Variable Speed Pump Control: A Final Review

By Chad Edmondson

If this series on variable speed pump control has taught us anything it’s that there is no one-type-fits-all control solution.  Control strategies must be applied based on the system design as well as what is practical for the given situation.

If we are talking strictly operating efficiency, DemandSet control operating in pressure independent mode, as described in our recent blog, Variable Speed Pump Control Using DemandSet Control Technology, is the best variable pump control strategy. It saves the most energy and it hits all the marks in terms of meeting ASHRAE 90.1-2013. But there are certain caveats. You must have a system that includes all pressure independent control (PIC) valves and a flow meter must be located near the pump.

DemandSet control could be overkill on many smaller systems, especially when pump motors are 5HP or less.  (ASHRAE 90.1-2013 doesn’t even require variable speed control on these smaller sized pumps.)  In such cases, a reasonable control solution would be Area Control, which uses a true measured set point and a differential pressure (DP) sensor. This method can be used on a new or retrofit system without incurring too much expense. 

When pumps are greater than 5 HP and there is an existing BMS to read position of the control valves, Area Control with reset is a good option. However, reset only works if you can read the position of the zone valves. If the system has PIC valves the valve position doesn’t change, so there is no reference for control.

Sensorless Curve Control is sometimes a suitable variable speed control option, and certainly an improvement over constant speed in terms of efficiency, but if air temperatures are critical, it may not be a good choice. Systems with a large amount of diversity and a large control area are probably not good candidates for sensorless control because there may be frequent and significant misses, resulting in occupant discomfort. Also, it is important to remember that not every pump is suitable for sensorless curve control.  If the horsepower line follows the same slope as the pump curve, then the pump flow would be indeterminable where the pump curve and horsepower lines overlap.  

Full System Flow Curve Control uses a flow meter to read flow versus the estimate of sensorless curve control, which is based solely on the theoretical control curve. This is a good option in that it can be used on all pumps and with any drive manufacturer.  It can also be applied to existing pumps already installed or in a retrofit application. But again, systems with high diversity and large control area may experience misses.

All the above is just a “skeletal review” of what we’ve covered over this entire series. If the light bulb isn’t lighting, go back and review, starting with ASHRAE 90.1-2010/2013: Impact on Variable Speed Pump Control in Chilled Water Systems. It should all come together. If not, feel free to leave a comment or question. We are happy to respond.

We also suggest downloading this two page cheat sheet which we have developed to help you determine the best pump control strategy for a given project: