.

 

[mas985]

Where do you get your information? The intelliflo uses less energy at the same flow of any single speed pump.

 

[PoolNewb2020]

Where do you get your information? The intelliflo uses less energy at the same flow of any single speed pump.

Not flow, but RPM. If a pump is small enough that is has to run at max RPM or very close to it, what's the point of VS? Why the need to mandate it beside probably some heavy lobbying?

 

[mas985]

No VS pump really has to run at full speed except for high flow rate water features (e.g. spa, waterfalls) which aren't run that often. For circulation, you shouldn't need more than 15 GPM per skimmer. But I reiterate, a VS pump at the same flow rate will use less energy than a single speed at the same flow rate. So no matter what the flow rate requirement maybe, the VS will use less energy.

But most VS pumps are above 1.5 THP. You are better off with a larger VS pump and running it at lower RPM as it produces much lower noise than a smaller pump at higher RPM. You really aren't paying much more for the higher capability.

As for the mandate, that falls on the PBs. For years they put pumps that were much larger than needed on the pools. They have even been reluctant to use two speeds. Had they accepted these technologies earlier, there might not have been the need for a mandate.

 

[JamesW]

Here's my estimate of flow vs. power usage by restriction vs. reducing the rpm.

A = Intelliflo VS watts at 3450 rpm with restriction.
B = Intelliflo VS watts with variable rpm.

GPM........A.............B
40.........2,000........172
60.........2,300........495
80.........2,500......1,045
100.......2,800......1,867
120.......3,000......3,000

Note: IntelliFlo VF Flow and Power vs Flow Pump Curve available in the Intelliflo vf manual.

Flow vs. power for reduced rpm estimated from the pump affinity law.

As you can see, reducing the flow by restriction uses way more power than reducing the flow by lowering the rpm.

 

[setsailsoon]

So, maybe the new standard design will be a single large VS pump that runs high only for the spa. Then lower speeds for everything else. Based on Jame's numbers looks like it might work well and save having an extra pump that mostly just sits there and rusts.

 

[PoolNewb2020]

As you can see, reducing the flow by restriction uses way more power than reducing the flow by lowering the rpm.

Yes, my little experiment showed similar results.

 

[JamesW]

And if you get with the flow restriction valve and cut the power consumption in half, as I was able to do with my system, you get.
Ball valve:$12.65
Savings per month $30
Break even: 13 days
Total saving over 120 months: $3600

Savings are there for those that, for whatever reason, have a single speed pump.

In my opinion, using a valve to restrict the flow is not a good choice.

I don't think that you could get the power to half without cutting the flow to basically zero.

Going to a smaller impeller would be a much better option to reduce power and flow.

You can go to a two speed pump or variable speed.

 

[Darin]

I actually did a write up on the savings of variable speed by utilizing a vfd. It can be found here. VFD, Real World Results.
Just remember that the pump affinity law is theoretical, there is heat and noise losses, thus using an amp clamp will give your "real" results. Still using a variable speed, whether a conventional 3 phase or mag motor will give quick payback. Now is it worth it to you to just throttle your single or two speed motor? Well that is up to you as you will save money, just not as much. If you have the ability to save money now, then buy a VS system, if you do not then throttle your present pump after all you will save money. Whenever it does die replace it with a VS.

 

[mas985]

Just remember that the pump affinity law is theoretical, there is heat and noise losses, thus using an amp clamp will give your "real" results.

Technically, affinity laws only apply to pump wet ends and not motors & drives. Unfortunately, too many have used the affinity equations to estimate motor power savings which is a misuse of the affinity equations. This is because motor efficiency changes with RPM so the affinity equation cannot be used unless the efficiency is known as well which in most cases it is not. However, the affinity equations do fairly accurately represent hydraulic power, head and GPM for which they were originally intended.