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Most residential pools are not equipped with flow meters. Commercial pools are — they need to verify a certain flow to meet codes and legal responsibilities — but for decades, through the single-speed pump era, there’s been little need for one, because what good is knowing your flow if you can’t change it?
But now that the variable-speed era is upon us (it has been creeping up on us for a long time), flow meters have seen their role grow in importance because they allow operators and service pros to use the capabilities of the VSP to their full capacity. They let users dial in flow exactly where it needs to be in order to maximize the savings VSPs were invented to deliver.
Getting the most out of a VSP circulation pump means running it at the lowest possible speed that still achieves the filtering objective. That varies from pool to pool, and even from time to time.
It’s up to the service pro or homeowner to decide how much filtration is needed, and that decision can be affected by ambient conditions, pool usage or even the desire to use less chlorine. (People looking to use less chlorine this summer to help cope with the shortage have the option of adding more filtration.)
But whatever the filtration objective, a flow meter can help achieve it more precisely, and that precision offers savings.
“A flow meter is like a speedometer for your car,” says Paul Hackett, electrical engineer by training and owner of H2flow Controls, Sylvania, Ohio, “and if you had a car that only had two speeds, zero or 50 mph, which is what a single-speed pump is like, you really wouldn’t need a speedometer.
“But as soon as you have variable-speed pumps, you need to have a speedometer to optimize the energy savings it provides. A typical VSP on the market is going to give the user several preset speeds. Take the Intelliflow — just for an example — it offers a maximum speed of 3450 rpm and a minimum of 450 rpm. So the range is 450 to 3450, with preset speeds at 750, 1,500, 2350 and 3110. But these are just presets. They’re not calculated for your pool.”
While a pump’s preset lower rpm is more efficient than full speed at the job of circulating water, Hackett says, it’s not the most efficient setting. The most efficient setting is the one that gives you the lowest flow sufficient to turn over your pool the desired number of times.
“What many users do is say to themselves, ‘I think I’ll use that 2350 rpm preset level, because 1500 is probably too slow for me, and 3110 is probably too high.’
“And if we run that pump at 2350 rpm, we’re going to consume $518 a year in energy* (see energy calculation assumptions below), and that’s a huge savings versus a single-speed pump. The user will make back the cost of the VSP in a couple of years at that speed.
“So in that sense, the user is very happy. But if the user really looked at the flow needed to have a clean pool and measured it, the user could have slowed the pump down to 1725 rpm, where it’s going to consume only $216 a year in energy. That’s over $300 in savings and a lot of electricity, and you would only know how to do that if you had a flow meter.”
The amount of filtration on a residential pool is to some extent a subjective decision. The more you turnover your pool every 24 hours, the more water reaches the filter, according to this set of figures:
1 turnover = 63% of the water in the pool reaches the filter
2 turnovers = 86% of the water in the pool reaches the filter
3 turnovers = 95% of the water in the pool reaches the filter
4 turnovers = 98% of the water in the pool reaches the filter
[Note: If you had two vessels, and you simply pumped the water in pool No. 1 through a filter and into pool No. 2, you would be able to filter 100% of the water every day with one turnover. But because you have to pump clean water back into the pool with the unfiltered water, you get the rates above.]
The commercial standard is four turnovers, although many homeowners are happy with three turnovers as that fourth turnover only filters another 3%.
“So, for a 20,000-gallon pool, four turnovers would require a flow rate of 55 GPM for 24 hours. But three turnovers only requires 42 GPM, which again, will filter 95% of the pool volume per day. So now you can use your fl ow meter to fine tune your speed to get you exactly the 42 GPM you need, giving you plenty of filtration and still giving you the maximum savings of energy and utility bill costs,” Hackett says.
As for performance of the flow meter, it doesn’t matter where in the system you put it, it will read the same. A lot of people get flow meters confused with pressure gauges, Hackett says, but flow and pressure are different. “They ask me, ‘If I put it before the filter, am I going to have a different flow rate than after the filter?’ And I try to tell them, ‘No, the people in the back of the bus go the same speed as the people in the front. The flow is the same throughout your system. I mean, if you have 50 gallons a minute before the filter, and you don’t have 50 gallons a minute after the filter, where’s that water going?’”
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