Mitigating Hot Spots in HVAC for Office Interiors

Picture Credit : https://www.askewsltd.com/blog/true-effects-heatwave-workplace/

Qpro has been appointed as the MEP consultant for a leading client for the fit-out project, which includes the HVAC low-side works. The builder has provided the high-side infrastructure, comprising chillers, pumps, a cooling tower, and smart AHUs. The AHU system is designed to feed the entire floor space, with VAVs installed for zone control.

However, a hot spot was found at the farthest 12-pax workstation. We identified the following potential causes contributing to this issue.

The AHU modulates its operation based on input from a return air temperature sensor located in the return air path of the AHU room. However, due to the short cycling of the return air, the AHU senses a return air temperature of approximately 19°C to 21°C, which does not reflect the actual conditions in the space. As a result, the AHU reduces fan speed to maintain the setpoint. This reduction in fan speed leads to insufficient airflow and inadequate cooling at the farthest locations.

Secondly, there are two main branches, of which the Differential Pressure Transducer (DPT) is fixed in only one of the branches. As a result, the temperature and pressure are not maintained in either branch. Since the Supply Air (SA) and Return Air (RA) temperatures are sampled at the same location, the EC fan controller cannot control the temperature properly.

The problem was mitigated by installing two DPT sensors, one in each branch. The average of the inputs from these two sensors can be used to regulate the Electronically Commutated (EC) fan speed. If the EC fan controller does not support two pressure inputs, an alternative solution is to use a Direct Digital Controller (DDC). The DDC can process the inputs from both sensors, calculate the average, and then provide the averaged pressure input to the EC fan controller.

Additionally, the return air path near the AHU has been modified to prevent short cycling of the return air, ensuring more efficient system performance.