How I sized the pump for use with OC-1 in a domestic inground swimming pool
Why such a small pump?
Why such a large one? Perhaps there is a perception that a more powerful pump is better than a less powerful one. I can see no intrinsic merit in forcing water through pipes at great speed. It all depends on what the pump is required to do. I can see five main tasks for the pump:
1) To disperse chemicals throughout the total pool water
2) To enable backwashing through the filter
3) To vacuum to waste
4) To power any other device in the pool
5) To circulate the water through the filtering process
1) Chemical dispersal is achieved with virtually any circulation rate: indeed adding chemicals to the water and then stirring with a few brush strokes will do just as good a job.
2) Backwashing to waste (for sand filters) requires a flow of 48 m³/m²/hr. With a filter area of 0.18 m² the flow needs to be 48 * 0.18, which is 8.6 m³/hr. This is easily met by the HPS031 at around 7.5 mtrs head. The backwash head on my system is nowhere near that. There are no specific backwash figures for OC-1 that I could find, but I am assuming that if it's good enough for sand then OC-1 won't be a problem.
3) Vacuuming to waste requires about 15 to 18 m³/hr. This is not quite achieved by the HPS031, but the previous pump at 0.33 HP was fine, and I don't want to rip the liner off the floor. Backwash and vacuum figures are taken from NSF/ANSI 50 recommendation for residential pools, which you can Google.
4) Other ancillaries - I don't have any.
5) Filter flow rate. Although OC-1 is stated to accommodate flow rates from 10 to 60 m³/hr, the optimum flow rate is given as 25 m³/m²/hr. The 20" OC-1 filter has a media area of 0.18 m²: 25 * 0.18 gives a flow rate of 4.5 m³/hr. It would be hard to find a pump that delivered such a low flow rate, certainly the HPS031 at a head of say 6 mtrs exceeds that easily. This is slightly misleading as I am trying to match that flow rate, not exceed it. The optimum flow rate for the 20" filter is confirmed by OC-1 as 5 m³/hr.
The power of swimming pool pumps is questioned, somewhat radically, by the US Dept of Energy. The US has, it would be fair to say, far more, larger, and more frequently used pools than in the UK. In a survey of pools in Florida the DoE found that 'a 0.75 horsepower or smaller pump is generally sufficient for residential pools.' (I don't know if there has been a survey on pump size in Florida, but if there has my bet is that the average size is rather more than 0.75 HP.) Furthermore my old pump at 0.33 HP managed for around twenty years to circulate, filter, backwash and vacuum with no apparent lack of power. And with a larger filter with OC-1 the resistance will be even less.
What is the Flow Rate?
Many retail sites have a flow rate chart for the HPS pumps, and this is what I used initially, and it confirmed that the 1/3 HP pump would be more than adequate for my needs. After a conversation with a Certikin technician he sent me the HPS brochure, which had flow rates wildy different from the charts.
For example the chart gives a rate of 11 m³ at 8 mtrs head, whilst the brochure gives 6.3 m³, almost half. The flow is still adequate, but where did the chart originate from, and why is it so far out? The sticky label on the pump (see below) gives a flow rate of 9.5 m³ at 6 mtrs head, which corresponds to the Certikin brochure.
Variable Speed Pumps:
Did I consider a variable speed pump? Yes, in as much as I looked at them online. The cost rules them out really, at four or five times the cost of the small single-speed pump. I also have the impression that variable speed pumps have a rating of around 1 HP or so upwards, and can be set to a lower speed and power. But why have an expensive and powerful pump which is set to run at a low less powerful rate the majority of the time? There is also the complexity of the variable speed electronics in somewhat hostile conditions, another costly point of possible failure not found on a single speed pump.
The Certikin HPS031M Pump:
So the HPS031 is the pump to go for? Yes, and I duly ordered one. When it came the sealed box was festooned with HPS031M lettering as one would expect. And the sticky label on the top confirmed this, with a P! of 480 watts and a P2 of 300 watts. (As far as I know P1 is the startup power and P2 the running power.)
But the riveted alloy label on the side tells a different story. Here the P1 figure is 610 watts and the P2 400 watts, and the HP as 1/2. Which is correct? I've really no idea. Was the alloy label picked from the wrong bin? I shall check the running watts when the pump is installed.
The pump has been running for a few days, and a test meter shows that it is using 635 watts, nothing like any of the labels. I am perplexed.
Yet again there is confusion. All the calculations, research and reasoning and in the end I don't know what pump I have, what HP it is, and I'm not even sure of the flow rates. Nothing in this job makes any sense.
To connect everything up I decided to use IBG Praher Flexfit flexible pipe and compression fittings. These are clunky, and not particularly flexible, but I was fed up with the cut back a little more and replace everything ethos of welded fittings, if there is a need to adjust anything half and inch or so. Perhaps this - my old pump house - explains my dislike of welded fittings:
The flexible PVC pipe has the continental 50 mm diameter, unlike the nominal 1.5" (48.5 mm) of the usual UK white ABS rigid pipe. Although I am sure the pipe comes out of the moulding machine in a straight line, it is stored in reels, shipped to the retailers, cut and delivered rolled up quite tightly. Consequently every length used is curved and shows no inclination to straighten up, which makes for a bit of a struggle and puts some pipework under tension. The pipe is not really flexible except in longer lengths, but does seem indestructible. Very warm weather does enable the pipe to straighten, mostly.
Although I was constrained by the emergence of the old pipework through the concrete base, and the new pipework isn't particularly elegant, the refit enabled the number of elbows in the pool house to be reduced from approximately 20 to 13.
Apart from being clunky, the compression fittings are very expensive. As usual, everything arrived next day except one fitting, which I had to wait two weeks to arrive.
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If you have any questions, comments or criticisms at all then I'd be pleased to hear them: please email me at kes at kcall dot co dot uk.
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