September 2010

Volume 19, Number 9

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Article Contents

High-Efficiency, Variable-Speed Pumps from Wilo and Grundfos

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Pumping uses a tremendous amount of electricity. In looking at efficiency of hydronic heating systems, we have long focused on the fuel used to heat the water being circulated, without paying much attention to pumping energy. Fortunately, that’s beginning to change.


Residential Stratos ECO circulators from Wilo.

Photo: Wilo USA

Two of the world’s leading pump manufacturers, Wilo (pronounced “veelo”) from Germany and Grundfos from Denmark, have introduced to North America advanced, high-efficiency, variable-speed, ”smart” pumps that can reduce pumping energy use by over 75%. Wilo offers the Stratos for commercial applications and the Stratos ECO for residential applications. Grundfos offers the Magna line for commercial applications and the Alpha line for residential applications.

Smart pumps

The Wilo and Grundfos smart pumps share two important features. First, they have integrated variable-frequency drives (VFDs) to modulate the pump speed or flow rate. They use a very clever technology to set that flow rate—analyzing their own rotational speed and current draw and, from this, computing where the pump is on its “pump curve” and optimizing pump speed accordingly. For larger pumps, separate VFDs have often been added; the revolution here is that the VFD is directly integrated into the pump and that it’s available for smaller pumps.

Second, these pumps have permanently charged, electronically commutated motors (ECMs) that reduce energy use by about 50%, according to Rick Fredette, of Urell and Company, the Northeast sales rep for Grundfos. He notes that pumping accounts for about one-fifth of the electricity used in most commercial and institutional buildings, making these savings very significant.

In addition to these features, the Grundfos Magna and Alpha circulators also include a unique “AutoAdapt” smart logic that automatically and continuously adjusts circulator performance to match the changing needs of the hydronic heating system. According to Fredette, AutoAdapt looks at the system every seven seconds, learns the operating patterns, then optimizes performance.

Why variable-speed smart pumps?

The biggest problem with water circulation in hydronic heating today is that pumps are significantly oversized and pump far more water than necessary. Steve Thompson, Wilo’s vice president for building services, estimates that 99% of pumps are oversized. Primarily by reducing the flow, these Wilo and Grundfos smart pumps offer a number of important benefits:

•Reduce electricity use. Electricity savings of 70%–90% can be realized, according to representatives of both Wilo and Grundfos. Dave Yates, president of F. W. Behler, Inc., a plumbing, heating, ventilation, and air-conditioning contracting firm in York, Pennsylvania, carried out extensive testing of Grundfos Alpha circulators when he replaced 11 constant-speed circulators in his own home (10 zones plus primary loop) with two Alpha circulators. Before making the change, he measured electricity consumption for water circulation for a full heating season, then measured that electricity for a heating season with the new system. “Our physical comfort levels improved, and our thermal-transfer electrical consumption was reduced by more than 90%,” he told EBN.

•Optimize water flow through radiators. A constant-speed pump sized to circulate the proper amount of hot water when all the zones are fully open will pump a lot more water than necessary (or pump that water too fast) when some zone valves are closed. This not only uses more pumping energy than necessary but also reduces heat-transfer performance.

•Eliminate multiple pumps. By using thermostatically controlled, electric zone valves to shunt hot water to separate zones, rather than installing a lot of individual circulators—one for each zone—as is typical practice today, the number of pumps can be significantly reduced. “It changes the way people do their circulators,” says Fredette.

•Improve boiler efficiency. By carefully regulating the hot water flow in a hydronic heating system—and reducing that flow when less heat is being extracted by radiators—return water temperature will be lower and boiler performance will improve. This is especially important with condensing boilers, which require a delta-T (difference in temperature between outgoing and return water) of about 30°F (17°C) to operate in a condensing mode. Using an analogy of a train carrying heat (Btus) from the central station (boiler) to outlying stations (heating zones), Thompson explains that “we slow the flow down and allow more Btus to get off the train.” Without a variable-speed circulator to slow down the flow of hot water, according to Thompson, condensing boilers often lose about 10% of total efficiency during the swing seasons (spring and fall) when the return water temperature is higher than optimal.

•Increase pump and zone valve life. You can extend the operating life of a pump by operating it at a slower speed most of the time. According to Thompson, “If we decrease the speed of a pump by a factor of two, we increase its life by 700%.” According to Elliot Shyer, of Heat-Tech Associates in Torrington, Connecticut, the regional sales rep for Wilo pumps, by reducing pressure in the system, zone valves also last longer.

•Reduce pump noise. In a hydronic heating system served by a single, constant-speed pump, when few zone valves are open, the system pressure rises, and that increases pump noise. VFD pumps are a lot quieter. As with the condensing boiler efficiency problems, the noise problem is greatest when little heat is being called for. “It’s those darn shoulder seasons that are driving people crazy,” notes Thompson.

Wilo Stratos line


Wilo Stratos circulator for commercial applications.

Photo: Wilo USA

The Wilo Stratos pumps have been available in Europe since 2001 and in the U.S. since 2007. Eight different sizes of Wilo Stratos pumps offer flow rates of 31–270 gallons per minute (gpm). Power consumption for the smallest Stratos ranges from 9W to 85W and for the largest from 40W to 1,550W. The residential Statos ECO 16F and 16FX pumps have a maximum flow rate of 15.5 gpm and consume 5.8W–59W. Pumps are rated in the European Union on an efficiency scale of A through G; these all achieve the top rating of A.

