Supply air temperature reset strategies

Many air handlers deliver supply air at a fixed 13 °C (55 °F) all year. That temperature makes sense on a hot, humid design day, but for much of the year in Canada it means cooling air more than necessary and then reheating it at the zones, or running chillers when outdoor air could do the job. Supply air temperature (SAT) reset raises the setpoint when loads are low, reducing reheat, extending economizer hours and saving cooling energy.

This guide explains the main reset strategies, the energy trade-offs, and the limits that keep reset from causing comfort or humidity problems.

Why reset saves energy

Reheat reduction

In VAV systems with reheat, interior and perimeter zones at minimum airflow often need heat even when the air handler is cooling. Each degree of warmer supply air reduces reheat. Constant volume reheat systems benefit even more, since every zone receives full airflow all the time. See constant volume vs variable air volume systems.

More economizer hours

A higher SAT setpoint means the economizer can meet the full cooling load at higher outdoor temperatures, delaying mechanical cooling.

Chiller efficiency

Warmer supply air may allow warmer chilled water, improving chiller efficiency.

The fan energy trade-off

In VAV systems, warmer supply air means more airflow is needed to meet the same cooling load, which increases fan energy. Fan power rises steeply with airflow, so aggressive reset during warm weather can cost more in fan energy than it saves in cooling. Good reset strategies account for this, usually by resetting most when loads are low and outdoor air is cool, and holding a low SAT when it is warm.

Energy codes reflect this. NECB and ASHRAE 90.1 require supply air temperature reset for many multiple-zone systems, with exceptions in certain cases. Confirm the requirements of the edition adopted in your province.

Reset strategies

Outdoor air temperature reset

The simplest strategy resets SAT linearly between two outdoor temperature points. For example, at outdoor temperatures above 21 °C (70 °F), SAT is 13 °C (55 °F); below 15 °C (60 °F), SAT rises to 16 to 18 °C (61 to 64 °F). It is easy to program and stable, but it ignores actual zone loads. A building with high internal gains may overheat in interior zones on a cool day.

Return air or space temperature reset

SAT is reset based on average return air or space temperature. This responds to load but averages away the hottest zones, which can leave critical zones short of cooling.

Zone demand reset

SAT is reset based on the zone needing the most cooling, typically the zone with the most-open damper or the highest cooling loop output. This keeps every zone satisfied, but a single undersized or faulty zone can hold the whole system at minimum SAT.

Trim and respond

ASHRAE Guideline 36 uses trim and respond logic with zone cooling requests. When few zones request cooling, the SAT setpoint is trimmed upward in small steps; when requests exceed a threshold, it responds downward. Importance multipliers let operators reduce the influence of problem zones. Guideline 36 combines this with an outdoor air temperature limit: the reset range is active during cool weather and SAT is held at the minimum in warm weather, which addresses the fan energy trade-off. See ASHRAE Guideline 36 high-performance sequences.

Comparison

Limits and safeguards

Humidity

Raising SAT reduces dehumidification. A coil delivering 16 °C air removes far less moisture than one delivering 13 °C air. In humid weather, the reset should be limited by a space or return humidity sensor, or by outdoor dew point, so the system returns to a lower SAT when moisture rises. ASHRAE 62.1 requires systems that dehumidify to limit space RH to 65 percent at design conditions. See dehumidification strategies for commercial buildings.

Fan and terminal capacity

Warmer air requires more airflow. If VAV boxes reach their maximum airflow or the fan reaches full speed, the zones will lose cooling. The reset should respond to these conditions through zone requests or a fan speed limit.

Heating mode and minimums

In heating-dominated systems, especially those with DOAS or perimeter heating, SAT may also be raised to provide neutral air. Avoid supply temperatures so warm that zones needing cooling cannot get it.

Special spaces

Data rooms, labs, and spaces with critical cooling loads on the same air handler may need to be excluded from averaging or given higher importance.

Implementation steps

1. Confirm the system type and identify which zones are likely to drive the reset.
2. Select a strategy and define the reset range, typically from a minimum around 12 to 13 °C to a maximum around 16 to 18 °C, adjusted for the building.
3. Define outdoor air and humidity limits.
4. Define how zone requests are generated and how rogue zones are handled.
5. Program rate limits so the setpoint changes slowly and loops remain stable.
6. Trend SAT setpoint, actual SAT, fan speed, zone requests, reheat valve positions and space humidity.
7. Review trends after a few weeks of operation and adjust parameters.

Verifying performance

Trends should show SAT rising during cool, low-load periods, falling during warm or humid weather, and zones remaining satisfied. Common faults include resets disabled after occupant complaints, a single zone with a failed damper holding SAT at minimum, and humidity limits missing. These are typical findings in energy audits and retro-commissioning.

Key takeaways

• SAT reset reduces reheat, extends economizer hours and can improve chiller efficiency.
• In VAV systems, warmer air increases fan energy, so reset is most valuable in cool, low-load conditions.
• Zone demand and trim and respond resets respond to actual loads; outdoor air reset is simpler but blind to zones.
• Humidity, fan capacity and critical zones must limit the reset.
• Trend and review reset behaviour after start-up, and fix rogue zones rather than disabling reset.