After years of legislation, rebates, seminars and word-of-mouth, the industry has begun to embrace the benefits of the pump affinity law. However, some pool professionals still must be dragged kicking and screaming into a new era of energy efficiency.
This is especially true in commercial pools, where turnover requirements seem to blind operators from making any kind of adjustments to their pump operations.
“Our lack of knowledge is the only thing that’s slowing us down,” says Mark Duda, owner of Duda Pools in St. Johns, Fla. “It’s a very lengthy time to get it in play, but there’s a lot of excitement for it, especially with the economy.”
Enter variable frequency drives. While many are unfamiliar with the phrase, VFDs are simply the technology within variable-speed pumps. However, the drives can be purchased separately and retrofitted onto existing installations, creating a wealth of savings for operators and homeowners.
Although the more immediate applications appear to be on commercial projects, drives can be successfully implemented into residential pool pumps as well. Installation rarely takes more than a day, and with a few caveats, a simple retrofit should successfully cut electric bills in half.
The VFD advantage
While most technicians know that cutting a motor speed in half can cut energy in 1/8 — though in real applications the fraction is closer to 1/5 or 1/6 — not everyone knows how this is actually achieved. Unlike two-speed pumps, VFDs literally control the frequency of the electrical power going to the pump. By changing the frequency of the waves going to the motor, VFDs can slow down a motor’s rotation, reducing flow and electrical usage.
“You’re getting constantly clean power,” Duda says. “I have not seen a negative yet, except for cost, but your payback is really quick.”
Residential drives can cost as little as $400, although the return is considerably slower. Depending on the size of the motor, commercial VFDs may peak above $5,000, but the cost recovery is realized much more quickly.
Drives either come equipped with their own timers or can be interfaced through a standard pool timer. Furthermore, drives can utilize flow meters to dictate turnover regardless of the filter conditions.
Best of all, VFDs can be retrofitted safely onto most high-efficiency motors, provided they are rated as “inverter duty.”
“If the motor is designed for VFDs, if it has the appropriate rating, it wouldn’t void the warranty,” notes Doug Whiteaker, principal at Water Technology in Beaver Dam, Wis.
While the VFD may create additional heat — which is why the inverter duty motors that have Class F insulation are recommended — they also have a number of protective features to safeguard against current and voltage fluctuations.
“The motor will continue to get the nameplate voltage until the voltage gets too low or too high for the VFD, and it will trip to protect itself,” says Justin McCollum, commercial sales manager for Sanford, N.C.-based Pentair Water Pool and Spa. “It will restart once the voltage has gone back into operating range.”
Also, most VFDs have internal motor protections, so external motor overloads are not required.
Of course, there was a time not too long ago when some VFD products had a shaky reputation.
“Back about 10 years ago, they had a lot of consistency and durability issues with the drives,” explains Bob Kappel, regional sales manager for Bradley, Ill.-based Siemens Water Technologies Corp. “But now they’re used in processes all over the world. Modern drives have been getting better and better every year.”
Heightened awareness of the product has helped build interest in the commercial sector while more affordable products have helped VFDs enter the residential market.
VFDs can be used on both residential and commercial pools, though they may have a more immediate impact on the latter.
However, some commercial designers and operators are resistant because most states require that pools maintain a six-hour turnover even when the facility is closed.
“Many people believe the only economy that you can get out of VFD is modulating flow rates based on peak hours and off-peak hours, [but] that’s really not true,” Kappel says.
While reducing flow during off-hours presents an additional avenue of energy savings, almost any large pool that utilizes a throttle valve could benefit from a VFD.
Operators often use throttle valves for waterfeatures such as deck jets or waterfalls, and reduce flow accordingly by 10 percent or more. While the throttle valve reduces flow, the motor is still working to full capacity, creating unnecessary flow at the impeller.
By retrofitting on a VFD, throttle valves can be turned into maintenance valves when techs need to isolate the water flow. During normal operating conditions, the valves remain open, allowing the VFD to regulate flow and reduce electrical demand.
“We can get anywhere from 12- to 20 percent energy savings just by [opening] the throttle valve,” Kappel says. “It’s a far more efficient way to operate the pump motor.”
The application to large commercial pools is especially apt considering VFDs have soft-start and soft-stop capabilities. By gradually increasing and decreasing the motor’s speed, operators can protect against the physical shock of water hammer, which can split pipes in larger hydraulic systems.
Depending on the design, commercial pools can realize a return on investment within a year of installing a VFD. Even if the installation requires a new motor in an area with affordable energy rates, the payback still can be realized within five years on larger motors.
But what about residential pools? The technology has been widely used because of variable-speed pumps, but using separate drives is still somewhat of a novelty.
“It’s pretty hard to compete when you have a pump that’s already built onto it,” Duda says. “There are some less expensive drives now, and I think when more hit the market it will be more feasible.”
However, some techs have found success already. Bob Nichols, owner of Precision Pool Service in Glendora, Calif., has retrofitted a programmable drive onto his own pool pump. Even with accompanying software, the installation was affordable.
“I’d say wholesale, it’s about $600,” Nichols said. “I’m searching for people all the time who are interested in [VFDs].”
It may take a continued proliferation of smaller VFDs and greater awareness among pool professionals, but the residential market appears to be increasingly viable for the technology.
VFD installations are relatively simple with a qualified electrician. Most retrofits can be done within an hour or two, with some considerations of equipment design and wiring.
Remember that for most installations, the pump motor should be three-phase and rated as inverter-duty.
The VFD also needs to be chosen carefully to accommodate the pool.
“Sizing is critical — you need to know the amp draw, horsepower and voltage of the pump in order to size the drive properly,” McCollum says.
The motor’s nameplate should give you the necessary information to make the right match, he adds.
For the wiring, you may need to run the VFD through relays, depending if you are using an external timer to program different flow rates. Furthermore, be sure to mount the drive within a reasonable distance of the pump.
“[Be] cognizant to keep the conductor runs between the drive and the motor as short as possible,” Kappel explains. “Reflective wave forms can come back and back-feed through the line, [causing] excessive heat build-up.”
Keeping the runs under 50 feet would be an ideal installation, he adds.
The physical installation of the VFD is often just a matter of replacing the pump motor’s starter.
“You’re just disconnecting the incoming power and the outgoing power and putting this in place of the starter, then reconnecting those wires,” Kappel says.
The wires may have to be shortened or lengthened depending on the pump motor installation.
Many commercial operators prefer installing units with a bypass. If for some reason the drive fails, the bypass allows the pump to resume at full power, which is another reason to leave the throttle valves on.
Also be aware of adjacent instrumentation that may be affected by the drive. Electromagnetic interference produced by the VFD can have a strange effect on nearby equipment.
“I’ve see digital flowmeters that are mounted right next to the drive, and the radio frequency from the drive is causing weird symbols to appear on the LED screens,” Kappel says.
Furthermore, RMI harmonics from the drive can wreak havoc on medical equipment. If the drive is installed, for example, in a hospital therapy pool, extra filters will have to be installed to prevent interference with the electrical grid.
Certain VFD products may require additional tweak to an installation for software interface. Consult the manufacturer for details.