HVAC's "Dirty Sock Syndrome"... What's that smell?

Are your residents complaining, or have they complained in the past, about a bad smell coming out of the HVAC vents? This can happen in a fairly large percentage of HVAC units and is typically most noticeable at the change of the seasons when the system changes from cooling to heating or vice versa. It’s commonly referred to as “Dirty Sock Syndrome”.

“Dirty Sock Syndrome” is a phrase coined by HVAC contractors nationwide to describe a microorganism growth problem in HVAC units and has plagued HVAC Original Equipment Manufacturers (OEM) and Independent Contractors for far too long. All OEMs have acknowledged the problem without having a long-term, effective, and efficient solution.

Since OEMs replaced copper with aluminum in coil construction, normal environmental bacteria and fungus have found a perfect habitat to grow and thrive. As microorganisms grow, they produce an off-gas, which builds up around the coils and in the Air Handling Unit (AHU). When the AHU blower first turns on, it pulls this cloud of off-gas from around the coils, through ductwork and the registers, and into the conditioned space. This “dirty sock” smell is typically noticeable for only a few minutes until it dissipates into the air. However, the puff of smelly air comes out every time the blower turns on, making the problem more noticeable since the inhabitants do not build an immunity to the smell. In addition to the smell, if the inhabitants are allergic to the species of microorganism that is growing in the AHU, they could have minor to severe allergic reactions to this puff of off-gas entering the space several times per day. Some species can be extremely harmful to anyone in the contaminated environment.

So, is “Dirty Sock Syndrome” a manufacturer’s warranty issue or an installation problem?

The answer is actually neither. A manufacturer’s warranty covers deficiencies in material and workmanship. The coils are not deficient, in most cases, the system is functioning properly, just as designed, and is conditioning the environment. If this is the case, we also know the installation contractor performed their services effectively. The problem is not an equipment or an installation problem, it’s an environmental problem.

So what’s the solution?

One solution some OEMs and Contractors are doing is removing the coils from the infected unit, sending them to a manufacturing or treatment facility to dip the coils in a multi-stage process. This provides a coating on the coils that inhibits microorganisms from growing on the surface. This process is expensive, leaves the environment unconditioned for several weeks which in turn produces very unhappy homeowners or residents, and doesn’t solve the root cause of the problem. Similarly, some contractors are coating the coils on-site with a polyurethane-like coating, which does inhibit the growth of microorganisms on the coils, but again does not solve the root cause of the problem. If the microorganisms are growing anywhere other than on the coils, the smells will still come through the registers.

The recommended solution is to treat the whole system, from returns to registers, which should include a root cause analysis and a load calculation. When going with this solution, make sure the OEM or Contractor you choose offers a warranty and ask how many visits they’ll need to make. The answer should be one visit! 

This article was written by Brady Lancaster with Signal Hygiene. More information about their “Dirty Sock Syndrome Solution” can be found here.