Applying variable frequency drives (VFDs) to existing constant-volume heating, ventilation and air conditioning (HVAC) pumps transforms them into variable flow equipment. This improvement is a well-established and popular energy conservation technique, especially among building managers eager to qualify for utility energy efficiency incentives.
The savings from energy retrofits to constant volume pumps—especially in commercial and industrial (C&I) settings such as office buildings, universities and hospitals—are often significant because of the number of pumps involved and energy saved.
Thanks to affinity laws, which reveal that a reduction in motor speed exponentially cuts energy consumption, reduction of energy is immediate and substantial.
Some utilities offer “prescriptive rebates” to win the interest of building managers. Typically, these entail pre-determined dollar amounts, usually per motor horsepower (hp) based on assumed energy savings.
The challenge is that the actual kilowatt-hour (kWh) savings may fail to materialize. The problem is not with the technology. Rather, it is one of human error. When an installing contractor buys a VFD for a new or existing pump motor, the installation process is anything but prescriptive.
Not only does the contractor have to mount and wire the device near the pump, but he must also decide where to place the differential flow sensors in the pipe, drill into the pipe to install the sensors, program the drive, then start it up.
The mounting and wiring is something most mechanical contractors can easily perform. The sensor installation and drive programming, however, is another matter entirely.
Contractors need substantial expertise to install differential flow sensors in the proper place and to understand the application and the model of VFD well enough to accurately program it.
Unfortunately, that expertise is not widely available, so many projects are installed incorrectly or with no modification to the drive’s factory defaults to match the specific pump or system requirements. This means that the utility, on average, is not going to see the kWh savings it prescribes, or is desired by facility managers.
“It’s all too common for us to see bolt-on VFDs set up incorrectly or even disabled altogether,” said Jake Marin, HVAC program manager for Efficiency Vermont, which was founded in 2000.
“A properly set up pump plus VFD can operate very well, but we need a simpler, more reliable option if we are going to see true market transformation.”
The key to addressing this problem is for industry experts to focus on the need for and provide integrated solutions.
Every major pump manufacturer offers a line of self-sensing pumps with an integrated VFD. These pumps do not require sensors installed in the pipe and come with the VFD factory-tuned to that specific pump and preprogrammed for sensorless variable flow pumping.
All the contractor needs to do is physically install the equipment in the hydronic system.
“With self-sensing pumping, there are no holes in the pipes for tubes or taps connecting remote differential sensors together with the pump. Self-sensing pump system design offers great accuracy for variable flow, higher energy efficiency, lower install costs and improved system stability,” according to an August 2017 Pumps & Systems article titled “Optimizing Commercial Hydronic System Performance.”
It is recommended that utility managers turn their attention to the value of these effective, off-the-shelf solutions. Efficiency Vermont is one utility that has fully embraced this concept.
The Efficiency Vermont program is providing enhanced rebates for pumps that “consist of a factory prepackaged and preprogrammed pump, drive, motor and integral controls package capable of varying speed and flow without the use of external sensors.”
By incentivizing integrated pumps—rather than separately sold components, fully dependent on qualified field installation and commissioning to operate properly and efficiently—utility experts can expect to see a substantial improvement in the real world energy savings for their