According to Thompson, interest in the Wilo Stratos pumps has been particularly high in places with high electric rates, such as New York and Alaska. Manufacturers of boilers, solar water heating systems, and ground-source heat pumps are also looking to integrate these pumps into their systems to improve overall performance, according to Thompson.

Grundfos Magna and Alpha lines

In the Grundfos Magna line, three different pumps are available in both cast iron and stainless steel. The smallest uses 25W–450W to circulate up to 95 gpm; the largest uses 35W–900W to circulate up to 170 gpm. In the Grundfos residential line, the stainless-steel Alpha is available in three sizes with flow rates up to 12 gpm with variable electricity consumption from 5W to 45W.

Bob Reinmund, senior product specialist for Grundfos, told EBN that well over a million of these pumps have been sold in Europe in recent years. The pumps were then modified for the North American market and tested for two years before introduction here starting around 2008. “It has been very well received,” he said.

Higher cost

According to Fredette, the residential Grundfos Alpha pumps are about three times as expensive as standard constant-speed circulators, while the Magna pumps cost about 50% more than standard circulators. Steve Thompson at Wilo said that the Stratos and Stratos ECO pumps cost two to two-and-a-half times that of standard circulators. The payback for these pumps (assuming electricity savings only) can range from eight months to about three years, according to Fredette and Thompson.

Dave Yates told EBN that looking forward, virtually all of his hydronic work will incorporate smart circulators. “The additional cost is not an issue when presented properly to consumers,” he said. “The ROI [return on investment] beats the stock market and real estate hands-down, and simple payback is well within a timeframe consumers tend to look for.” He added that in areas with deregulation, electric rates may be going up as much as 50%. “The higher our kWh rate, the greater my ROI.”

Case studies

Prism Engineering in Burnby, British Columbia, investigated the energy savings with smart pumps for BC Hydro, publishing the findings in a report, “New Pump Technology Pilot Project,” dated May 2009. The study compared two nearly identical high-rise residential towers in Richmond, B.C. Each building originally had a 1.5 hp pump to serve all perimeter heating needs. In one of the buildings, this pump was replaced with a Wilo Stratos pump. Metering was installed in both buildings to measure electricity consumption for pumping.

Measured power consumption for the existing pump averaged 1.1 kW throughout the test period, while power consumption for the Wilo Stratos pump averaged 0.45 kW in the winter months and 0.3 kW in the spring. The researchers projected annual savings of 76%.


A 1-HP pump that ran nearly continuously at All Souls Church in Brattleboro, Vermont, was replaced with this Grundfos Magna VFD “smart” pump, which senses pressure and circulates only as much hot water as needed.

Photo: Alex Wilson

I was involved with one of the first installations in New England of the Grundfos Magna pump. As part of a major heating system upgrade of the 7,000 ft2 (650 m2) All Souls Church in West Brattleboro, Vermont, failed zone valves were replaced so that hot water flow could be shut off to zones in the building where heat is not needed. Mechanical engineering firm Kohler & Lewis Engineering of Keene, New Hampshire, recommended the Grundfos Magna, because it would automatically modulate the flow of hot water through the hydronic loop depending on how much heat was called for. The new pump replaced a one-horsepower pump that ran nearly continuously throughout the heating season.

While the heating system upgrade at the church was only completed in February 2010, there have been no problems so far, and the new pump is remarkably quiet.

Some are skeptical

While the benefits of variable-speed, ECM, smart circulators are touted by many, not all experts are as enthusiastic. “VFD is another magic box,” says Henry Gifford, an HVAC contractor in New York City. He sees this new generation of pumps as an alternative to actually sizing pumps. “Probably less than 1% of piping systems have any calculations done,” Gifford told EBN. He says that fixed-speed pumps, which he prefers to use, work fine with hydronic systems that use zone valves, and he claims that the current draw drops when they are pumping less water.

Gifford described a recent building he worked on in New York City, a five-story apartment building with relatively high loads due to poor insulation and significant air leakage. This building uses a single 1/8 hp pump (a Taco 0010F), which is rated at 127 watts. Because it’s an inductive load, though, Gifford says the power factor is significantly less than one, so “the true wattage is probably less than 100.” Because the power consumption of the pump drops when it is doing less work, he says the average usage is probably less than 50 watts. He argues that the cost and complexity of a VFD pump is hard to justify for such minimal savings.

To Gifford’s point, in the BC Hydro study described above, a caveat explains that the design flow and head were not confirmed for the base-case pump: “… it is possible that measured energy savings could be partially a result of oversizing of the base pump.”

Right-sizing unlikely

While very significant savings could doubtless be achieved by right-sizing circulator pumps, that is unlikely to occur, and the VFD smart pumps from Wilo and Grundfos provide an excellent way to achieve significant savings when older pumps are replaced or new hydronic systems are installed. “The fact that these pumps deal with oversizing is a big deal,” says Dan Lewis, P.E., of Kohler & Lewis. Energy consultant Andy Shapiro, who also advised on the project, adds that you still need to size pumps, “but it provides comfort in knowing that a bit of oversizing won’t be a penalty.”

Sales of more efficient pumps in North America are hampered by the lack of a standardized efficiency rating for pumps, as exists in Europe. That will be changing. Steve Thompson of Wilo is chair of the Fluid Pumps committee of the Air-Conditioning, Heating, and Refrigeration Institute (AHRI), which is working on such an energy rating. Thompson hopes the standard will be released in 2012. Once pumps carry accurate ratings for energy efficiency, we will likely see incentive programs that subsidize their purchase.

– Alex Wilson

For more information:

Grundfos USA

Olathe, Kansas


Wilo USA

Melrose Park, Illinois


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August 30, 2010