Reducing pump energy

Pumps move water through heating, cooling and condenser loops, and like fans they often run long hours. In many older buildings, pumps run at constant speed year round, with the excess pressure throttled away by balancing valves and three-way control valves. That design is simple but wastes energy whenever loads are below peak, which is nearly all the time.

Pump energy reduction follows the same logic as fan energy: move less water, against less pressure, more efficiently. This guide works through each lever and the pitfalls that come with them.

What determines pump power

Pump input power is:

Pump input power (W) = flow (L/s) x pressure rise (kPa) / overall efficiency

Overall efficiency combines the pump, motor and any VFD. In imperial units, brake horsepower equals flow in USgpm times head in feet, divided by 3,960 times pump efficiency.

The affinity laws say that for a given system, power varies roughly with the cube of pump speed. Real systems have a fixed pressure component, such as a differential pressure setpoint or a static lift in an open cooling tower circuit, so actual savings at reduced flow are smaller than the ideal cube law suggests. Even so, variable flow systems use far less energy than constant flow systems at typical loads.

Move less water

Design for a larger temperature difference

Flow is inversely proportional to the temperature difference across the coils. A chilled water system designed for a 7°C (12.6°F) difference moves considerably less water than one designed for 5.5°C (10°F), with smaller pipes and pumps. Heating systems designed for low-temperature heat pumps often use larger differences as well, subject to coil selection.

Fix low delta T

Many chilled water plants operate with a much smaller temperature difference than design, a problem known as low delta T syndrome. The plant must pump more water to deliver the same cooling. Causes include three-way valves, coils with fouled surfaces, incorrect setpoints, and control valves that are oversized or leaking. Fixing them reduces both pumping and chiller energy.

Convert to variable flow

Replacing three-way valves with two-way valves, and adding VFDs to distribution pumps, converts a constant flow system to variable flow. Plant arrangement matters: chillers and boilers need minimum flows, which can be protected with bypass valves or decoupled primary-secondary piping. See primary-secondary vs variable primary flow.

Stop pumps when not needed

Heating pumps can be shut off above a warm-weather cutoff, and chilled water pumps can stop when there is no cooling load, provided freeze protection and minimum flow requirements are respected.

Reduce pressure

Differential pressure setpoint and reset

Variable speed distribution pumps usually maintain a differential pressure setpoint somewhere in the system. Two choices matter:

• Sensor location: a sensor near the most remote coil allows a much lower setpoint than one at the pump discharge.
• Setpoint reset: rather than holding a fixed design setpoint, reset it downward based on control valve positions so the pump runs at the lowest pressure that keeps the most open valve satisfied.

Our guide to variable speed pumping and differential pressure setpoints explains how to tune both.

Remove unnecessary resistance

Trim or slow oversized pumps

Pumps are frequently oversized because of conservative design pressure drop estimates. Once balanced, they deliver excess flow or are throttled. Trimming the impeller to the actual required duty point, or permanently setting a VFD to a lower maximum speed, captures savings that throttling throws away. Measure the system curve before trimming.

Move water more efficiently

Pump selection

Select pumps near their best efficiency point at the expected operating range, not only at design. For variable flow systems, multiple smaller pumps in parallel can keep each operating closer to its best efficiency. The US Department of Energy's Pump Energy Index (PEI) and similar metrics help compare clean water pumps; Canadian efficiency regulations for pumps are maturing, so check current federal requirements.

Motors and drives

• Premium efficiency or electronically commutated motors on smaller circulators.
• Integrated variable speed circulators with built-in pressure control for small zones and terminal loops.
• VFDs sized and programmed with sensible minimum speeds to avoid overheating motors or losing minimum flow.

Condenser water and cooling towers

Condenser water pumps in open systems have a static lift component that does not fall with speed. Savings come from optimized flow during low load, careful selection of tower and chiller minimum flows, and correct sizing. See condenser water systems and cooling towers.

Procedure for a pump energy review

1. List all pumps over a chosen size: duty, motor power, control method, operating hours.
2. Measure flow, differential pressure and electrical input on the largest pumps.
3. Compare measured duty with design and with actual load requirements.
4. Check balancing valve positions and control valve types.
5. Trend system delta T and differential pressure against load.
6. Implement controls changes first (schedules, setpoints, resets), then mechanical changes (trimming, valves, VFDs).
7. Rebalance after changes and verify savings with trends.

Frequently asked questions

Do I still need balancing valves with variable speed pumps?

Often yes, although pressure independent control valves can reduce or eliminate the need. Systems must still deliver design flow to each coil at full load.

Can I put a VFD on any pump?

Most centrifugal pumps can run on a VFD, but the system must be variable flow for meaningful savings. Adding a VFD to a constant flow system with three-way valves mainly allows fine-tuning of a fixed speed.

What about chiller minimum flow?

Each chiller has a minimum evaporator flow. Variable primary systems protect it with a bypass valve and controls. Check the manufacturer's limits and rate of change of flow before reducing flow.

Accurate flow measurement is the foundation of any pump retrofit; our water balancing work provides it.

Key takeaways

• Pump energy depends on flow, pressure and overall efficiency; flow has the largest effect.
• Converting to variable flow and fixing low delta T reduce the water moved.
• Differential pressure sensor location and setpoint reset reduce wasted pressure.
• Trim or slow oversized pumps instead of throttling them.
• Measure before and after; rebalancing is part of every pump energy project.