Pump Quit - Will An Adequately Sized 2 Spd Replacement Work For Me?

HTM

New member
Apr 2, 2021
2
SE Pennsylvania
I am new to the forum - finding it informative with knowledgeable and helpful members.
The pool and equipment came with the house. Both are old and basic. Exception is Speck S90 230V 3/4" pump thatI installed 12 years ago to replace a larger pump that wasn't working well.
Our pool use and needs are basic and limited. Open June - September. I do the upkeep, which now includes needing a new pump to
replace the Speck S90. It operated very well until late last season when motor lost output. Very reduced water flow. Some internal pump
components also found to be worn, so we are opting for a new pump, possibly a 2 spd rather than VS. Leaning toward 2 spd as it's simpler overall and we could run it at reduced speed the majority of time provided 1725 rpm operation offers adequate circulation and filtration.
I foresee running the new pump at reduced speed almost entirely, say 12-14 hrs/day more or less, depending on what's needed for a full turnover. Full speed operation for an hour or two after back washing the filter or shocking. However, I have three concerns:
1. I've read that DE filters do not work very well with long hours at relatively low motor speeds (1725 rpm). Is that accurate?
2. Performance curves for small 2 spd pump/motors generally chart flow from maximum 20-25 ft total head to zero. And flow rates are
relatively low even with very low total head. When our DE filter is clean, pressure is around 12 psi, or roughly 28 ft total head
(12 x 2.31).
With this amount of "real world" total head, is it possible that 1725 rpm would move zero to very little water? Or, what sort of flow
would be realistic for a 1-1/2 hp 2 spd pump at 1725 rpm?
3. A step up in motor size could improve low speed water flow, but the size of the return line is a constraint. I don't want to stress the
filter components or the motor.
Thanks in advance for comments and suggestions.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
44,406
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Welcome to the forum!
A two speed motor might work well for you. Do remember that as of July 19, 2021 all primary pool pumps must be Variable Speed if over 1.1 THP. So if you can get a two speed installed now, you will need to change it to VS the next time that motor fails.
Filters work best at low flow rates.
No idea on performance curves. Head varies dramatically on flow rate and other items.
You will not stress any components at 1725 rpm of a 1 1/2 hp pump. At half rpm, the hp is approx 1/4.
I suggest you read ABC's of Pool Water Chemistry.
 

HTM

New member
Apr 2, 2021
2
SE Pennsylvania
Thank you for the comments so far.
I probably should have titled my post: Water Flow @ 1725 RPM - Is It Sufficient?
Data sheet/performance curves for Speck S90 attached. This may better illustrate the reason
for my doubts about adequate water flow @ 1725 rpm.
In our pool, the pump is gravity fed. Return line rises 4 ft over a 50 ft run to the pool wall.
With our 3/4 hp single speed Speck pump, typical total head was a little less than 30 ft.
In the 1725 rpm performance curve, Model S90-III, which is 56 frame 1-1/2hp pump, flow begins
@ 20 ft total head. The 2 hp model S90-IV produces flow @ 25 ft total head.
I had assumed that a representative 2 spd pump operating at slow speed would have zero flow
at total heads higher than 20-25 ft total head, and there would be no possibility of it working in
our 30 ft total head application.
However, I had not figured on a reduction in total head with a change to low speed operation.
Is there an accepted formula for estimating the new total head? I assume it's more involved than
a 50 pct reduction.
 

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