Pump replacement question

[ajrose2579]

I need to replace my pool pump. The motor arched and sparked the last time I tried to start it. I have been researching variable speed pumps versus single speed pumps. I have a 300 lb. sand filter and 1.5 inch piping. The previous owner put a 1.5 HP Supastream pump which I can't find information for on any U.S. website. I believe the pump was pumping at a flow rate exceeding the filter capacity. So now to my question. I only need the pool pump for my pool(no other features). Why don't they just make a single speed pump that will run at a lower RPM and allow for the same savings as using the lower setting on a variable speed pump? What am I missing?

If I need a pump that will pump 45 gpm (20000 gal pool) to turn the pool over in 8hrs. can I just buy a 3/4 or 1/2 HP pump that supplies that flow rate?

All the blogs say a variable speed pump will save money because you can run at a lower rpm for longer and get the same turnover. So if all I am going to do is run the variable speed pump at a lower RPM for longer than why not just have a single speed pump that runs at a lower RPM??? What do I need the multiple speeds for?? I don't get it. Please help me understand.

I do know that for a sand filter backwash I would need a higher flow rate but I am thinking about replacing it with a cartridge filter which would support my questions

Thanks in advance

20k pool with swg. augusta, Ga..

 

[WASP]

The higher or full speed settings are used when running water features or possibly cleaning devices when more pressure and flow is required. You can buy a two speed pump and run it at the low speed for filtration. A variable speed,pump allows you to fine tune the pumps speed for the specific features being used.

 

[zea3]

Your thinking is correct. If you don't have any water features to power, and don't use pressure side pool cleaners you only need a higher hp to vacuum. A 2 speed 1hp pump will give you plenty of suction to vacuum the pool when needed and low speed will turn over the water adequately.

 

[ajrose2579]

So assuming I purchase a robot pool cleaner and a cartridge filter I can purchase a very low flow(>45 gpm) 1/2 hp pump? The electrical savings will be equivalent to a variable speed pump( not including the new equipment obviously)? Any issues with swg operation at low flow rates?

I found a 1/15 hp spa pump (up to 80 gpm with no head loss). Would that be crazy?

FYI: my intent is to make the pool as maintenance and trouble free even if it cost a little bit to get there. I think the initial setup of the pump and filter has made it a royal pain!

 

[JVTrain]

I would still look at a 2 speed pump with 1 HP or slightly less. There is not much cost increase to get a pump in a two speed versus a single speed. When set on low speed, it will consume the energy of a ~1/3 HP motor or less but you have the option to run on high speed to manually vacuum your pool or to circulate a bit more if you desire. The flexibility you gain is worth it.

I seriously doubt the 1/15 HP pump is strong enough to push water through your filtration in any meaningful way.

 

[duraleigh]

I found a 1/15 hp spa pump (up to 80 gpm with no head loss). Would that be crazy?

Yes, it would. "No head loss" I assume means no resistance whatsoever which doesn't exist.

Do some more homework but don't try too hard to re-invent the wheel.......others that do this for a living have already been there.

 

[Kiss4aFrog]

I'm with JVTran. Look at a 2 speed pump with 1 HP. You want something with a bit more than you need not something that just barely meets your minimum. There are bends, the filter, height ... a number of things that if you don't have some extra in your pumps ability you might end up with something that doesn't get the job done.

 

[JoyfulNoise]

I think part of the OP's original question is why don't single speed pumps operate at a lower RPM, and thus, a lower energy use?

The answer to that question is one of electrical and mechanical engineering. Electrical power in the US is supplied at a frequency of 60Hz. A 60Hz, 2-pole electric motor spins at a synchronous speed (unloaded speed) of 3,600 RPMs. When a load is applied to the shaft, there is "slip" that occurs (essentially the spinning shaft is trying to catch up to the rotating magnetic fields) and the RPMs drop slightly to 3450 RPMs. In a 2-speed motor design, the number of poles on the stator is selectable and uses either 2-poles for high speed (3450 RPM) or 4-poles for low speed (so the RPM's drop in half to 1725 RPM). In a variable speed motor, I believe the number of poles in the stator is fixed but the drive frequency of the motor is modulated by the external electronics so that, rather than having a fixed, 60-Hz electrical power source, the electrical power is delivered with variable frequency.

So, in effect, if you buy a single speed pump, you are limited to 3450 RPM no matter what horsepower you select and you adjust your electrical power draw based on HP not motor speed. This is why a 2-speed motor is a much better option - it allows you to utilize the pump at a lower speed where you get good filtration performance and still have access to higher speeds when you need to do vacuuming.

 

[ajrose2579]

Thank you everyone for the responses. Joyfulnoise you nailed it.

So which costs less to operate. A 1 hp pump at half speed or 1.5 hp variable speed at low setting? Long term savings being the driving factor.

 

[JoyfulNoise]

Thank you everyone for the responses. Joyfulnoise you nailed it.

So which costs less to operate. A 1 hp pump at half speed or 1.5 hp variable speed at low setting? Long term savings being the driving factor.

That's kind of an apples-to-oranges comparison. Generally speaking, it depends on your utility rates and lowest speed you can actually operate a VS pump and still get good filtration. Just because you can run a VS pump at 600rpm does not mean it will push water to any purposeful end. Also, in general, VS pumps tend to have a much longer ROI because your power loss at low speed start to get dominated by the electronics of the variable frequency drive and not the motor itself and, since VS pumps tend to cost a lot more than 2-speed pumps, it takes longer to realize your savings.

If you're interested in pump hydraulics and pool hydraulics in general, please read the following thread by TFP Expert mas985 - Hydraulics 101 - Have you lost your head? . I believe you will find a lot of your answers in that thread.

 

[ajrose2579]

Will do thanks again!!! Merry Christmas to all!!!