System Effects of High Efficiency Filters in Homes

Occupant concern about indoor air quality (IAQ) issues has led to the increased use of more effective air filters in residential heating and cooling systems. A drawback of improved filtration is that better filters tend to have more flow resistance. This can lead to lower system airflows that reduce heat exchanger efficiency, increase duct pressure that leads to increased air leakage for ducts and, in some case s, increased blower power consumption. There is currently little knowledge on the magnitude of these effects. In this study, the performance of ten central forced air systems was monitored for a year. The systems used either a Permanent Split Capacitor (PSC) or a Brushless Permanent Magnet (BPM) blower. Each system was operated with a range of filter efficiencies ranging from MERV 6 (the lowest currently permitted in ASHRAE Standard 62.2) up to MERV 16. Measurements were recorded every ten seconds for blower power, filter pressure drop, supply and return plenum pressures together with plenum and indoor temperatures. These detailed continuous measurements allowed observation of filter loading effects as well as the initial change in system performance when filters were swapped. The results of the field measurements were used in simulations to examine more general system performance effects for a wider range of climates. The field tests showed that system static pressures were highly influenced by filter selection, filter loading rates varied more from house to house than by MERV rating and overall were quite low in many of the homes. PSC motors showed reduced power and airflow as the filters loaded, but BPM motors attempted to maintain a constant airflow and increased their power to do so. The combined field test and simulation results from this study indicate that for MERV 10-13 filters the effects on energy use are small (<1%) over a wide range of performance conditions and climates. However, using higher efficiency MERV 16 filters leads to problems in terms of potential for significantly increased energy use (>5%) and usability. In systems using low MERV filters that are already close to blower performance limits the addition of a MERV 16 filter pushed the blowers to their performance limits.