As air filters continue to increase in efficiency, one opposite reaction often overlooked is the decrease in system efficiency that they may cause. Let’s take a closer look at how much filter some fans can and cannot afford.

In reality, a majority of residential fans rated are still rated at only .50-in. w.c of maximum total external static pressure. With the pressure drop of today’s coils, these fans can often barely afford the filter the manufacturer includes with the unit. If you substitute a more “efficient” filter, the resistance may exceed the capacity of fan to move 400 CFM per ton. When airflow is substantially decreased below 400 CFM per ton, BTU delivery and system efficiency decreases at a similar rate.

So, can a “high efficiency” filter turn a 16 SEER system into a 9 SEER system? Yes, this poor system performance is verified across the country many times a day. So, how many systems have you trashed lately by upgrading the filter in the name of IAQ improvement? Remember, the first two principles of Indoor Air Quality are temperature and humidity control. Installing the wrong filter can have an immediate negative impact on both.

Could restrictive filters be the number one cause of poor indoor air quality? The only way to answer that question is one system at a time.

The good news is that it takes less than five minutes to test filters and evaluate their effect on the systems you install, service and sell.

To measure the pressure drop over a filter is a very simple test, and the tools needed are not very expensive. This test will ideally, hopefully, maybe someday, become a standard practice in our industry. Once you learn how to take this test, you’ll wonder how you ever looked at a system with out checking filter pressure drop.

First, The Tools

There are several good manometers on the market, but beware, some are poor quality or inappropriate for measuring pressure drop. The right digital gauge should have a range near 1-in. to 5-in. water column. A gauge with a larger range may be inaccurate at low pressures. It should have a positive and a negative pressure port and come with a static pressure tip and pressure test hoses. A good analog gauge is a 0-in. to 1-in. Magnehelic gauge. Including all the accessories you need (carrying case, drill bit, sheath and hole plugs) the kit will cost less than $200 per tech.

How To Measure Pressure Drop

Here’s a brief summary of this five minute test:

1. Simply drill a 3/8-in. test hole into the ducting or equipment either side of the filter. Look before you drill, we all hate that hissing noise a nicked coil can make.

2. Turn on your digital manometer or level and zero your analog gauge.

3. Attach a pressure hose to each pressure port on the manometer and insert a static pressure tip into the opposite end of each hose.

4. Insert the static pressure tips into the holes you drilled on each side of the filter, and face the tips into the airflow.

5. The pressure drop over the filter magically appears on you manometer. Note: If the gauge reads below zero, swap the hose connections on the meter, and the reading will be corrected.

6. Read and record the pressure drop. That’s it.

How To Interpret The Reading

Here’s a rule of thumb to get you started: Ideally, the pressure drop over the filter should not exceed 20% of the rated maximum static pressure of the fan. Here’s an example: If a fan is rated for .50-in. multiply by 20% or .20 to identify the filter pressure drop for than fan should not exceed .10-in. That’s why I said earlier that fiberglass or hog hair filters are about all many fans could afford.

If the pressure drop exceeds 20%, you have to make some changes in the system in order for it to work properly.

What To Do About It

The odds are that once you start testing, you’ll uncover a filter problem in the first three filters you test. Unless you were previously experienced with filter pressure drop measurement. Also, a single filter slipped into the intake of an air handler is not usually the best solution for filtration, so please consider some of these options.

The first response is to throw away the restrictive filter and replace it with a low pressure drop model. This solves the problem and immediately improves system comfort and efficiency. Well done.

But what if you just sold a 4-in. thick pleated filter cartridge and housing last spring? Most fans can only pull a maximum of two tons of air through the larger cartridge type filters. The best solution is adding another return duct to the system and installing a second cartridge filter on the other side of the return plenum or furnace. Divide the return airflow between two filters and the problem is usually solved.

The principle to reducing filter pressure drop is that as you increase the filter surface area, the pressure drop decreases. If the filter manufacturers would share this information with us, they would sell two to three times more filters.

If you double the filter surface area in a system, the pressure drop over the filters will reduce by more than 50%. So, the idea of making a “V” shaped filter rack, or adding several return air filter grilles is the right idea and will usually solve the problems you uncover by measuring filter pressure drop.

Some systems don’t offer an easy fix, but repair is not optional. The fact that high static pressure reduces system performance is not an opinion or a new idea. It’s simply how a system has always responded and will always respond to a restrictive filter that the fan cannot afford.

Remember pressure in a system is invisible. The only way to “see” pressure and address pressure problems in a system is to measure it. As Dominick Guarino says, “If you don’t measure, you’re just guessing.”

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company with technical and business level membership organizations. If you're an HVAC contractor or technician interested in a free report on 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, articles and downloads.