Is a Variable Speed Pump Worth the Investment

ross939April 24, 2012


I live in Sonoma, CA. I have a small in-ground pool in my backyard. It is 12' x 20' with a six foot deep end and about 3' shallow. I estimate it to be about 10,000g. The pool was built in 1978 and I had it re-plastered about three years ago. I would like to replace the original equipment pump this spring. I have an 8 year old Sta-rite cartridge filter. I am wondering if a variable speed pump is worth the extra expense? I have no heater or solar so I don't anticipate I need much more than 3/4 hp if I go with a single speed pump. We swim from about April to September. I cover the pool and shut everything down during the off season to save energy. I run a Polaris 360 Vac off the main water outlet from the pump so possibly looking at 1 hp. I'm having a tough time swallowing the $800-$1000 cost of a V.S. Pump, while I can get a Quiet Flo Plus 1hp for around $300. Any suggestions are welcome!

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Note: Should read "Whisper-flo" Plus 1-hp pump in above psost, not "Quiet-Flo".

    Bookmark   April 24, 2012 at 9:29PM
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How much does your electricity cost per kilowatt?
Is the pool plumbing with 1.5" or 2", not including the Polaris line which is 1.5" usually?

What is your goal in making the change?


    Bookmark   April 25, 2012 at 12:22AM
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Thank you for your response. Our costs per ka hr in the Bay Area is $0.202, I believe we are on a sliding rate system where if decreases in non peak hours. Pool plumbing is 1.5" except all the above ground pipes around the sta-rite filter, which were converted to 2". No dedicated line for Polaris, runs off the main return line coming from the filter. My goals are two fold. Pump and impeller are nearly 34 years old, so Im afraid it can die any time. Second, we need to get a handle on our household expenses. We've been here 8 years and in that 8 years inflation has skyrocketed, food, gas utilities, kids, and our salaries have not. I run filter only 4 hours or less in the non peak hous at night and then we have it on when we
swim. Thanks again.

    Bookmark   April 25, 2012 at 11:09AM
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While I feel your pain with expenses, running at night for you is not a good idea. The sun energizes the chlorine in the pool and causes it to gas off. It must have that replaced during the day while it is happening or your water can go unprotected.

Lets say your pool gets sun from 8 to 6 most days. No one goes swimming till 2. That's 6 hours of the pool gassing off its chlorine without it being replenished. How long do you suppose it will take to do that? What if a guest didn't tell you he had an ear infection or an eye infection. Now you can get it. What will the doctor cost? The meds? Did you save anything? Nope. What if it happened to a family member or another guest?

If it works, great, but IMHO, the Polaris has no business being there. I think you would be better served with a robot, not a sweep. Sweeps, particularly pressure side ones, should have their own line.


    Bookmark   April 25, 2012 at 7:21PM
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Thanks, Scott. I'm sure you are right on, there. Will reset my timer to cycle at least 3 hrs during peak heat times, 3 hours on peak times.

    Bookmark   April 25, 2012 at 7:43PM
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That being said, I would expect the operating costs to increase to where they should have been to begin with. It's from there that you need to calculate whether a VSP makes financial sense or will a two speed pump be good.

I have found that in real world calculations, you should be able to realize as much as a 75% reduction on a pool's circulation costs. That is because they are using fewer RPMs to move water and generally speaking, most single speed pumps move the water at a much higher speed in the pipes and equipment than is needed.

All three of the major brands have energy usage calculators. Using them with the Schedule 40 pipe size charts on velocity and the gallons per minute you need for your gear to work will help you to calculate which type of pump works best for you.

Please note that tablet feeders generally need the water moving at about four or five feet per second to function correctly. Going with a slower speed will result in them not dispensing enough. Salt cells generally need 25 gallons per second (GPS) but like 30, and heaters generally like it at 40. These speeds satisfy the flow switches on these devices which are there for safety's sake. Filtering is generally better with slower water velocities also. Note that at slower speed, the pressure gauge on a filter, the measure most use to determine if a filter needs cleaning, generally uses a 10 PSI change with single speed pumps. With the slow speeds of a VSP, that pressure will drop a lot to as little as a couple pounds per square inch.

Trying to tell the whole story!


Here is a link that might be useful: Schedule 40 water velocity, friction losses, and flow rates charts

    Bookmark   April 27, 2012 at 4:08AM
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Been watching this thread with interest, as we just signed up for our area's Variable Pricing Plan for electricity during June 1 - Sept 30. Didn't want to hijack the thread, but here goes anyway :)

I was going to have a timer installed for my single speed hot tub and pool pumps (both 1.0 hp, I think), so that they did not run during 2-7pm peak times. Our price per kWh is variable up to $0.46 during those hours, and the rest of the time and on weekends and holidays will be $0.045 per kWh - a huge difference. Just in the short time we've owned the house, we calculate even at off-season pricing (currently $0.084/kWh for shoulder season) it's costing about $90 minimum per month to have just the pool pump on 24/7 - the spa pump needs repair so it's not on as the kWh usage overnight is about 1.25 - 1.5 kWh when we don't have any appliances/ac/etc on, just the pump.

My thought was we don't need the pumps running 24 hrs per day, and I wanted to cut it back to whatever is necessary for our 38K gal pool (about 800 sf surface area) so have been reading on TFP about scaling it back until the water gets cloudy then adding an hour back. I'd planned to have any and all hours of "on time" necessary to be during off-peak times. Is that not safe to do?

Off gassing of chlorine scares me because I'm sure I don't understand it. How does having the pump on add chlorine to the water during sunny times of day? I would think it's just circulating the chlorine that's already in the pool, which will be burning off anyway if you don't have enough stabilizer in the pool.

    Bookmark   April 27, 2012 at 12:10PM
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Chlorine, when energized, escapes the water and enters the atmosphere. This is off gassing.

UV light from the sun will energize it.

Cyanuric acid, aka CYA or stabilizer keeps the chlorine sort of busy so even if it does get some energy from the UV, it isn't enough to keep it from "playing" with the CYA in the water.This also can keep it from doing work though so having too much CYA prevents Free Chlorine. aka FC, from doing it's jobs of oxidation and killing biological pathogens.

Generally speaking, you need about 5 to 7% of the ppm count of CYA for FC with salt cell equipped pools and 8 to 10% for all other forms of feeding chlorine to a pool. Too much CYA, and the elevated levels of FC are very difficult to make happen on a regular basis. If the CYA were 150 ppm, can you see trying to maintain a residual of 12 to 15 ppm? Drop the CYA to 30 ppm and 2 ppm of FC is much easier and costs less. Please note that salt cell equipped pools often have somewhat higher levels of CYA and FC to reduce the cell's ON time so it lasts longer.

Generally, pools, when on, they either have a tablet feeder or tabs in the skimmer (generally not a good idea, BTW) or a salt cell which won't work without water moving.

Pools get the most use during the day. Activity will cause some gas off too, in addition to the demands that sweat, tanning lotions, killing bio-badies, etc... place on the residual FC in the pool. These can deplete a pool pretty quickly if the FC isn't replenished as it's being used. You need to keep that residual FC in there. Running at night when nobody is there doesn't help the swimmers if it isn't there when they are.

Without the UV of the sun, aka night, the FC stays put. The water is safe. I do recommend keeping the pump running whenever there are swimmers using the pool, day or night.


    Bookmark   April 27, 2012 at 7:34PM
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