Why is it important to measure accurately the temperature difference between the supply and return of the chilled or condenser water?

The temperature difference between the chilled/condenser water supply and return temperature (°C) is used to compute the net cooling load delivered to the building, or amount of energy consumed by AHUs/FCUs (air handling units/ fan coil units).  As an illustration, for a chilled water plant operating with a chilled water supply and return temperature difference of 5°C, a seemingly acceptable error of ±0.5°C can in fact lead to a 20% error in the final computation of the system cooling load.

In addition, many energy audits conducted on existing buildings have shown that a large number of existing chiller plants experience a low delta T syndrome (i.e. temperature difference of about 2 °C to 3 °C), especially when they operate at part load conditions. In such circumstances, the error in the cooling load computation is even greater.

A maximum uncertainty of ± 0.05°C end-to-end for each temperature measurement is thus required, to allow for optimisation of the plant operation. Since the lifetime of a typical chiller plant is 15 to 20 years, it is important to install highly accurate temperature sensors for long-term accurate trending of the operational efficiency of the plant.