As filters continue to increase in efficiency, one parallel fact is that usually the pressure drop over the filter also increases. According to Fan Law 2, as pressure drop increases, the airflow will decrease at twice the percent that the pressure increases. So, be wary of some high efficiency filters. And learn to install filters that your fans can handle.

One practical standard taught at NCI is that in a gas furnace with a constant speed fan and an air conditioning coil, an ideal pressure drop through a filter is about 20% of the fan rated total external static pressure. For example, if a furnace has a fan rated for .50-in. of total external static pressure, ideally, the pressure drop over the filter would not exceed .10-in.

Although a single filter with a pressure drop of less than .10-in. doesn’t do much filtering, it leaves adequate pressure to allow the rest of the components in the system to have their fair share of the static pressure budget. In an ideal system, 40% of the pressure would be allocated to an external (or optional) cooling coil, and the remaining 40% would go to the supply and return duct systems.

However, when determining the available pressure drop of a filter in a heat pump air handler, the coil pressure drop is included with the equipment, and the need to reserve 40% of the static pressure for an external coil is not necessary. Because of that, the practical standard for filter pressure drop may be changed to up to 50% of rated fan static pressure.

In other words, if you’re sizing a filter in an air hander without an external coil and the fan is rated at .50-in., then 50% of the available pressure can be budgeted for the filter. In this scenario, an ideal filter pressure drop may be .25-in. This pressure drop can handle many of the pleated, cartridge, or packaged filters offered by several manufacturers today.

There are exceptions to every practical standard. You may spend 75% of your pressure drop on a filter, as long as the total external static pressure remains at or below the rated air handler pressure. If you’re able to measure delivered airflow in and out of a system, who cares what the static pressure is? (within reason, of course)

The best way to decrease filter resistance is to increase the filter surface area. This can be done by fabricating filter racks which allows you to install twice the amount of filters in a “V” configuration instead of a single filter installed at 90 degrees. Another method is to install return air filter grilles. This allows for generous filter surface areas, often as much as 4 or 5 times the surface areas as installing a single filter in the inlet of the air handler.

Bottom Line, measure the system total external static pressure and filter pressure drop of every system that you install or service. It takes less than five minutes and provides essential system performance information that you can’t afford not to know.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a free procedure showing you how to measure filter pressure drop, contact Doc at robf@ncihvac.com or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, technical articles and downloads.