1-speed vs 2-speed math

[UaVaj]

there are some 1-speed vs 2-speed thread with excellent info, however this thread is for the expert to check my math.



INFO SPECIFIC
* pool is 30,000 gallon. filtration target is a 1 turnover per day. vacumm target is 2 hour per day. *
pump is a hayward super pump. with a 2HP motor and with a 40' head. yield is 98gpm.
south florida electricity is $0.11 per kilowatt/hr.
note: there are obviously other factors that comes into play. however since this is on the same pool, same plumbing, same everything except motor. all those factors should negate each other.

recently bought a 1-speed motor (from the local pool shop) about 2 weeks ago (never heard of 2-speed or variable-speed. yep first pool).

1-speed motor on hand is century centurion pro ST225. 3450rpm. 230v @ 10.2amp. 2346watt.
cost $0.26 per hour pumping 5880 gallon.

2-speed motor in consideration is a century 2-speed B2979. 3450rpm/1725rpm. 230v @ 11.0amp/1.6amp. 2530watt/368watt.
full speed - cost $0.28 per hour pumping 5880 gallon
low speed - cost $0.04 per hour pumping 2940 gallon.



THE MATH (this is what need to be checked)
1-speed motor to push 30,000 gallon. 5880 gallon per hour. takes 5.1 hour. costing $1.32. both filtration and vac requirements met.

2-speed motor to push 30,000 gallon. 11760 gallon first two hour for vac. costing $0.56. 18240 gallon remaining needs filteration. motor need to run for additional 6.20 hours. costing $0.25.

a saving of $0.51 per day.



SIDEBAR
not convince that this motor (B2979) is capable of running 1725rpm @ 1.6amp (15% of full speed amperage draw).
it's predeccessor B979 required 4.1amp at low speed. let use 4.1amp correction for the low speed draw.

11760 gallon first two hour for vac. costing $0.56. 18240 gallon remaining. motor need to run for additional 6.20 hours. costing $0.64.

an adjusted saving of $0.12 per day.



COMPARISON
online shipped: 1-speed is $225 (ao smith) vs 2-speed is $317 (century). difference of $92.
savings of $0.51 per day. going to take 6 months to break even.
adjusted savings of $0.12 per day. going to take 26+ months to break even.

 

[jblizzle]

Admittedly, I did not trudge through all that, but I feel you are over thinking things

The 2 speed pump on low will move half the water for a quarter the power cost as on high. The amp numbers you are using are not the actual running amps I don't think.

 

[UaVaj]

amp numbers in calculations are from manufacturer spec sheet.


given the simple task of - 30,000 gallon filtration with 2 hour of vac.
1/2 the water for 1/4 the power - is simply NOT adding up.

 

[JasonLion]

The spec sheet always lists peak possible amps, real world amps are generally rather lower (depending somewhat on your plumbing setup). Low speed will be exactly half the water moved for roughly 1/4 the power, as jblizzle said.

Also, the goal is to keep the water clean, which has more to do with total run time, than with the amount of water moved. So two speed pumps can almost always be run for less than twice as long yet still keep the water just as clean.

Regardless, break-even will be rather less than a year, which is excellent, making the two speed pump well worth the small extra cost.

 

[Isaac-1]

Pump electric power consumption defies common sense, for example adding outlet side restriction will not only lower the volume of water pumped it will also lower the amount of electricity used, this is somewhat against common sense which would tell you that restricting the output should make the pump work harder and therefore draw more electricity, but it just does not work that way. (as a side effect of this rule the spec sheet is going to have wide error variation from one installation to the next) Having said that a good rule of thumb is that a 2 speed pump will consume 1/4 the amount of power on low speed as it does on high speed and move 1/2 as much water, this is a fundamental physical law of centrifugal pumps, any variation from this will come from minor differences in efficiency of the motor at different speeds.

Now down to a couple of side points, shooting for turn overs per day is fine for design estimation, but don't let an estimating tool be your target, in our experience many if not most pools can manage effective filtering at far less than 1 turn over per day, although finding out exactly how much less takes a bit of trial and error, and may change with the season due to pollen, etc. Secondly a 2 HP 2 speed pump is a BIG pump for a 30,000 gallon pool with no water features, I suspect a 1 or 1.5 HP 2 speed would likely serve you better depending on your exact pool setup, speaking of which we could give better answers if you would put that information in your signature. see http://www.troublefreepool.com/content/165-getting-started

 

[mas985]

Pump electric power consumption defies common sense, for example adding outlet side restriction will not only lower the volume of water pumped it will also lower the amount of electricity used, this is somewhat against common sense which would tell you that restricting the output should make the pump work harder and therefore draw more electricity, but it just does not work that way.

There is a very good reason why a centrifugal pump exhibits this behavior.

A pump creates more than one type of energy; kinetic (flow rate) and potential (pressure or head). The product of these two components is proportional to the total energy delivered to the water which is sometimes called hydraulic HP. More specifically, the equation for hydraulic HP is

Hydraulic HP (HHP) = Head (ft) * GPM / 3960

This is then related to motor brake HP by

Brake HP (BHP) = HHP / Pump Efficiency

Which in turn is related to electrical HP by

Electrical HP (EHP) = BHP / Motor Efficiency = Volts * Amps * Power Factor / 745.7

So the electrical input power is directly related to the product of pump head and flow rate. This is the reason that centrifugal pump's power decreases with decreasing flow rate. The flow rate decreases faster than the head increases along the head curve therefore the electrical power decreases with decreasing flow rate. BTW, this only happens for pumps with low specific speed (i.e. certain shape of head curve) which is typical of centrifugal pumps.

 

[UaVaj]

found a very nice calculator and it "verified" the calculations.
http://www.centuryelectricmotor.com/applications/twospeed.aspx

regarding the math. the longer the run time. the bigger the savings. folks running 24/7 will see the biggest difference.
however at that spectrum. you are already in the hole regarding electricity usage. because you are running the pump 24/7. so any little bit in electricity reduction helps.



as for the end goal of keeping the pool clean. per the "Pump Run Time Study". 2 hour morning and 2 hour evening of run time per day (4 hours or less) should be sufficient run time for circulation, for skimming and for vacuuming.

1-speed = 4 hour (full speed) run time per day
vs
2-speed = 6 hour (full speed 2hr / low speed 4hr) run time per day. (to keep this exercise constant. volume of water should be equal. hence 6 hrs.)

a net saving of $0.04 per day. (true apples to apples comparison of operation cost)

in adjusting for "only" 4 hours run time either way per day. an adjusted net saving of $0.25 per day.



when all said and done. using pump run time study as the base line.
1-speed benefit: 2hr more vacuum and filter 5880 gal more or 2-speed benefit: save $0.21.