Metals can corrode whenever they are in contact with substances that have acidic or basic (alkaline) properties. Acidic substances have high concentrations of hydrogen ions, and basic substances have high concentrations of hydroxide ions.
We can measure a substance’s acidity or alkalinity on the pH scale; acids are between 0 and 6 on the pH scale (think lemon juice and coffee), and bases are between 8 and 14 (think soap and bleach). Water, H2O, is in the middle of the scale at 7 and is able to behave as an acid or a base.
As such, water can interact with other molecules in the environment to make acids that corrode your A/C coils, much like when you leave a cheap nail outside during a rainstorm and find it covered in rust a few days later. The main problem here is that the coils carry refrigerant, which is what moves heat around. When those coils corrode, they may eventually leak refrigerant and make HVAC systems unable to keep us as comfortable as they should.
Acids Are the Silent Saboteurs
Acids don’t just show up out of nowhere; they’re formed from the very environment in which your HVAC systems operate. Common acids include the following:
- Carbonic acid — when carbon dioxide interacts with water
- Chlorides — from salts, building materials, and cleaning products
- Acetic and formic acid — from off-gassing furniture, paint, and cleaning supplies
- Nitric acid — from some combustion appliances like furnaces
Even weak acids can cause harm by making the coil’s surface more vulnerable to stronger acids down the line.
Preventing Acid-Induced Corrosion
The best way to fight acids is to stop them before they can do damage. The three main facets of indoor air quality—filtration, ventilation, and humidity control—actually also help prevent acid formation.
Four-inch MERV 13 or greater media filters are great at catching pollutants, even volatile organic compounds (VOCs) that off-gas from furniture and paints. That’s because these filters have a high surface area. True activated carbon filters are also good because they adsorb pollutants like VOCs; the VOCs stick right to the surface.
Introducing fresh air via a controlled pathway (such as through a ventilating dehumidifier) dilutes concentrations of carbon dioxide (which we and our pets exhale) and nitric oxides, both of which may lead to acid formation. Humidity control is another key aspect, and the goal is to get indoor relative below 60% to minimize the likelihood of dew forming on surfaces. Controlling moisture helps us keep water molecules from interacting with VOCs and carbon dioxide. That’s where dehumidifiers shine.
Avoid using bleach for DIY drain cleaning, as it’s an alkaline substance that can corrode copper. Instead, you can use water and a shop vacuum for cleaning drains. To prevent bacterial growth without the adverse effects of bleach, you can pour a cup or two of white vinegar down your drain monthly.
If you notice corrosion on your coil, it may not be a cause for concern. While reddish corrosion could look concerning, that’s galvanic corrosion. It may lead to leaks over time, but most leaks happen because of formicary corrosion, usually caused by acetic and formic acids. These look like ant nests. Preventing acid formation with good filtration, ventilation, and humidity control is the best thing you can do—and keep an eye on your HVAC system’s performance if you do notice corrosion.
If you’ve noticed that your A/C or heat pump isn’t getting you cool or warm, Kalos can send one of our techs out to help. Call or text us anytime at (352)-243-7099, and we will schedule a time for one of our technicians to visit and get to the bottom of your HVAC system’s problem.
