scope:

INTRODUCTION

High-efficiency filtration in forced air heating, ventilating, and air-conditioning (HVAC) systems is used to protect building equipment and occupants, but can also influence building energy use. Filters with a high MERV (Minimum Efficiency Reporting Value, as defined by ASHRAE Standard 52.2-2007) typically have a greater pressure drop than a filter with a lower MERV. The energy consequences of a greater pressure drop due to filtration are well known for large commercial systems, where fan and motor controls typically maintain required airflow rates. A higher pressure drop filter causes the fan motor to draw more power to overcome the pressure drop and deliver the required amount of air, thus increasing energy consumption (Chimack and Sellers 2000; Fisk et al. 2002). This association between energy use and filter pressure drop is widely assumed to hold true for smaller residential and light-commercial systems, but operational differences between small and large systems suggest very different energy consequences.

The central difference is that increasing the pressure drop of a filter in most residential HVAC systems generally causes diminished airflow, although evidence is limited. Parker et al. (1997) measured a 4 to 5% airflow rate reduction when replacing standard disposable filters with high-efficiency pleated filters in residential air conditioner field tests. Diminished airflow generally decreases cooling capacity, power draw of the compressor, and system efficiency. Parker et al. (1997) predicted by computer simulations and laboratory tests that a 5% reduction in airflow from a value recommended by most manufacturers of 400 CFM ton^-1 (193 m³·h^-1·kW^-1) to 380 CFM ton^-1 (184 m³ h^-1·kW^-1) would decrease sensible cooling capacity by approximately 2%. This suggests that a system would run 2% longer to meet the same cooling load. In laboratory experiments, Rodriguez et al. (1996) tested 3.5-ton (12.3 kW) air conditioners and reported approximately 6 to 7% reductions in efficiency and total capacity associated with a 10% reduction from the recommended airflow rate. Palani et al. (1992) measured the impacts of low airflow on a 3-ton (10.5 kW) air conditioner in a series of laboratory tests as well and found similar reductions in capacity for comparable reductions in airflow. The same studies showed that more drastic energy consequences occur when flow reductions are extreme. Parker et al. (1997) reported that system cooling energy consumption could increase by 20% if flow diminishes approximately 40% from 400 CFM ton^-1 (193 m³·h^-1·kW^-1).

The previous investigations show that if the presence of a higher-efficiency filter diminishes airflow, sensible cooling capacity will decrease, suggesting an increase in energy consumption due to increased system runtime. However, fan and compressor power draw also generally decrease, potentially limiting negative energy impacts. In addition, a change in filter pressure drop can affect duct leakage by changing the pressure around duct leaks. Although we know of no direct research of the implications of filtration on duct leakage, there is extensive literature on the energy consequences of duct leakage in residential and light-commercial systems (e.g., Modera 1989; Modera 1993; Parker et al. 1993; Jump et al. 1996; Walker et al. 1998; Withers and Cummings 1998; Siegel et al. 2000; Francisco et al. 2006).

One of the central challenges of associating energy consequences with filtration is the complexity of these interacting effects. The magnitudes, and even the signs, of many of these effects are not well characterized, but are likely very systemdependent and are affected by such parameters as the fraction of the system pressure drop associated with the filter, the fanspeed setting, and the intersection point of the fan and the duct curves. To explore these effects in real systems, we monitored residential and light-commercial forced air cooling systems at multiple sites in Austin, Texas. Measured parameters included system airflow rate, power draw, cooling capacity, pressure drops across filters and coils, and duct leakage. Periodic measurements were made over the course of a year at each site with readily available filters with different MERV categories, as rated by the filter manufacturer. The purpose of this research was to assess how filter MERV and the corresponding measured pressure drop impact energy use in smaller airconditioning systems. The specific goal is to allow system designers and users to evaluate the consequences associated with higher-efficiency filtration.