Most commercial hydronic systems built in the last few decades use two-way control valves and variable speed pumps. As valves throttle in response to falling load, the pump slows down to match, and because pump power falls steeply with speed, the savings can be large. NECB includes variable flow requirements for larger hydronic systems for exactly this reason.
How much energy a variable speed pump actually saves depends almost entirely on how it is controlled. The location of the differential pressure sensor and the value of the setpoint matter more than the drive or the pump. This guide explains the principles and the practical choices.
How variable speed control works
A variable frequency drive (VFD) changes pump speed. A controller compares a measured variable, usually differential pressure somewhere in the system, to a setpoint and adjusts speed to hold it.
When loads fall and valves close, system resistance rises and differential pressure starts to climb. The controller slows the pump to bring pressure back to setpoint. The pump follows a control curve rather than the system curve, and the shape of that control curve determines energy use.
The affinity laws say flow varies with speed, head with speed squared and power with speed cubed. Those relationships apply strictly only along a system curve that passes through zero head at zero flow. Because a differential pressure setpoint holds some head even at low flow, real savings are less than the cube law suggests. The higher the setpoint relative to total head, the smaller the savings. See pump selection and system curves for the underlying curves.
Where to put the differential pressure sensor
| Sensor location | Control curve | Energy use | Notes |
|---|---|---|---|
| Across the pump | Nearly flat (constant head) | Highest | Simple but saves little; mostly used for small systems or as a fallback |
| At the plant, across supply and return mains | Moderately flat | Moderate | Easy to install and maintain; setpoint must cover all pipe losses to the far end |
| Near the hydraulically most remote load | Steeper, follows system curve more closely | Lower | Classic recommendation; setpoint only needs to cover the remote coil, valve and branch |
| Multiple remote sensors, lowest-signal select | Follows the most demanding branch | Lower | Useful for buildings with several risers or wings with different load patterns |
| Sensorless (pump-internal estimate) | Programmed curve | Varies | Built into many modern pumps; accuracy depends on the pump's model and the system |
The hydraulically most remote load is the one with the highest pressure requirement, not necessarily the farthest in distance. It is often the coil at the end of the longest run, but a large coil with a high-pressure-drop valve closer to the plant can be the critical one. Balancing data should confirm which branch it is.
Choosing the setpoint
For a remote sensor, the setpoint should equal the pressure drop at design flow across the components between the sensor taps: typically the coil, control valve, balancing device, strainer and branch piping. For a pressure-independent control valve, add the valve's minimum operating differential pressure.
Setting it in the field
- Balance the system first, so that design flows are established and the critical branch is known.
- With all valves at design flow (or simulated design conditions), measure the differential pressure at the sensor location.
- Confirm the critical coil receives design flow at that pressure.
- Set the controller to that value, not to the design engineer's early estimate.
- Document the setpoint and the method in the balancing report and the sequence of operation.
A setpoint left at a conservative design value, or raised by operators in response to a single complaint, is one of the most common reasons variable speed pumping systems underperform. The guide on reducing pump energy covers other contributors.
Setpoint reset using valve position
Even a well-chosen fixed setpoint is only correct at design load. At part load, the critical coil needs less pressure. A valve-position reset strategy uses BAS data to lower the setpoint until at least one valve is nearly wide open:
- Poll all control valve positions at a regular interval.
- If no valve is above a threshold (for example 85 to 90 percent open), lower the setpoint by a small step.
- If one or more valves are near fully open and calling for more, raise the setpoint by a small step.
- Keep the setpoint between configured minimum and maximum limits.
This is the trim and respond logic described in ASHRAE Guideline 36, applied to pumps. It works best when the BAS has reliable valve position data, and when "rogue" zones that always demand full flow (because of a bad sensor or an undersized coil) can be identified and excluded. Ignoring rogue zones defeats the reset.
Minimum speed and minimum flow
Pumps need a minimum speed to maintain lubrication and cooling of the motor and to stay out of unstable regions of the curve. Chillers and boilers also have minimum flow limits. Systems therefore need either a minimum flow bypass controlled by a flow meter or valve, or a design that guarantees a minimum number of open valves.
A permanently open bypass or three-way valves at the ends of mains defeat variable flow and contribute to low delta-T syndrome. Control the bypass so it opens only when needed.
Multiple pumps in parallel
When two or more variable speed pumps run in parallel, staging logic decides when to add or drop a pump. Running two pumps at reduced speed is often more efficient than one pump near maximum speed, because each operates closer to its best efficiency point. Staging based on speed, flow, or calculated efficiency all work if tuned properly. All running pumps should run at the same speed to avoid one pump dead-heading against another.
Common problems
- Sensor located at the pump when a remote location was designed.
- Setpoint far above what the system needs.
- Sensor tubing plugged or air-bound, giving false readings.
- Control loop poorly tuned, causing pump hunting.
- Pumps stuck at minimum speed because the setpoint is satisfied by an open bypass.
- Pump VFDs left in hand mode after service.
Trend pump speed, differential pressure and valve positions. A pump that rarely drops below 80 percent speed in mild weather deserves a closer look.
Frequently asked questions
Is sensorless pump control good enough?
For simple systems it can work well. For larger systems with diverse loads, a remote sensor or valve-position reset generally gives better results.
Can I add a VFD to an existing constant speed pump?
Often yes, if the system has or can be converted to two-way valves. Check motor compatibility with drives and the effect on chiller or boiler minimum flows.
Key takeaways
- Variable speed savings depend on sensor location and setpoint more than on the drive itself.
- Place the sensor near the hydraulically most remote load, confirmed by balancing data.
- Set the setpoint from field measurements after balancing, not from early estimates.
- Valve-position reset captures additional savings at part load.
- Verifying setpoints and pump sequences is a core part of control system verification.
Talk to an engineer
Have a project or a problem this guide touches on? Tell us about the building and we’ll be in touch within one hour.