Chilled water and condenser water reset strategies

A chiller's efficiency depends heavily on lift: the difference between the refrigerant pressure in the condenser and in the evaporator, which follows the difference between condenser water and chilled water temperatures. Raise chilled water temperature or lower condenser water temperature, and the compressor has less work to do. Reset strategies take advantage of this by moving setpoints away from design values whenever loads and weather allow.

This guide covers chilled water supply temperature reset, condenser water reset, the trade-offs with pumps and cooling tower fans, and the limits that protect comfort and equipment.

Why lift matters

At design conditions, a typical water-cooled plant might supply chilled water around 6 to 7 °C (43 to 45 °F) and receive condenser water from the tower around 29 to 30 °C (84 to 86 °F). Most hours of the year are far from design. As a general rule of thumb often quoted by manufacturers, each degree Celsius reduction in lift improves chiller efficiency by a few percent, although the exact value depends on the chiller type and operating point. Variable speed centrifugal chillers in particular gain substantially from reduced lift at part load.

Chilled water supply temperature reset

How it works

The chilled water supply temperature (CHWST) setpoint is raised when cooling demand is low and lowered when demand rises. Warmer chilled water reduces chiller lift and may reduce losses in distribution piping.

Reset signals

The pumping trade-off

In variable flow systems, warmer chilled water reduces coil capacity per litre, so cooling coil valves open further and pumps move more water. Pump power rises steeply with flow. At some point, the extra pumping energy outweighs the chiller savings. Effective sequences reset chilled water temperature first at low loads, and as loads rise, coordinate with the pump differential pressure reset, often by resetting differential pressure first and then chilled water temperature, or by optimizing both together. See variable speed pumping and differential pressure setpoints.

Limits

• Humidity control: warmer chilled water reduces the coil's ability to dehumidify. Limit reset during humid weather using space humidity or outdoor dew point.
• Critical loads: data rooms, labs or process loads on the same loop may need cold water regardless of building demand.
• Delta T: warmer supply water and wide-open valves can worsen low delta T in some systems. See low delta T syndrome.
• Chiller limits: confirm the maximum leaving temperature allowed by the manufacturer.

Condenser water temperature reset

How it works

Instead of holding tower leaving water at a fixed temperature such as 29 °C (85 °F), the setpoint is lowered when outdoor wet bulb temperature allows, reducing chiller lift. The tower's ability to produce cold water depends on the outdoor wet bulb temperature and the tower's approach.

Common strategies

1. Fixed approach reset: setpoint equals outdoor wet bulb plus a design approach, limited between minimum and maximum values.
2. Load-based approach: the target approach varies with chiller load, since the tower can achieve a closer approach at low heat rejection.
3. Optimization based on total power: the controls look for the condenser water setpoint that minimizes combined chiller and tower fan power, using equipment models or real-time search.

The tower fan trade-off

Lower condenser water temperature saves chiller energy but costs tower fan energy. At some point, the fan energy needed to drop the water another degree exceeds the chiller savings. Variable speed tower fans make this trade-off continuous. In general, running more tower cells at lower fan speed is more efficient than fewer cells at high speed, provided minimum flow per cell is maintained.

Minimum condenser water temperature

Chillers have a minimum entering condenser water temperature or minimum lift. Below it, oil return, refrigerant management and expansion device control can be affected. Older chillers often require relatively warm condenser water, while many modern machines tolerate much colder water. Always follow the manufacturer's limits, especially during start-up in cold weather, when tower bypass valves or start-up logic may be needed. In Canada, cold-weather tower operation also requires freeze protection. See cooling tower selection and winter operation.

Coordinating plant resets

The best plants coordinate chillers, pumps and towers rather than optimizing each in isolation:

ASHRAE Guideline 36 includes plant sequences in newer editions, and many owners use plant optimization software. See chiller plant sequencing and optimization.

Implementation steps

1. Confirm chiller manufacturer limits for leaving chilled water and entering condenser water temperatures.
2. Identify critical loads and humidity-sensitive spaces.
3. Select reset signals and define minimum and maximum setpoints.
4. Install or verify accurate sensors for chilled water, condenser water, outdoor dry bulb and humidity, and valve positions.
5. Program rate limits so setpoints change gradually.
6. Trend setpoints, temperatures, chiller power, pump and tower fan power and valve positions.
7. Review energy and comfort results over a season and adjust.

Plant reset optimization is a common outcome of energy audits, and in many plants it costs little to implement because the necessary sensors and drives already exist.

Key takeaways

• Lower chiller lift through warmer chilled water and colder condenser water improves chiller efficiency.
• Chilled water reset must be balanced against pump energy and dehumidification.
• Condenser water reset must be balanced against tower fan energy and chiller minimum lift limits.
• Demand-based resets outperform simple outdoor air resets.
• Coordinate chillers, pumps and towers as one system, and verify with trends.