Heat pumps are HVAC devices that transfer heat from a lower-temperature fluid to a higher temperature fluid. In the 1970s, heat pumps were touted as the energy efficient HVAC systems of the future. Everyone, including electrical utilities, rigorously promoted heat pumps, birthing a new market. New markets typically have new problems, and for heat pumps the nature of the problem related to odors. A particular foul smell was so prevalent that the moniker Dirty Sock Syndrome was coined and universally accepted.  

A thorough research of this subject, including that which is published by ASHRAE (2008, 2011, and 2012 Handbooks), suggests that the Dirty Sock Syndrome odor emanating from heat pumps stems from the decay of accumulated organic debris by microorganism activity — similar to sewers, plumbing traps, and other rotting, decomposing organic material.

The Nose Knows

Our sense of smell provides the perception of odor, mediated by the olfactory nerve. Many terms are used to describe decaying organic material but generally it depends on concentration. Common organic odors that people are used to, such as their own body odor, are less noticeable to individuals than uncommon odors such as the dirty sock odor. Not only is the smell of decomposition unpleasant, it also generates a sense of illness in many people. This is possibly because the smell indicates unhealthy conditions that people should get away from or fix.

Our sense of smell is also a big factor in the sensation of comfort. Odorous compounds act as a stimulus, triggering reactions that include nose, eye, and throat irritation. Perception of odor and of irritation is unique to each person and varies based on exposure and frequency, concentration, and duration before becoming a nuisance.

HVAC Sources

The dirty sock odor is not only linked to equipment types, but in-home occupation, activities, and use, which include pets, cooking, indoor laundry, and cleaning styles and materials.

Cooling coils, and other a/c components, are not made in sterile environments, meaning that microbes cover all of their parts along with a small amount of lubricant. During the first cooling event after installation, condensate brings life to the microbes, which initially use the lubricant as an initial food source. Recirculated air contains a variety of organic materials and microbial species that aide ongoing decay and microbial proliferation. Microbial decay (rotting) is the process by which organic substances are broken down into simpler forms of matter — including gas molecules and acids. Even the carcasses of the original organisms begin to decompose shortly after death, setting the stage for a cumulative cycle of microbial growth.

Twenty plus years of data collection from numerous investigations has manufacturers, academia, and practitioners in agreement that the dirty sock odor can only be the result of microbial activity. Microbial activity is related to water activity and temperature Can microbes do well in a coil plenum? As seen in your home refrigerator, once initiated, mold and bacteria will proliferate at relative humidity (rh) levels down to 30 percent and temperatures of 40°F and lower. Often the activity is visual, however, when unsure, most of us “smell” the item to aide in our eventual determination. A “bad” smell is decay — rot — decomposition.

Heat Pump Problems

During certain times of the year when a heat pump may warm in the morning hours and then cool at noon and later hours, it can produce condensate from noon on to fuel growth for up to 16 hours — only to release odor and toxins during the next morning’s warming hours. Heat pump coil temperatures are often not sufficient to kill proliferating microbes. Those that may die simply serve as a future food source for continued proliferation.

These conditions can be observed in some auto a/c units. After a warm day the cooling coil may be 80+°. When the car is started and the a/c comes on, the dank, moldy smell that occurs lets us know how much growth occurred during the day. When heating is performed, it’s usually by a coil that’s downstream of the cooling coil, so only a moderate warm up of the cooling coil occurs, just enough to release additional products of mold, bacteria, and their volatile organic compounds (VOCs).

When we research Dirty Sock Syndrome, we find that residential contractors have written the following — some dates back to the 90s and some is current:

• Once the problem is identified, start with a thorough cleaning of the evaporator coil with a non-acid coil cleaner. This usually prevents complaints for the rest of the season unless weather conditions force the system back and forth from heating to cooling.

• The problem is not brand specific, all manufacturers acknowledge complaints. When changing out systems with new product and/or different brands, the complaints return. Units have been removed from one house, cleaned and installed elsewhere without complaints.

