The filter on the right was treated with PuraClean, an electrostatic coating filter spray originally developed for NASA to keep spacecraft ventilation systems cleaner.

There is no doubt that consumers want healthier homes today. An HVAC system air filter upgrade is one way contractors can simultaneously increase customers’ residential IAQ and generate more business.

The central air conditioning/heating unit is the perfect appliance for providing cleaner IAQ, but few consumers realize it. Many consumers buy an electric-powered room air cleaner, which is as logical as using a window air conditioner in a central air conditioned home. Therefore it’s the contractor’s responsibility to educate consumers and suggest methods to improve IAQ. For any comprehensive approach, this must include ventilation and purification.

Despite the fact that the central air conditioning/heating system is the place to begin any residential IAQ improvement, the fiber filter supplied with most residential HVAC systems has a typical Minimum Efficiency Reporting Value (MERV) rate of 1-4. These low quality filters leave plenty of room for improvement, because they capture less than 20 percent of the contaminates in the 3-10 micron range. Consequently, the evaporator coil functions as a filter itself and inevitably becomes excessively clogged. Additionally, these filters do nothing against smaller, respirable particles from contaminants such as smoke and pollen, which can lead to problems for families with allergies or asthma.


The first and most logical choice is to move the customer to a better filter with a higher MERV rating. A higher MERV improves the filtration effectiveness by trapping more particles via the brute force of a tighter weave (such as a pleated filter). This is an acceptable solution if the system blower can tolerate the increased static head on the overall air circulation system. Unfortunately, there is no simple answer to determine whether the existing system can handle the additional pressure drop of the improved filtration since the total pressure drop of the indoor coil, air distribution ductwork (both supply and return), registers, and the filter must be accounted for and accommodated by the system blower.

If available, a hand-held anemometer can measure the effect the filter has on the airflow into the structure. Fan speed may be modified to minimize this effect at the cost of increased noise. In general, however (and neglecting the effect of the latent cooling), a reduction in airflow results in a proportional increase in the temperature difference required across the coils.

If the pressure drop caused by the filter is significant, then the airflow rate over the evaporator coil is reduced, due to the fan’s head-flow curve. Head (or pressure drop) x Flow (or air volume) is equal to a constant, which in this instance is the power of the blower. Therefore, if the pressure drop across the filter goes up, the flow rate across the evaporator goes down, thus the evaporator has to produce colder temperatures to provide the same cooling capacity.

Sensible cooling = (mass flow rate of air) x (heat capacity) x (temperature drop across the coils)

If the flow rate is lowered, the temperature drop must be increased to maintain the same cooling capacity. Generally, the system is 10 percent less efficient for every 10° colder the evaporator must run. Therefore, using a filter with too tight a weave (too much pressure drop) can decrease a/c efficiency.

Incidentally, dirty filters actually perform better than clean filters because particles accumulate into areas that were once open spaces. Unfortunately, dirty filters function similarly to filters with a tighter weave; the increased pressure drop of a dirty filter increases operating costs, and in very extreme cases, could cause evaporator icing.

Without knowing the specifics of any system, however, it is safe to assume that moving from a MERV 1-4 filter to a tighter-weave MERV 6 rating should not impose a significant problem. This is true especially when considering that there isn’t much difference between an excessively dirty evaporator - due to the poor performance of the less-efficient filter - and the airflow resistance of a MERV 6 filter. A move to the even tighter weave of a MERV 12 filter, however, might not be tolerated by the system, without impacting efficiency.

To improve filtration above a MERV 6 rating without a pressure drop requires alternative approaches. Two potential options are electrostatic and electronic filters.


Electrostatic filters trap more particulates with the use of friction, which creates an electrostatic charge as conditioned air flows through the filter. These electrical charges can build up on an electrostatic filter surface rather than dissipate. The electrically charged filter then attracts dirt particles via electro-forces, in addition to the mechanical filtration, and removes them from the recirculated airstream.

