Variable speed pool pumps?

[trader4]

In the Polaris pool cleaner thread the subject of variable speed pool pumps came up. Which got me thinking about them again because a friend who has a pool with a single speed pump is considering options. Doing a bit of research, here's what I have concluded and I wanted to see if anyone has addional thoughts.

First, it looks like because of the design of the variable speed permanent magnet motors, they are inherently more efficient so you use about 15-20% less energy even at full speed.

Second, the biggest savings in energy come from slowing the pump down, reducing the flow resistance. That means you can run the pump for say 4 times as long, at 1/4 the flow rate, move the same amount of water, but save 70 to

80% or so in energy.

However, the price of these pumps is very high. A complete new pump runs from $900 to $1200 dollars online. The next obvious problem is that they typically have only a 1 yr warranty. I found an online story where a guy bought a Pentair Intelliflow one where one of the two small control boards failed shortly after the warranty ended. He says Pentair will not sell the replacement board as a seperate part or take pumps back for repair. He would have to buy a whole new pump. It looks like Jandy, another major manufacturer, does sell parts, but the equiv part for their pump would be $400.

So, this got me to thinking. It appears to me the biggest part of the energy savings can be obtained by just going to a dual speed pump. And since my friend has a working pump that is about 5 years old, I'm thinking the most cost effective solution is to just replace the motor with a dual speed motor. We could do that for under $300.

With the variable speed pump, assuming we bought the lowest cost one, by my calcs, with the energy savings it would take almost 4 years to break even. With the dual speed motor, it would take less than 2 years. The pool is seasonal, used 3.5 months a year. And it looks to me like the additional savings in energy going with the true variable speed pump is only about $50 more a year over the dual speed. And then what happens if the fancy electronics on this motor, which sits outside all year, fails at say year 3, 5 etc?

The only other advantage of the variable is you can fine tune it to run at exactly the speed you want, but I don't see a compelling need for that. Also, there aren't any utility rebates available here that would alter the equation. So, it seems for this app, the variable speed makes no sense. If you had a pool running year round, higher cost electric, rebates, etc, then the results might be different.

So, swap out the motor for the dual speed for $300? Any other thoughts, issues I missed?

[gfretwell]

This still does not make sense to me and it has become law in a lot of places. If you can really keep your pool clean with a 1/4th hp pump, why not just buy a 1/4th hp pump? I run my 1HP on a timer that assures one turn over a day and runs strong enough to vacuum at the same time. I am not sure how a variable speed pump would save me a dime.

[DD_BobK]

T4-

Looks to me like you covered all the issues..... dual speed is right choice. Fancy electronics (unless cheap to replace) will eat up any energy cost savings when the board fails. :(

cheers Bob

[hrhofmann]

I don't have a pool, but the OP seems to have his ducks right in line.

[DD_BobK]

You can probably 95% of the time get by at 25% power (low speed running)

BUT there might be sometimes when you need full system flow.

In SoCal we have seasonal winds that can super load a pool with dirt, leaves & other debris. To clean the pool up quickly, it might take a couple days running

24hrs at full RPM. At low speed it would take a lot longer. :(

But for "typical" / "low dirt load" many hours at low speed could do the job.

Two speed pumps give you enough flexibility to tailor motor speed to pool cleaning requirements.

Kinda goes along with the concept of "side stream" / "kidney loop" filtering of a hydraulic oil tank as opposed to full flow filtering.

It's all about the total energy used to accomplish the job.

At first glance dual speed pumps might seem silly but they do the job AND save money.

cheers Bob

[trader4]

Agree with the advantage to full speed if the pool has some problem, eg it's startup after winter or it's dirty and you have a pool party this afternoon, etc.

The higher speed would probably be needed to backwash the DE filter. Doubt that low speed would do the job. And also if the pool gets too full from rain and you want to lower it would be better to get it done in 20 mins instead of an hour. But overall, I think the backwashing is the one thing I don't think you can get around. Plus a dual speed motor is only $100 more than single.

[gfretwell]

So what's wrong with only running the big pump long enough for a full turnover (6 hours in my case) and turning it off?

[Ashton Crusher]

I looked at the same issue a few months ago. I need to have the pump run at high speed at least part of the time in order for my sweep system to work. To do that the least expensive option would be a 2 speed motor installed on my current pump. But to automate it I also would need to buy the dual timer control for it which is another $100. By the time you add it all up it will take several years to break even. So I'm still on the fence. My current pump and motor is 20+ years old and when it goes out I will think about this issue again.

[trader4]

The problem is that you will be using a lot more energy than if you moved the same amount of water, but did it slower over a longer time. Instead of moving that water in 6 hours, if you cut the flow rate down to 1/4 and ran it 4 times longer, ie 24 hours, you'd reduce energy usage by about 70%, while still moving the same amount of water. Most of that is from the fact that the amount of force it takes to move water through piping is an exponential function of the flow rate.

[trader4]

Yeah, that's another issue. In my case, I think it can be done with a simple switch inside the timer box panel, which has room. That would allow manual control of the speed. And in this case, pretty sure that's all that's needed. High speed for draining, backwashing, fast cleanup, which would be only occasionally. Low for all circulation. I also saw a pool parts place online that had a motor end cap with a switch in it to select speed for A O Smith motors. But it indicated that it had been discontinued. Not sure if anyone else has a similar device, but it's not the most convenient place to have a switch anyway.

The sweep/pool cleaner point is a good one. This system has a seperate booster pump for the Polaris and I would think it would be enough to work with the pump on low. If not, then would have to put it to high when using that too, but the Polaris is manual now as well.

quoted text -

[Mark]

AGREED! Mark

[JimT]

If I'm reading correctly it looks like the board on the Pentair VS is about $650.

Cheap compared to the display panel on my Prius. $3600. Used I got one for $300.

Jim

[trader4]

Nothing wrong with the simple 2 speed pump. That was the point of my original post. It looks to me like you save the most on energy usage by going with the variable speed, permanent magnet motor. That saves about 70%, most of it because of the lower speed, the rest because the motor is more efficient regardless of speed.

On the other hand, it looks like I can save about 57% by going with just replacing the existing single speed motor with a dual speed one. Can do that for $300, vs $900 for the variable speed pump.

The difference between 57% and 70% savings, given the pool is run 3.5 months here, amounts to about $50. So, it would take

12 years to recover the additional $600 cost of the variable speed. That plus the possibility of failure of the fancy drive electronics, etc lead me to conclude the best solution is the 2 speed motor.

That's an interesting idea. If you rig it together with a 2 speed pump so that solar is used just for low speed, might be a cost effective solution. If it has to be able to run the pump at high speed, IMO it will be economically unviable because you'd need a lot more panels. On the other hand, by my calcs, with the 2 speed pump running on low, my electric usage will go from $340 a season down to $150. At $150 a year in usage, I think it would take a long time to pay back the solar system cost even for the smaller array running the pump on low, probably rendering it unviable here in the north with a 3.5 month season.