Occasionally a heat pump unit will develop a smell problem called "dirty sock syndrome." This is a plague that causes problems for about one to two percent of all units, but it’s most common in humid and warm climates. It is a frustrating problem that can be difficult to fix. Keep in mind that all heating systems will cause an odor the first time they are used, and often for the next few start-ups as well.
What Is "Dirty Sock Syndrome"?
Most experts agree that dirty sock syndrome is caused by bacteria forming, although some people suggest that it is a fungal problem. The microbe grows on the heat pump coils and propagates because the coils are usually wet with condensation. The smell is most noticeable during the defrost cycle.
The small is usually described as a dirty or "locker room" smell, similar to the odor that comes out of a car air conditioner when it has not been used for a while.
How to Fix It
Before you assume the smell is caused by "dirty sock" syndrome, check to see if the odor is present all the time; if it is, it is caused by something else, not your heat pump. Dirty sock syndrome only smells when the indoor coil cools and the bacteria releases its smell into the air.
There are a number of recommendations for limiting this problem. One of the most common, but short term fixes is to use a bleach and pine solution on the coils to try and kill the bacteria. A tea tree oil solution should also work.
A thorough cleaning of the coil should bring the system back to normal for the rest of the heating season, but must be redone each summer season and sometimes more than once. Running the heat pump unit in "Auto" mode will reduce the humidity in the house, also reducing the bacteria formation in the coils hopefully before it becomes a problem.
A newer fix is to attach a UV germicidal light system at the coil and drain pan area. These emitter tubes generally last a little over a year before needing to be replaced. This is said to be a permanent solution, so if it doesn’t work, as a last resort, the coil itself may have to be replaced.
Finally, if it doesn’t seem as though coil is not the problem, make sure the smell isn’t being caused by debris and organisms in the drain pan or filter.
Other Heat Pump Smells
If the smell is more of a burning or electrical burning smell, you should call a professional repair shop to send out a technician. This problem could be weak fan motor or capacitor. The first time you fire-up a heat pump it may give off a slight electrical smell, but this is nothing to worry about unless the smell gets worse, or you see smoke.
Changing the brand of heat pump will not usually fix the smell problem as the problem is often limited to certain environments, rather than units.