• The problem seems more common in heat pumps as in most gas-fired furnaces coil temperatures exceed 160° which kills microorganisms. But in heat pumps, coil temperatures during heating are between 105 and 130°, just a nice temperature for microorganism activity.

• One solution is to coat the coil with an antimicrobial agent. Another is to remove the coil and ship it to a manufacturer of a coating service. The coating material contains an antibacterial agent with a warranty; the smell hasn’t come back so far.

• UV is the way to handle the problem but it isn't ideal because UV only kills what it can see and it can't see everything. There are nooks and crannies the light will miss.

• The only known remedies are to clean or replace the coil, use drain pan treatments, or in extreme cases, replace the entire unit — and even that didn’t always solve the problem. Generally speaking, however, frequent cleanings with bleach did reduce the odor to acceptable levels.

• The problem mostly occurred during “shoulder” seasons like fall and spring, when the units would be used for heating at night and cooling during the day.

Dirty Sock Solutions

A thorough coil cleaning will make a difference, but the problem will usually return. The problem has not been reported with gas-fired furnaces when the coil is downstream of the furnace.

Antimicrobial agents temporarily disrupt microbial growth, but they require continuous application because dead carcasses and the agent’s inert ingredients provide a nutrient source for new growth. ASHRAE research has shown that antimicrobial coatings are effective until organic debris builds up sufficiently to insulate the agent from the microbial activity. UV-C always works when it is installed and operated correctly. Bleach works as long as it gets applied on a routine basis. If the goal is to eliminate coil and/or equipment change-outs, the only way to do that is with continuous cleaning, and UV-C lamps are currently the best solution to provide 24/7 cleaning of coils and drain pans.

Accepted methods for the complete killing of microorganisms include heat, chlorinated compounds, and UV-C. Heat is used in several types of food production (canning) to kill microorganisms of all kinds. It has limited application in residential systems, and it leaves carcasses behind that will serve as a future food source. Chlorine (bleach), works amazingly well as it not only kills microbes, it disassociates their carcasses (with sufficient concentration and exposure) to limit them as a potential food source. This is evidenced in swimming pools when the water can appear crystal clear after treatment. But bleach is corrosive, and requires routine application…and there is the odor.

Like heat, UV-C kills microbes of all kinds and types and, like bleach, disassociates their carcasses and other organic materials. However, UV-C produces very little heat, no odor, and leaves no secondary contamination behind — and it operates continuously, which means no build-up of microbial matter occurs, ever.


For these reasons, as described in the ASHRAE Handbooks, UV-C energy is ASHRAE’s recommendation for killing microorganisms on cooling coils, drain pans, and on other surfaces, and for degrading any and all remaining organic debris to return the coil to as-built performance characteristics.

Described performance, however, requires that a UV-C product be installed and maintained correctly. The amount of UV-C output means less than where the lamp is located. To get the results expected by following ASHRAE recommendations requires that the lamp be located just downstream of the cooling coil and be operated 24/7/365. Operating continuously means that even 1 microwatt of UV-C wavelength energy will add up to 2.3 million microwatts-seconds in a month’s time. That’s more than enough to kill anything on coil and drain pan surfaces, etc. It’s also enough to kill and remove growth and debris already on a coil if installed as a retrofit.

UV-C will do some amazing work, but there are a few things that will help prior to its installation, where possible:

Clear drain lines and traps.

Correct drain line angles.

Make sure traps are sufficient and primed.

Seal water and air leaks.

Today, UV-C products are inexpensive, profitable, and because of the annual re-lamping requirement, they put you back in touch with your customer a year later. According to many contractors, they are less expensive than a proper coil cleaning procedure and/or antimicrobial application, and when coil cleaning can’t be done, often UV can be installed. Most systems require less energy than a 30W light bulb and the opportunity for the homeowner to save a/c operational energy is very high — best of all, no dirty sock odor!

Publication date: 9/8/2014

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