Electrostatic filters that are drop-in replacements for conventional fiber media filters aren’t 100 percent electrostatic. They are partially composed of electrostatic fibers. The more electrostatic fibers a filter has, the more effective it is. These electrostatic filters can be washable or disposable. However, it can be difficult to effectively wash reusable electrostatic filters. Filter manufacturers normally don’t disclose the filter’s quantity of electrostatic fibers, but they do typically provide the effective MERV rating.

The other approach is to create an electrostatic filter by coating a conventional inexpensive metal, disposable fiber, or reusable filters with an electrostatic coating such as PuraClean® filter spray, a patented product originally developed for NASA to keep spacecraft ventilation systems cleaner.

Independent testing has shown that electrostatic filter sprays provide a 200 and 1,200 percent filtration efficiency improvement for 3- and 7-micron particles, respectively.

Like electrostatic filters, electrostatic sprays make conventional fiber filters more efficient and raise the MERV rating without decreasing airflow.

For example, with 9-micrometer particles, an electrostatic spray increased a conventional media filter from 36 to 92 percent (like upgrading from a MERV 6 filter to a MERV 11), according to independent tests by Re- search Triangle Institute (RTI), Research Triangle Park, N.C., RTI is one of several approved labs to conduct the “Gravimetric and Dust Spot Procedures for Testing Air-Cleaning Devices Used in General Ventilation for Removing Particulate Matter,” as outlined in ASHRAE Standard 52.2, and the earlier 52.1 standard, for determining the performance of air filters.

While sprays like PuraClean use both the electrostatic strategy and a “tackifier,” there are also filter sprays on the market that are only tackifiers. A tackifier makes an ordinary filter sticky (tacky), so captured dirt doesn’t fall off the filter and back into the airstream. It does not improve filtration, it only improves the retention of the captured dirt.


Electronic filters use the same principle of electrically charging dirt particles passing through a filter. In the case of an electronic filter, however, the charge is not built by static electricity (electrostatic) charge, but by a rather high-voltage electric charge created by an electronic high-voltage “transformer” that draws power from an input source. Therefore, the electronic filter must be connected to a power source, although its power consumption is minimal.

Electronic filters use a high-voltage, electrode arrangement that charges the dirt particles, which are then attracted to a grounded surface (typically referred to as a ground plane, collection plate, or ground electrode).

This filters very well, but if it’s not cleaned periodically and maintained, the collection device becomes coated with an insulating layer of dirt particles. Therefore, conductivity and effectiveness decrease, allowing charged dirt particles to pass through the HVAC system. Instead, they attach to register grilles, walls, and ceilings, which could lead to customer complaints.

Also, the charging of the dirt particles by the high-voltage electrode can cause ozone formulation. However, most manufacturers claim the levels are very small and should not increase system corrosion or lead to increased allergic or asthma problems. However, customers should be educated by the contractor on these devices’ potential maintenance challenges and/or medical implications.

The products discussed here can have a broad range of price points. Whatever method of advanced filter technology is employed, contractors who properly apply these principles can be confident that the customer’s IAQ has been improved without sacrificing system efficiency, and can therefore add another IAQ element to their services.

Sidebar: What's a MERV?

The measure of a filter’s effectiveness is the Minimum Efficiency Reporting Value (MERV). Most filters are labeled with a MERV rating number, which measures a filter’s ability to trap particles ranging in size from 3 to 10 microns.

Residential filters commonly have MERV ratings of 1-12. The higher the MERV rating, the more efficient the filter is, and the more particles it can filter. MERV is an industry standard rating, so it can be used to compare filters made by different companies. Below is an explanation of what a few of the MERV rating numbers mean:

• A MERV rating of 6 means the filter is 35-50 percent minimally efficient at capturing the measured particles.

• A MERV rating of 8 means the filter is 70-85 percent minimally efficient at capturing the measured particles.

• A MERV rating of 11 means the filter is 85-95 percent minimally efficient at capturing the measured particles.

The information was taken from the free online manualIndoor Air Quality and Mold Remediation Service Techniques - A Desktop Reference and Training Guide for IAQ and Mold Remediation, by Robert P. Scaringe; it is available at

Publication date:07/19/2010