How to Pick a Centrifugal Pump Part 10: Over-Speeding an HVAC Pump Using a VFD

By Chad Edmondson (JMP) and Norman Hall (RLD)

Most--if not all--consulting engineers fear under-sizing pump head in a hydronic HVAC application. 

Pump affinity laws, Btuh coil heat transfer calculations, and ASHRAE flow tolerance recommendations all tell us that a slight miss on the pump head may result in only a minor difference in the pumping system’s ability to satisfy load. Still, no one wants the owner to receive a balance report indicating a shortage of flow compared to the schedule.  

There are many examples of specifications instructing the reader to select a pump at less than the maximum impeller so the designer has “room to move” if needed. Most of these “hand-me-down” statements create a lot of confusion and flawed pump selections. In addition, by not selecting a pump at or close to the maximum impeller published, you give up the possible best efficiency point (BEP) of the pump. 

How can you protect yourself and still take advantage of the efficiency associated with greater impeller diameters?  

The answer is using a VFD (variable frequency drive) to run the pump at a higher speed and hertz, a technique known as over-speeding the pump. 

Selecting a Pump for Over-Speeding with VFD

Look at the two selections below. The selection on the top is the better selection but many engineers would choose the one at the bottom so they can increase the impeller if they need to. But, there is more to the story….

Part 10 pumps selection 1.jpg
Part 10 Pump selection 2.jpg

If we need to increase the impeller so the flowrate increases by 5%, we will run out of horsepower and have to increase it from 40 HP to 50 HP.  

What about the better curve on the bottom? If we increase the speed of the pump by 5% or up to 63 Hz, thee1510-5EB curve will now look like this:

Part 10 Pump Section 3.jpg

The horsepower of the pump and the amp draw of the variable frequency drive (VFD) will be acceptable to operate this pump at 1860 RPM. This provides 10% additional head capacity for the future, while sustaining overall pump efficiency. We checked the horsepower and found it to be non-overloading all the way to 1450 GPM at the higher speed of 1860 RPM. This is a great pump selection!

One More Tip for Writing Your Specification …

We also suggest removing all specification statements like, “85% of maximum impeller” or “capable of increasing the impeller by one diameter” or “pump selected for future larger impeller.” Instead, consider adding this to your specification: 

“Pump and motor shall have the ability to operate at 5% over the scheduled speed using a VFD without affecting the warranty or causing damage to the pump or motor.”

Just make sure your selection has enough horsepower to operate at the point created at the new speed should you ever need it. We have seen many specifications indicating the potential need to increase the impeller without addressing the horsepower and frame size. Remember -- changing an impeller might change the motor horsepower and the pump shaft size, the bearings, base and even the pump dimensions. Just make sure you have enough horsepower in your selections if you are using the suggested specification addition!