Acid, alkaline, or neutral cleaners?

May 11, 2000
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There are a number of coil cleaner choices for the technician. In one sense, this is good. It holds the promise of letting you select just the right product for the specific cleaning challenge you face on each job.

However, there are a couple of problems with this abundance of selections.

Making a truly informed choice would require knowledge of soil and surface conditions that may not be available; formulation details that the cleaner manufacturer for competitive reasons may not want to release; and a grasp of chemistry well beyond the scope of most high school chemistry classes.

Moreover, few technicians are willing to carry a wide selection of cleaners to each job. As a result, most try several products and settle on a favorite that they use on most jobs. Some carry a couple of different products that they use for specific applications, such as an acid for condensers and an alkaline for evaporator coils.

Is this approach correct? Is there a better approach, short of becoming a chemical engineer? Some understanding of cleaning technology will at least lead to a more informed choice.

Types of products

The terms “acid,” “alkaline,” and “neutral” refer to positions on a scale of pH ranging from 1 to 14. (Actually, a solution can have a pH below 1 and above 14, but this range is most commonly used.)

A pH of 7 is neutral. Numbers below that are in the acid range, and those above indicate an alkaline solution. We often think of a pH of 7 as being desirable because the fluids of a human body are close to this value. Thus, if we experience contact with a solution either above or below that point, we may find it irritating.

If the pH of a solution that contacts unprotected skin is significantly above or below 7, it can severely damage human tissue. Often, we refer to products that are highly acidic (below pH 3) as corrosive, and those that are highly alkaline (above pH 11) as caustic.

Those are pretty scary words and rightly so; such products are usually treated as hazardous chemicals, requiring special shipping and handling procedures. The labels on such products normally direct the use of gloves, eye protection, and sometimes respirators, for safe use.

Why would someone choose to use products with such a high potential for injury? The answer is simple. They work.

The types of soils we encounter in air conditioning systems can be extremely difficult to remove. Grease- and oil-based soils are responsive to high alkalinity, and are often encountered on evaporator and other internal air handler surfaces. The oils come from human and animal skin flakes, cooking, and other inside sources.

Particulates, smoke, and other environmental soils tend to be responsive to acid solutions. This makes acid-type cleaners especially popular for exterior applications.

In addition, any solution with a pH at either end of the spectrum reacts with metals. While this reaction creates a significant mechanical (bubbling) action that contributes to cleaning effectiveness, it also removes a small layer of the metal the solution reacts with.

These cleaners tend to be relatively inexpensive, because the ingredients that impart both high and low pH are cheap and plentiful. So, it is possible to formulate effective cleaners by using less of the more expensive ingredients.

Chemical concerns

Very high- and low-pH cleaners carry other concerns. The chemical reactions between these cleaners, the soil, and metallic surfaces may produce irritating or toxic fumes. Not only can these harm the technician using them, they can be carried through the air ducts and into conditioned spaces if not used very carefully.

Even where the fumes are not sufficiently concentrated to be an actual danger, their odors can lead to occupant confusion that results in evacuation of a facility, and corresponding loss of productivity and poor publicity. Some persons have claimed injury in the face of noxious odors, even where there is no clinical evidence of harm.

Finally, the minor etching of heat exchanger fin surfaces that these chemicals cause can interfere with the orderly flow of condensate water from the surfaces. The microscopic pits that are etched in the surfaces can hold water and cause it to bead up. Those beads of water can then be carried into the ducts by the airstream.

Such excessive moisture in a duct will result in growth of bacteria or fungi, and an indoor air quality (IAQ) problem can result.

Stay neutral?

With the apparent disadvantages of acid and alkaline cleaners, more neutral cleaners would seem to be an ideal solution. This, however, depends on several factors.

First, few neutral cleaners are really neutral. Formulations with pH anywhere from 5 to 11 have been marketed as “neutral.” This is because, although they are not technically neutral, such formulations may not have the strong reactivity that the very acid or alkaline ones do.

A strictly neutral formulation may not have very good cleaning power. Some reactivity is desirable.

Finally, there are many variations in neutral (or pH-balanced) cleaners. Some use high levels of solvents to cut oily soils. These solvents may give off fumes that are as objectionable as the fumes from corrosive or caustic mixtures, and can also be air quality threats.

The best neutral-type cleaners are detergent based. That means one or more detergent surface-active ingredients are formulated together so that they provide a high level of cleaning activity and very low toxicity.

Some of these products are also formulated with detergents that are readily biodegradable, and thus are not harmful to lakes and rivers into which the rinse water may be carried. Such products are increasingly in demand and may be mandated for use in factories or other commercial locations covered by strict environmental regulations.

Such products are considerably more expensive to manufacture. Often, however, they are highly concentrated and their final-use dilution may not be excessively expensive to use.

Exercise prudence

More often, high-alkaline or high-acid cleaners are just not prudent options. Modern, high-efficiency systems use components that are less tolerant of the extreme formulations used in the past.

The following sticker was found on an air handler from a major equipment manufacturer:

“To avoid damage to plastic pan, do not use strong cleaners (pH 9.5 or greater). Clean the coil with mild detergent and water. Rinse coil and pan thoroughly with clear water.”

The technician who ignores this label and uses an alkaline-, acid-, or solvent-based cleaner is taking a risk. That person may be open to a claim that they caused equipment damage or an IAQ problem.

If it can be proven that they used an improper chemical, they could be liable for replacement of equipment, or worse. Also interesting is the caution to “Rinse ... thoroughly with clear water.” This raises a question about the cleaners that are marketed as “self-rinsing,” which rely on the condensate to remove soil loosened during cleaning. It is doubtful that such an approach can really remove all the soil.

The difficulty of removing heavy soils from inner coil fin surfaces has led some experts to recommend replacement of any coil that has not been maintained carefully since installation, and as a result is contaminated to the point where airflow is restricted below the manufacturer’s specifications.

In the process of testing coil cleaners as part of product development, we cut a lot of coils apart following cleaning. Even after thorough cleaning, coils that appear clean on the surface still have noticeable organic material deposits on fin surfaces. These deposits contain spores that will grow when the coil becomes wet.

Since these deposits are as much as an inch from the coil surface, it is impossible to tell by visual inspection if they are there following cleaning.

We are still investigating the impact that such growths have on IAQ. Many feel they may be a major reason why problems are sometimes so difficult to eliminate. The more we learn about IAQ, the more we become convinced that a proper coil hygiene strategy is vital to any maintenance program.

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