HVAC Filtration

Filters are used in residential heating, ventilation, and air conditioning (HVAC) systems to protect equipment and to remove particulate pollutants that can pose a serious health hazard. There is an increased interest in filtering indoor air to improve air quality as an alternative or compliment to simply providing more air. This interest comes from many sources: health professionals (such as the American Lung Association), regulatory bodies (California Air Resources board) and engineers/designers (ASHRAE Standard 62.2).

While there is no doubt that filtering air can remove particulate pollutants we need to account for other effects. Better filters have more airflow resistance resulting in more fan power and energy required to move air and the potential to degrade heating and cooling system performance by changing airflows over heat exchangers. Filters come in a range of performance and there is a tradeoff between filtration effectiveness and the energy used to move the air through the filter as its airflow resistance increases with increasing filtration effectiveness.

Filters are commonly rated by their Minimum Efficiency Reporting Values (MERV) value, which is based on an ASHRAE Standard Test method to allow comparisons between different filters. The lowest level of filters are about MERV 2 and are the common glass fiber filters we are all familiar with that mostly just protect HVAC equipment from large particles to prevent fan and heat exchanger fouling. The next step is MERV 6 (the minimum required by ASHRAE Standard 62.2) that do a good job of filtering large particles. But to get substantial filtration of particles that we believe are key for human health needs MERV 13 or better.

In this study we performed long-term field monitoring of ten homes where we measured the change in energy use of HVAC fans and heating/cooling equipment with different filters and how performance changed with time as filters loaded. Filters were changed periodically to swap high/low MERV filters in the same system. The results have shown that higher MERV filters do indeed require more energy for HVAC system operation. However, the changes in performance can be quite specific to individual systems depending on their airflow resistance in addition to the filter and where the resulting total system airflow resistance puts the operating point of the system fan. Changes in energy use also depend on the type of fan used in the system.

Lastly, the rates of filter fouling are highly variable – implying that more guidance is necessary here for occupants on acceptable filter lifetimes. The results of this study will be used to inform future potential changes to residential energy codes in California (Title 24) and to the national residential ventilation standard: ASHRAE 62.2.