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One common and frustrating thing a technician encounters is the same nasty equipment year after year. This may sound familiar. You perform maintenance each year, but by the next maintenance visit, dirt and grime are in the equipment again. Have you ever wondered why this happens? Should a properly designed and installed system get dirty, or could something else be going on?

HVAC system maintenance is important. However, if conditions exist that speed up the need, there’s likely a filtration failure. Some symptoms are obvious; others are not. Let’s look at three common signs of air filtration failure; what causes them; and how you can test, diagnose, and repair them.

Dirty Blower Assemblies

The most common sign of filtration failure is dirt accumulation on the blower assembly. You probably see it so often you don’t think twice when you clean the blower motor and wheel. Another common location is the secondary heat exchanger on a condensing furnace. It is the first stop for any dirt discharged by the blower into the air stream on a high-efficiency gas furnace.

Results of dirty blower assemblies often show up as airflow and heat transfer issues. A dirty blower wheel significantly reduces fan airflow capacity. It’s estimated that 1/8-inch of dust on the vanes of a forward-curved fan reduces airflow by as much as 30%. Results of a dirty secondary heat exchanger often appear as a high temperature rise across the furnace and excessive flue temperatures on a combustion analyzer.

Nasty Indoor Coils

Another common sign of filtration failure is a nasty indoor coil. One of the toughest parts of a maintenance call is pulling the indoor coil for a visual inspection. These doors usually aren’t in the easiest places to access. In some gas furnace applications, they are nearly impossible to open. If coil cleaning is necessary, the difficulty level and time involved is compounded.

Plugged drain lines are a side effect of nasty indoor coils. However, we often don’t make the connection between the two. Condensate off the indoor coil is distilled water. It should not have dirt in it that changes into slime. Instead of addressing the problem directly, we install various levels of condensate protection and happily blow the gunk out of the drain line each year. What if we could prevent the gunk from getting trapped in the first place?

Nasty indoor coils are a leading cause of water damage, compressor failure, and IAQ (indoor air quality) issues. The longer an indoor coil operates with a filtration failure, the lower the heat transfer ability of the coil. The dirt acts as an insulator and reduces how well refrigerant in the coil absorbs heat from air passing across it. Over time, this leads to floodback, coil freezing, and eventual compressor failure. Any dirt carried into the condensate drain ends up collecting at the lowest point and provides a perfect breeding ground for biological growth. Eventually, the restriction grows bad enough that it results in a service call — or, worse, a collapsed ceiling or flooded basement.

Smoking Heat Strips

As we head into winter, the third sign of filtration failure makes a grand appearance the first time the electric heat strips energize. Homeowners usually discover the problem from the “burning” odor or a smoke alarm going off. This reaction can quickly send a homeowner into panic mode because they believe their house is on fire. Then you get the call for help.

While this problem is short-lived, it is still an issue. Unfortunately, we encounter it so much we forget to consider why it happens. Dirt should not accumulate on heat strips. Think back to how a heat strip package looks when first installed. It’s shiny and clean.

How Do These Filtration Failures Happen?

You might say to yourself, “I see this all the time. Why does it happen, and how do I fix it?” There are a couple of reasons why it happens, and even more ways to fix it. The most complicated problems usually boil down to industry fundamentals that we forget or ignore.

Most HVAC professionals understand that air takes the path(s) of least resistance. It’s the first of five airflow rules you once had memorized but may have forgotten. Filtration failures happen because air follows this rule.

As you diagnose these issues, try to think like air. It’s lazy, so air will always look for the easiest way to get to the blower inlet. A filter is an enormous airflow obstacle. Unless you design your systems so that there is no place else for air to go except through the filter at an acceptable speed, you’re guaranteed one or more of the problems above.

For air to flow, there must be an opening and pressure difference across that opening. In an HVAC system, the blower provides the pressure difference, and the duct system provides the openings. The blower inlet is the highest area of negative pressure in a duct system and will pull air from wherever it can get it. Problems occur when there are unintentional openings between the air filter and blower. Some of these unintended openings are:

• Poorly fitting filter racks and filter doors;
• Unsealed furnace base pans;
• Poorly fitting access panels; and
• Unsealed electrical connections.

Each of these openings allows unfiltered air to bypass the filter media. This effect amplifies when there is excessive air speed through the filter.

Our industry measures this air speed, otherwise known as velocity, in feet per minute (fpm). Most filter manufacturers like to see a filter face velocity of less than 300 fpm. As velocity accelerates above this limit, particulate in the air can be pulled through the filter, reducing its effectiveness. Undersized filter openings and restrictive air filter media are two common causes of this problem.

Finding the Cause

To find the source of these failures, start with a visual inspection and think like air. Yes, I’m asking you to be an airhead. Most of the visual clues are things you see every day. As you read the problems above, you probably thought, “I see that all the time.” If so, that’s great. You know where to begin. Look for ways air would move through an unintended opening and bypass the air filter.

Sometimes the problem isn’t so obvious, such as air velocity issues, and you will need to dig deeper. In these cases, you’ll need equipment information, like the blower speed settings, to measure what’s happening on the airside. A simple pressure drop measurement across the air filter provides a lot of insight into how filtration failure might happen.

Once you have the filter pressure drop measurement, compare it to the manufacturer-rated pressure drop at rated airflow or use National Comfort Institute (NCI) static pressure budgets to estimate how restrictive the filter is.

Part of a Bigger Solution

There are no silver bullets for filtration failures. Unfortunately, no single product will fix this problem. Instead, the solution is part of a bigger process involving proper design, filter selection, and installation. Once these pieces are in place, maintenance helps to keep the equipment in tip-top shape instead of being a Band-Aid to cover up poor design and installation.

It’s possible to have an HVAC system with no filtration failures. However, it isn’t easy or cheap.

SURFACE AREA: Any great air filtration product should have plenty of filtering surface area, a leak free cabinet and rack, and low pressure drop at a high MERV rating. (Courtesy of National Comfort Institute)

The best filtration solutions start with good filter design. The lower the filter face velocity you size for, the better. Next comes proper installation. Be sure the filter installation allows for full use of the filter surface area and there are no leaks between the filter and blower inlet. Filter location also makes a difference. Once you top these characteristics off with the right filter media, filtration issues will greatly reduce.

One air filtration product that recently grabbed my attention as part of the solution to this problem is the Dust Free Sixteen Air Cleaner my friend John Ellis helped design. From what I’ve seen, along with a well-designed and installed system, it is part of a practical solution for filtration failure. I encourage you to check it out once they hit the market.

Regardless of the filtration solutions you offer, make sure they work as promised. As you can see, there are many opportunities to solve these issues and better serve your customers.