This is the second in a series of HotMail articles by ‘Doc’ Falke, addressing specific repairs that substantially improve HVAC system performance.

As filter efficiencies increase, their resistance to airflow also increases. Measuring the pressure drop over the filters in the systems you service, sell, and install enables you to know if resistance to airflow is acceptable or excessive. When filter pressure drop is excessive, fan airflow decreases and system performance takes a dive. Let’s take a look at how to find and fix this problem, and verify your filter system meets the design intent and industry standards.

Why High Efficiency Filters Are a Problem

Due to effective marketing from filter manufacturers aimed at promoting high priced, high efficiency filters, inexpensive disposable filters are now often looked on as inadequate and unable to provide adequate filtration to keep the equipment clean. In reality, the majority of the fans used in residential HVAC systems today can only afford a low efficiency filter due to the resistance to airflow that higher efficient filters cause.

Also, minimum efficiency filters will keep the equipment clean, but will not stop other allergens that some families may require. If needed, you can use high efficiency filters, but filter surface area will need to be increased. We’ll discuss that later in the article.

Also, many “Green Filters” aren’t green at all. While they may clean the air better, if a single filter is used, most will decrease airflow below the specified amount. If using a variable speed fan, it may increase the fan watt draw as much as three times what the manufacturer specifies. Both these actions can seriously decrease operating system efficiency.

Each system fan is rated at a maximum total external static pressure. When the operating system’s total external static pressure exceeds the rated fan pressure, that fan often cannot move the required airflow to allow the system to operate as designed.

The way to prevent excessive filter pressure drop is to reduce excessive filter pressure through system design, or to measure the filter pressure drop on an existing system in the field.

How to Measure Filter Pressure Drop

Filter pressure drop can be measured in a minute or two if you have the right test instrument, accessories, and a drill. The right test instrument is a low pressure manometer with two pressure ports. If using an analog gauge, such as a Magnehelic® Gauge, use a 0-in. to 1-in. w.c model. If using a digital manometer, use a 0-in. to 5-in. w.c meter. You’ll also need a static pressure tip like a Dwyer A-101, and tubing to attach the manometer to the tip. You’ll need a drill and a 3/8-in. drill bit, preferably with a sheath (a 4-in. x 1-in. dowel with a ½-in. hole drilled though its center) to stop the drill bit once it has made the test hole. That prevents it from damaging anything inside the duct or equipment.

Drill two test holes in the duct or equipment: one on either side of the filter. Take and record a pressure reading on each side of the filter. Subtract the smaller pressure from the larger pressure. This is the filter pressure drop. Another way is to use two hoses with static pressure tips, and the manometer will subtract the two pressures for you. For a complete pressure drop procedure, see the contact information at the end of the article.

Compare the filter pressure drop to the pressure budget (see below). If pressure is higher than budget at the required system airflow, repairs or changes in the system are needed to bring the filter pressure drop within budget.

What Should Filter Pressure Drop Be?

In a gas furnace with a remote or external cooling coil, the filter pressure budget is 20% of fan rated pressure. For example, if this furnace maximum pressure rating is .50-in. w.c. (which is a majority of the systems in the country) the filter pressure budget is 20%. So, .50-in. w.c. x 20% = .10-in. w.c. You may say that’s a really low filter pressure drop, and you would be correct. Typically, a filter that would have this pressure drop would be a 1-in. fiberglass or a basic manufacturer provided filter. While you may want a more efficient filter, this type of filter is all this fan can afford.

Or, if a more efficient filter is needed, you’ll need to add additional surface area to reduce pressure drop.

If you’re interested in filter pressure drop budgets for other types of equipment see the contact information at the end of the article.

How to Reduce Filter Pressure Drop

The easiest way to reduce filter pressure drop is to install a new filter that has a lower pressure drop. Often this requires replacing a pleated filter with a less restrictive disposable fiberglass filter. Remember, the choice is between increased filtration and improving the operating efficiency of the system.

If the filter is a permanent filter, and it’s dirty, it can be cleaned to reduce pressure drop.

If your customer needs a high efficiency filter due to preference or health conditions and you need to reduce the filter pressure drop, you can reduce the pressure drop by increasing filter surface area by adding additional filters to the system.

This can be done by installing two filters in the return duct near the equipment in “V” configuration. It can also be done by installing additional filters throughout the system in return air filter grilles. Some contractors regularly install commercial style filter housings that hold four to six filters. This can reduce filter pressure drop by as much as 80% while using super high efficiency pleated filters.

When extreme filtration is required, some contractors have added return air fans between HEPA filter systems and the inlet of the equipment. When doing this, normally a belt drive return fan is used so pressures can be adjusted to assure a negative pressure is present when air enters the system fan.

There are also fan powered HEPA filter systems available that come complete with their own fan and are installed in a bypass duct so the filter exerts no pressure drop on the system.

One way to qualify your filter vendor is to inquire about their knowledge of filter pressure drops and learn about their ability to help you match the filters you design and install to the system fan. This certainly will differentiate the professionals from the filter floggers.

Verification

In reality, the only way to verify if a filter meets design is to measure its performance at verified maximum system airflow under live conditions. Measure both the filter operating pressure drop and compare it to the NCI filter budgets. Then measure operating total external static pressure and compare it to the rated maximum total external static pressure. When you measure, you know. When you don’t, you’re only guessing and hoping. Move beyond guessing and hoping and know for sure that your systems’ perform well.

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company and membership organization. If you're an HVAC contractor or technician interested in a free NCI Pressure Budget Table and a Pressure Drop Test Procedure, contact Doc at robf@ncihvac.com or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.