The relationship between filter pressure drop, indoor air quality, and energy consumption in rooftop HVAC units

Abstract

HVAC filters are commonly used to decrease exposure to particulate matter, yet little is known about the energy impacts and air quality consequences of high efficiency filters installed in commercial buildings. To explore these effects, system airflow, filter and coil pressure drop, fan pressure rise, and power draw were measured, and cooling capacity and compressor power were modeled for at least four filter pressure drops in 15 rooftop units equipped with and without fan speed control. Energy implications and clean-air-delivery-rate were estimated for a large dataset of filters divided into four efficiency (MERV) categories. Field measurements conducted on units without fan speed control showed that increased filter pressure drop decreased flow, cooling capacity, and power. For a unit with fan speed control, the same increase in pressure drop resulted in the same magnitude change of fan power but in the opposite direction, and other parameters were unchanged. Replacing MERV 8 with MERV 13/14 resulted in higher energy consumption (2–4%) during cooling mode for both unit types, energy savings during fan-only mode (8–13%) in units without fan speed control, and increased energy consumption in fan-only mode (11–18%) in the unit with fan speed control. Energy consumption increases were offset by improvement in clean-air-delivery-rate, especially for PM2.5 (2.9–3.8 times increase going from MERV 8 to MERV 13/14), with larger benefits achieved for the unit with fan speed control. A comprehensive understanding of the impact of filtration is essential to selecting the appropriate efficiency of filters that ensures low-energy use and a healthy indoor environment.