If the filter has never, or rarely, been changed, then the evaporator ends up acting as the filter, getting clogged up with lint, dirt, and other particulates. (Courtesy of Jeffrey Siegel and Lawrence Berkley National Labratory.)
The evaporator coil, or indoor coil, is often forgotten. It sits in an attic or some other unobtrusive place, cooling all the air in a building. And quietly it attracts mold, dirt, and other contaminants. This build-up accumulates gradually, but over time, a dirty coil can reduce airflow drastically.

Some say that for each 10% reduction in airflow, the efficiency of the equipment drops about 5%. When the airflow is reduced, that means less cooling is taking place, and building occupants may become uncomfortable. Then they start to complain.

It is recommended that evaporator coils be cleaned once a year (sometimes more, depending on the application), especially if the customer does not change the filter regularly. However, most coils are in hard-to-reach places, making it difficult for service technicians to access them, let alone clean them. It can also take a lot of time to clean a coil, which means it can be a costly procedure. That’s definitely not attractive to customers.

But a clean evaporator (and condenser) is going to increase the life and efficiency of the equipment probably more than just about any other procedure.

Convincing Customers

One of the main reasons evaporators often don’t get cleaned is that the homeowner doesn’t see them. Unlike the condensing coil, which sits outside in view, the evaporator is usually hidden inside the home.

“It’s an abstract to most customers. There’s feedback on the condenser coils outside, because the technician can get them shiny and clean, and it’s something the customer can see,” says William Dyle, general manager, Highside Chemicals, Gulfport, MS.

Unfortunately, it can be difficult for a contractor or technician to convince customers that they need to clean their coils. That’s why it’s important to educate customers about the reduced efficiency, wasted energy, and potential indoor air quality problems that can occur with a dirty evaporator.

Another point customers need to be told again and again is the need to change the filter. That in itself is one of the best ways to prevent a heavy build-up on an evaporator. If the filter has never or rarely been changed, then the evaporator ends up acting as the filter, getting clogged up with lint, dirt, and other particulates. “I have seen some evaporator coils that you could peel [the build-up] off like felt, it was so thick. I was surprised that any air was getting through them at all,” says Dyle.

According to Jeffrey Siegel, a graduate student researcher at Lawrence Berkeley National Laboratory, “Cleaning is not a solution to the problem of fouling. This is not an ideal approach because it has the potential to release very harmful byproducts and other materials into the airstream and because it can be very difficult to see, and therefore clean, the inside of an evaporator coil.”

Siegel notes that a better solution is to maintain filters and eliminate filter bypass, a relatively unstudied but frequent phenomenon where air bypasses the filter because of a poor installation. “In addition, the current focus on bioterrorism further means that we have to look at solutions that involve prevention (e.g., filtration), rather than cleaning after the fact,” he notes.

Cleaning Agents

If customers do not change their filters, however, there may be no other choice but to clean the evaporator. Numerous cleaning agents can be used, depending on the application. However, many contractors use dishwashing detergent, and that often is not the best choice to get a coil clean.

If the application has a lot of environmental tobacco smoke and/or grease, then a regular detergent probably won’t work. It will be necessary to use a heavy alkaline type of coil cleaner to really cut the grease and tar. Alkaline cleaners tend to be very mild and have little to no odor. But if that doesn’t work, there are solvents, enzyme cleaners, regular detergents, and acid-based cleaners.

There are some drawbacks to just about any type of cleaner. Enzyme cleaners work well, but they often take a long time to do the job. Acid bases are rather aggressive, and therefore aren’t usually recommended for evaporator coils. In addition, the fumes from an acid cleaner can cause some irritation to the person applying it to the coil.

Another issue that needs to be considered when getting ready to clean a coil is that the evaporator is cold. “Most cleaners don’t work at their optimum cleaning power at cold temperatures. It’s better to have warm or hot,” says Dyle.

Dyle says that his company has three different types of cleaners. First is the traditional liquid coil cleaner that comes in a concentrate. With this, technicians dilute the cleaner and spray it on with a normal pressure sprayer, then rinse thoroughly with water. If the coil is not very dirty, it may not even be necessary to rinse off the cleaner; the evaporator condensate will take care of it.

Second is a powder cleaner that needs to be dissolved in 1 gal of water, and that effectively yields 1 gal of concentrate. The main advantage of the powder is that it’s much lighter. In addition, it is a lot less dangerous as far as spills go, and its shelf life is much longer.

Third is the aerosol cleaners. These are usually available in 20-oz cans, and they’re solvent or alkaline based. These aerosols are sprayed on, and if the system is working well, the condensate on the evaporator coil will provide a thorough-enough rinse job.

No matter which type of cleaner you use, Dyle stresses the importance of cleaning in the right direction so the coil doesn’t become packed with dirt. “Clean it so that you’re pushing the materials out of the coil the same way they came in. In other words, clean from the air out, not back. You don’t want to push the material further into the fins, you want to push it back out.”

How do you know if the coil is really clean? According to the Florida Solar Energy Center, coil cleanliness can be measured via the air pressure drop across the coil. The coil should be cleaned to within 10% of oem specifications. Replacement should be considered if the air pressure drop is 1.5 to 3 times the oem specifications.

Publication date: 11/19/2001