Your questions answered: Energy codes and standards for pumps and hydronic systems

Additional questions from the April 28 webcast are answered, including those about using VFDs to trim pumps, energy savings, and designing based on weather data.

By Reece Robinson, Grundfos Pumps Corp. May 5, 2015

Reece Robinson, senior technical trainer at Grundfos Pumps Corp., tackled unanswered questions from the April 28, 2015, webcast on energy codes and standards for pumps and hydronic systems.

Question: What is the process of using a variable frequency drive (VFD) to electronically trim an over-sized pump?

Answer: I believe I introduced the term "electronically trim" during the presentation so I am obligated to explain. When a pump impeller is trimmed mechanically there is a loss in hydraulic efficiency. The distance between the impeller tip and the pump casing is increased thus resulting in added recirculation within the pump casing. A VFD simply reduces the speed of the pump to the required performance without an increase in internal pump recirculation.

Question: Did you say that pump throttling was preferred over impeller trimming from an energy conservation standpoint? That seems counter-intuitive.

Answer: I hope I didn’t say that! Actually ASHRAE Standard 90.1 recommends MINIMIZING throttling LOSSES first, then trimming pump impellers.

Question: As an example, how much energy is saved when using a pump with VFD operating at 80% to meet 300 gpm versus a constant flow pump being throttled back on discharge side of pump to meet the 300 gpm design?

Answer: Figure 1 shows a performance curve for a pump that can meet a 300 gpm capacity. Two speeds are represented, 100% which represents running with a fixed speed motor and 80% via control with a VFD.

For example, let’s say the original requirements were 300 gpm at 80 ft of head. When first switched on the pump operates at point A, which is the un-throttled condition (balancing and/or multi-function valves still open). This is also the intersection of the true (as-built) system resistance curve and the pump performance curve. Point B is the throttled condition once balancing and other valves are adjusted. Point C represents the operating point with an adjustable speed drive controlled pump running at 80% speed (48 Hz down from a base speed of 60 Hz). Line B-C represents the head energy (46 ft or about 20 psi) wasted by throttling. Line B’-C’ is the horsepower output reduction by using the adjustable speed drive (4.6 hp), which is a savings of 43%. In this case, a 20% reduction in speed saved 43% in energy.

Question: Do we design HVAC loads based on monthly average or maximum historical temperatures?

Answer: The most common and accurate way of determining HVAC loads is using software that includes weather data. This weather data is based on historical data, but does not include extremes. Modern HVAC load calculation software uses hour-by-hour weather data, so peak conditions can be determined for the entire building or connected load.

Question: Are there any guides in the codes for best way to run multiple pumps with VFDs, such as when to phase in the next pump-four pumps running at 50% versus two pumps at 100%?

Answer: Unfortunately, this is not part of the code. The best way to stage parallel connected pumps is efficiency. This will require flow, head, and power measurement (or at least a good calculation of performance). When pumps operate at high flow, approaching their "end of curve" the efficiency starts to drop. So even though a single pump can provide ample flow, it may be more efficient to switch on a second pump and share the flow. Some of this is covered in the on-demand webcast Efficient Pump Selection and Control.

Question: How does the cost saving realized by decreased piping sized compare by the resulting increase in design pump head, pump size, and power requirements (including electrical supply)? Does not the larger pump further decrease part-load efficiency?

Answer: Very good question and yes, because pipe sizes can be smaller for variable flow systems, there will be a small increase in pump head requirements. This is where looking at parallel connected pumps becomes important. For lower flow rates, operating a smaller pump can be more energy efficient. Keep in mind that running multiple pumps will only be required part of the time where as running one smaller pump may occur more often depending on the load profile. There is no rule of thumb here—pump selection should be done knowing what the flow profile will look like.

Question: Is there HVAC load software available for free?

Answer: On the Dept. of Energy website there’s a Building Energy Software Tools Directory that has a long list of available software ranging from freeware to a few thousand dollars.