The role of the central heating and cooling system continues to expand as homeowners look to improve the comfort, health and safety of their indoor environment. Besides conditioning the air, furnaces and AC units are expected to control humidity and remove harmful airborne contaminants.
Filters were originally added to HVAC system design in order to protect the evaporator coil and other critical components from the dirt and grime that degrades refrigeration cycle efficiency. Fiberglass filters trap relatively large particles, but they are ineffective at capturing the microscopic particulate matter that negatively impacts allergies, asthma and other chronic respiratory ailments.
Over the last decade, advanced construction techniques have dramatically improved the energy efficiency of modern buildings. Unfortunately, tight perimeters eliminate natural fresh air pathways, and pollutants accumulate in the living area with no means of escape. To address these problems, manufacturers have developed high-efficiency filters capable of trapping ultra-fine particles including tobacco smoke, bacteria and certain chemical fumes. While the improvements in filter capture ability are welcome, many homeowners do not understand the adverse impact that high-efficiency filters may have on airflow.
Static Pressure and Resistance
Static pressure is the total amount of resistance that moving air encounters as it travels through your HVAC system. It is measured in column inches of water, and a higher static pressure requires more energy to deliver the same quantity of air without a reduction in velocity. As the static pressure rises, the efficiency of the blower fan declines. If the static pressure ultimately exceeds the capacity of the fan, the air will stop moving altogether.
The amount of available static pressure from the blower is critical for ensuring that your HVAC system operates properly. Older systems were often purposely oversized since system design formulas were extremely inaccurate. While oversized equipment leads to short cycling, loss of comfort and higher utility costs, the one minor benefit was that the blower had excess capacity.
However, professional HVAC contractors now use ACCA Manual J to design systems, and they are able to match equipment capacity to precisely meet the indoor load. This provides exceptional efficiency and helps lower installation costs, but there is usually very little excess static pressure, especially in production housing.
The Problem with High-Efficiency Filters
Since the size of the blower dictates the volume of air that travels through the system, the total resistance must never exceed the available static pressure. Tract homes are often designed with return air grilles that are at the lowest end of air flow capacity, usually due to cost constraints. In general, most residential systems are designed with an available static pressure of .5 inches, which is barely enough to accommodate the evaporator coil, air duct components and a low-resistance fiberglass filter.
When the homeowner replaces the inefficient filter with a high-efficiency model, the system may not have the capacity to handle the added resistance. If the fan can’t move the required volume of air, comfort is compromised, and the unit must work longer and harder to satisfy the indoor load. This can raise utility costs and reduce the life of the equipment.
The situation is compounded when the filter is loaded with dirt and other debris. Entry level furnaces and air handlers use permanent split capacitor (PSC) motors that only have a single speed. While higher priced variable speed motors can adjust to overcome increased resistance, the efficiency of a PSC motor falls substantially as filter resistance increases. In some cases, reduced airflow can lead to more serious problems including coil freezing. When left unaddressed, a frozen coil can cause permanent damage to the equipment. Indoor air quality is also compromised since moving air will look for ways to travel around the filter, which allows contaminants to be drawn directly into the living area.
Installing a New Filter
Homeowners are advised to install filters rated with an air speed velocity of less than 300 feet per minute. Filters with a higher face velocity will allow particulate matter to pass through the medium instead of trapping it.
High-efficiency filters offer significant indoor air quality benefits, but it is important to determine the resistance of the filter and the available static pressure in the system before installing a model with a higher MERV rating. In most instances, consulting an HVAC professional before making the change is recommended.