The air filters we use in our heating and cooling systems are becoming more and more efficient every year. Filter marketing brochures compete to raise our awareness of the legions of dreaded unseen airborne enemies that threaten the peace and security of our homes. But that’s only a part of the story.

Because of the downside of this so-called progress, I occasionally succumb to the temptation to visit the filter aisle at the local Home Deport and hunt for this month’s champion air killer filter. I also have to admit offering some impromptu training sessions for homeowners who almost chose extremely restrictive filters.

In reality, most air handlers with a fan rated at 0.50-in. of maximum total external static pressure can barely afford the fiberglass or hog hair 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/ton. When airflow is substantially decreased below 400 cfm/ton, Btu delivery and system efficiency both decrease at a similar rate. In fact, a “high efficiency” filter can turn a 16 SEER system into a 9 SEER system.

So, how many systems have you trashed lately by upgrading the filter in the name of improved indoor air quality (IAQ)? Remember, the first two principles of IAQ 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 IAQ? The only way to answer that question is one system at a time.

The good news is that is 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. Be aware, however, as there are others that 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 Test Pressure Drop

Here’s a summary of the five-minute test:

1. Simply drill a 3/8-in. test hole into the ducting or equipment on either side of the filter. Look carefully before you drill, as the last thing you want to hear is the 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

I could write a book on this one. But to get you started, use this rule of thumb: 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 0.50-in., multiply by 20% (or .20) to identify the filter pressure drop for that fan. In this case, you’ll find that it shouldn’t exceed .10-in. That’s why I said earlier that fiberglass or hog hair filters are about all many fans can afford.
If you choose a god quality variable speed fan rated at 1.2-in. of total external static pressure, you fan can afford 0.24-in. of pressure drop over the filter. Get the picture? If the filter pressure drop causes the system total external static pressure to exceed the maximum, airflow plummets and so does the Btu output and performance of your system.

If the pressure drop exceeds 20%, you’ll have to make some changes in the system in order for it to work properly.
Try checking the filter pressure drop on your system at home of at the office and see what you find. If this is new for you, I wish you a pleasant journey as you add system performance measurement to your mechanical repairs and system evaluations.

What to Do About It

I guarantee you’ll uncover a filter problem in the first three filters you test. So, what are your options for a solution? Consider some of the following.

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 to add another return duct to the system and install 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.

Most systems could use another return duct to relieve high system static pressure anyway. Offering this solution will test your commitment to doing the right thing. Be sure to sell the additional filter as a system upgrade, not as an apology or a fix to last season’s mistake.

The best solution to this problem is to admit you didn’t know a single high efficiency filter cause so much restriction, then teach your customers exactly what you have learned. Take them to the filter and show them how you measure pressure drop, and explain how to interpret the readings.

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.

By now, some of you have also figured out this same test can be applied to cooling coils. The rule of thumb for wet coils is that their pressure drop should not exceed 40% of the fan’s rated total external static pressures.

As usual, we have prepared more information for you, if you could use it. There’s rarely enough room in an article or enough time for me to write on Saturday mornings to tell the whole story. If you’ve been reading over the years you know all the elements of system performance are interconnected and it’s impossible to address one measurement without touching on others. Please e-mail or call and I’d be happy to send you a technical report and procedure on how to measure pressure drops in a system.

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

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring and improving HVAC system performance. If you're an HVAC contractor or technician interested in a no cost one page test report and procedure on how to measure duct loss, contact Doc at robf@nationalcomfortinstitute.com or call him at 800-633-7058.