A temperature reset strategy can have a considerable impact on the energy use of HVAC equipment. Best of all, it requires minimal investment.
Most HVAC systems have a fixed temperature set point for Heating Hot Water (HHW), Chilled Water ((CHW) and Condenser Water (CW), or manually regulated floating temperature set points for the heating/cooling fluids which are not
optimised.
Temperature reset can allow for minimised energy consumption of HVAC equipment when operating in the part-load conditions that occur throughout most of the year.
For example, resetting HHW delivery temperature can save up to five per cent of energy consumed by condensing boilers, while resetting CHW delivery temperature can save up to 15 per cent of energy consumed by chillers.
Resetting CW delivery temperature can save up to 15 per cent of energy consumed by chillers, according to the HVAC Optimisation Guide released by the NSW Office of Environment & Heritage in partnership with AIRAH.
Heating Hot Water
Reducing HHW temperatures will reduce distribution losses (the heat lost or gained through the pipes and pipeline components) and slightly improve the thermal efficiency of conventional boilers while significantly improving the
efficiency of non-condensing boilers.
If the return temperature of a conventional (non-condensing type) boiler is reduced below 55°C, any small gains in efficiency will be negated due to boiler back end corrosion, also known as fireside corrosion, which will cause premature failure of the boiler. This must be avoided.
Where condensing boilers are installed, they are more efficient when the return temperature is maintained below 55°C which is the typical dew point of flue gases for natural gas-fired boilers. As such, it is important to maintain the boilers at as low a HHW flow temperature as possible, except for the relatively short periods of the year when full heating system capacity is required; typically during extremely cold weather and/or during early morning warm-up periods.
During periods of high heating demand, the HHW flow temperature should be reset to 80 85°C when condensing boilers have reached their maximum output. The HHW flow temperature should be reset to 80 85°C and the condensing boiler used as
the lead boiler before additional (non-condensing type) boilers are brought online.
Chilled Water
Typically, CHW is supplied in most systems at temperatures between 6 7°C for most systems under design conditions; however, this temperature can be reset upwards to around 10 12°C during milder weather, providing there are no adverse
effects such as the loss of de-humidification (humidity control).
Increased CHW temperature will reduce chiller energy consumption; however, it could require additional pumping energy and/or increased S/A flow. These increases should be considered when calculating the overall benefits from this
optimisation strategy.
For CHW systems that have long CHW circuits, the pumping power is significant.
Condenser Water
For every 1°C decrease in CW, chiller compressors consume 2 3 per cent less energy for fixed-speed compressors and 4 5 per cent less for variable speed compressors.
Cooling towers are typically designed to produce CW at temperatures that are 3 4°C higher than the prevailing ambient wet bulb temperature. This temperature difference is called the cooling tower approach .CW temperature reset is based on the modulation of the cooling tower fan speeds to track the ambient wet bulb temperature, which provides the lowest possible
condensing conditions.
Optimisation is essentially a balancing exercise between reduced chiller energy consumption (due o the lower condensing temperature) and higher fan energy consumption at the cooling towers.
In these circumstances, variable CHW flow, on secondary circuits, may be a more appropriate strategy for energy saving than CHW temperature reset and should be assessed on a case-by-case basis.
Generally, for short CHW circuits, CHW temperature reset is more energy efficient as savings in pumping power are smaller, relative to potential gains in chiller efficiency through CHW reset. Variable speed chillers typically have a better response to increased CHW temperature than fixed-speed chillers. Similar behaviour occurs when CW temperature is reduced.
During mild weather conditions, warmer than standard CHW temperatures can satisfy the cooling load. Raising the CHW temperature will reduce the electricity consumption of chillers.