Heating, Ventilation, and Air Conditioning (HVAC) systems are a standard solution to the hot and humid environment of the Southeast United States. HVACs draw heat and humidity back out of the home through refrigerant and condensate drain lines. Given how active these systems can be during the summer, drawing in dust and moisture, it’s no surprise that they can become clogged over time. But what exactly is blocking these lines? And could there possibly be anything living in there?

Algae comprise a large group of aquatic, non-flowering, photosynthetic plants (1). Algae can be as tiny as a single cell (which lump together in lakes or ponds) or as large as seaweed. Despite being plants, they lack roots, stems, leaves or any other organ that would typically be found in their land-dwelling counterparts. Googling “algae” loads images of swathing green films atop lakes and beaches but turn out your HVAC condensate line, and you’ll be greeted with a white slime. Believe it or not, there’s also algae in there.

The “greenness” commonly associated with algae is owed to the chloroplast within its cells. Chloroplasts are responsible for photosynthesis, the process of turning sunlight, carbon dioxide, and water into energy and organic materials; and we call organisms who do this phototrophs (2). However, some algae (like the white slime of a drainpipe) are heterotrophic, that is, they obtain energy and carbon through other means like food (3). The food, in this case, comes from the dust particulates that are drawn into the drain lines. They use this nourishment alongside oxygen (another difference from phototrophs that use carbon dioxide) to grow. Without chloroplast, these algae might lack their vibrant green color, but they can survive in the dark, confined spaces of the condenser line.

There may be an unknowable number of species of algae across the many HVAC systems of the southeastern USA. But this heterotrophic method of acquiring energy is something that would be shared by almost all algae. A single condensate pipe would have thousands of heterotrophs like colorless green algae, freshwater golden algae, and members of the phyla Euglenophyta and Dinoflagellata, which can all be differentiated by slight variations in their shapes and cell structures (4–6).

The commonality of these algae is their capability to grow in the dark and moist environments of HVAC condensate drain lines. And the hot and humid climate of the southeast United States creates the perfect environment for heterotrophic microorganisms to grow. While it might simply look like inorganic muck is clogging up your pipes, it’s truly a plethora of living, breathing organisms oozing out of those lines. Consider that the next time you put off maintaining your HVAC system.

How do I keep algae out of my drain line?

The safest way to keep your air conditioner drain line completely clean and clear of algae is with the use of chlorine bleach. The old method has been to keep a jug of bleach next to your unit and pour a little bit down the drain periodically. The problem is most people don’t remember to do it until it’s too late!

The most reliable way to treat your hvac drain line with bleach is with the use of a chlorinator device that uses 1 inch chlorine tablets.

The HVAC Chlorinator can be purchased at Amazon or Ebay.

References

1. Palmer JD, Soltis DE, Chase MW. The plant tree of life: an overview and some points of view. American Journal of Botany. 2004;91(10):1437–45.

2. Photoautotrophs and Photohetrotrophs [Internet]. 2021 [cited 2022 Apr 6].

3. Morales-Sánchez D, Martinez-Rodriguez OA, Kyndt J, Martinez A. Heterotrophic growth of microalgae: metabolic aspects. World Journal of Microbiology and Biotechnology. 2015;31(1):1–9.

4. Suzuki S, Endoh R, Manabe R, Ohkuma M, Hirakawa Y. Multiple losses of photosynthesis and convergent reductive genome evolution in the colourless green algae Prototheca. Scientific Reports. 2018 Jan 17;8(1):940.