An Examination of Electrostatic/ElectronicFilter Upgrades

Airflow, pressure drop are key considerations

An HVAC system air filter upgrade is one way contractors can simultaneously increase customers’ residential indoor air quality (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 will 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.

A comprehensive assessment must include ventilation and purification. The central air conditioning/heating system is the place to begin any residential IAQ improvement, yet, the fiber filter supplied with most residential HVAC systems has a typical Minimum Efficiency Reporting Value (MERV) rating of 1-4. Low quality filters leave plenty of room for improvement, because they’re essentially useless (capturing less than 20% of the contaminates in the 3-10 micron range). Consequently, the evaporator coil itself functions as a filter, and inevitably becomes 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.

A higher MERV rating improves filtration effectiveness by trapping more particles by virtue of a tighter weave (such as a pleated filter). This is an acceptable solution if the system blower can tolerate the increase static head on the overall air circulation system.

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

A hand-held anemometer can measure the effect the filter has on the air flow into the structure. Fan speed may be modified to reduce this effect at the cost of increased noise. In general, however (and neglecting the effect of the latent cooling), a reduction in air flow results in a proportionate 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% less efficient for every 10F colder the evaporator must run. Therefore, using a filter with too tight a weave (too much pressure drop) can decrease air conditioning 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, because 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’s 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 its MERV 1-4 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, but without a pressure drop requires an alternative approach such as electrostatic and electronic filtration.

Electrostatic Methods
Electrostatic technology is one strategy that traps more particulates with minimal airflow effect. Friction creates an electrostatic charge as the conditioned air flows through a filter. If an electrostatic filter surface exists, these electrical charges can build up on the filter rather than dissipate. The electrically charged filter then attracts the 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 cost approximately $20. Typically they aren’t 100% electrostatic, because they’re partially composed of electrostatic fibers. The more electrostatic fibers a filter has, the more effectiveness. These electrostatic filters can be washable, or disposable such as the 3M Filtrete 300 disposable filter. However, it’s difficult to effectively wash reusable electrostatic filters. Normally the filter manufacturers don’t disclose the filter’s quantity of electrostatic fibers, but they typically provide the effective MERV rating.

The other electrostatic filter 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 unique 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% filtration efficiency improvement for three and seven 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 nine micrometer particles, an electrostatic spray such as PuraClean, increased a conventional media filter from 36 to 92% (improvement of a MERV 6 filter to a MERV 11 filter), according to independent tests by Research 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’s 52.2 and the earlier 52.1 Standard for determining the performance of air filters.

While PuraClean uses 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 that captured dirt doesn’t fall off the filter and back into the airstream. It doesn’t improve filtration, it only improves captured particulate retention. Two examples of tackifier only sprays are Filter Plus™ and Filter Charger.

Electronic Methods
The electronic filter uses 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 rather a high-voltage electric charge created by an electronic high-voltage “transformer” drawing the power from an input source. The power consumption is minimal. Electronic filters use a high voltage electrode arrangement which charges the dirt particles that are then attracted to a grounded surface typically referred to as a ground plane, collection plate or ground electrode.

This strategy filters very well, however if not periodically cleaned and maintained, the collection device becomes coated with an insulating layer of dirt particles. Therefore conductivity and effectiveness decreases, allowing the charged dirt particles to pass through the HVAC system and attach to register grilles, walls and ceilings. This could lead to customer complaints.

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

Marketing Electrostatic Filters
It costs upward of $750 for the equipment and installation costs of add-on electronic filters, making them marketable only to the upper-middle or high end market.

Disposable filters with some amount of electrostatic fibers are a more affordable option for the middle and low-end markets, even though they can cost upwards of $20 or more per filter, and may be difficult to find in odd sizes.

A bottle of electrostatic spray with tackifier capable of treating up to 12 filters has a wholesale trade price near $10 and can easily be marked up to $25 or $35 retail, creating an extra $15 to $25 profit margin on service invoices. Since one bottle will treat filters for up to one year, it is a perfect add on to a pre-season tune up.

A great marketing tool for selling the electrostatic spray is treating one-half of a filter and letting a customer see the difference after two or three weeks of operation. The treated section will have visibly attracted more dust than the untreated section. A YouTube ( video demonstration under the search heading of “puraclean filter spray” can be shared with customers.

Whatever method of advanced filter technology is used, a contractor who properly applies these principles can be confident the customer’s IAQ has been improved without sacrificing system efficiency and can therefore add yet another IAQ element to their services.

What are MERV Ratings?
The only true 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 3.0 microns to 10.0 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.

  • A MERV rating of 6 means the filter is 35% to 50% minimum efficient at capturing the measured particles. ? A MERV rating of 8 means the filter is 70% to 85% minimum efficient at capturing the measured particles.
  • A MERV rating of 11 means the filter is 85% to 95% minimum efficient at capturing the measured particles.
  • MERV is an industry standard rating, so it can be used to compare filters made by different companies.

Note: Information was taken from the free online manual Indoor Air Quality and Mold Remediation Service Techniques—A Desktop Reference and Training Guide for IAQ and Mold Remediation by Robert P. Scaringe, and is available free at

Robert Scaringe is president of Mainstream Engineering (, Rockledge, FL. He’s been involved in the development of advanced vapor-compression heat pump systems for more than 33 years. He currently holds more than 70 HVAC-related patents and has written for more than one hundred technical publications.

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