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Question: Why does my heat pump smell musty when it defrosts? What is Dirty Sock Syndrome?

dirty sock syndrome 1

Question: Why does my heat pump smell musty when it defrosts? What is Dirty Sock Syndrome? 

Answer: We have had several people ask us this question including myself! I do not want anyone to continue to have this awful smell in their home so that is why I am writing this post. I installed a new packaged heat pump unit at my house in the Spring of 2021. Everything was going swell and dandy until this fall when the new heat pump started having to defrost to melt the ice off the unit. My wife said, “What in the world is that terrible smell??? It smells like soured laundry clothes!” You know when you forget to place the laundry in the dryer for several days and the clothes go sour!  Yes! It was a terrible smell that lasted while our heat pump was defrosting and for about 20 minutes after the defrost stopped! I asked myself how could this be happening with a brand new heat pump packaged unit??? How can this happen with new equipment??? Is it something that I did wrong in the installation?? I started thinking about how heat pumps defrosts and it makes sense how this smell can occur. I first Googled the subject, “Dirty Sock Syndrome” and found the following definition: “Dirty sock syndrome occurs when your heat pump goes into defrost mode, which stirs up microorganisms and leads to a foul smell. … Then when the heat pump goes into defrost mode, the indoor coil cools down and becomes moist, which reactivates the microorganisms.” I told myself that this problem makes sense on how it can continue to occur when the heat pump defrosts! The heat pump system switches the hot outdoor condenser coil (summertime) with the indoor evaporator coil in the wintertime. During the summertime, we do not have the dirty sock syndrome smell because the heat pump unit’s indoor coil is always cold, but in the wintertime when the heat pump has to defrost the hot indoor coil goes into air conditioning mode which makes the indoor coil cool, cold and the coil collects moisture. During the defrost cycle which usually lasts 10 to 15 minutes the indoor cool cold coil is accumulating water and moisture from your home’s indoor air. When the heat pump cycles back to the heating mode the cool, cold coil is immediately heated up which burns off and evaporates the water, moisture along with funky smelling dirt and bacteria off the indoor evaporator which produces the dirty sock odor! Yes, that makes sense, but how do I eliminate this problem? I have researched this and unfortunately, there really does not seem to be a long-term solution. Many people have temporarily stopped the problem by having the coil cleaned with a special cleaner every year like the dirty sock coil cleaner that we sell on the following page: Please click here if you are interested in the Dirty Sox coil cleaner that we sell. Some have installed Ultraviolet UV lamps to kill mode and bacteria on the coils. Please click here if you are interested in the UV lights we sell. 

I have just had Cataract surgery and have not been able to clean the coil on our heat pump yet. I will let you all know if the Dirty Sox spray coil cleaner that we sell works or not when I get the doctor’s OK to lift heavy objects again! What I have been doing with our heat pump on cold nights (when the outdoor temp is below 32 degrees) is that  I place the heat pump thermostat on “Emergency Heat” which cuts the outdoor heat pump off and heats our home off all electric heat. Lucky it has not been that cold yet here in Kentucky, but it is a lot better than having that awful dirty sock smell in our home! I am sure I will see how my emergency heat move has affected our next electric bill! LOL! I probably will not be laughing we I see our electric bill!

I found a really good article about how to fix Dirty Sock Syndrome from https://www.yourmodernfamily.com/dirty-sock-syndrome-cleaner/ 

On YourModernFamily.com they discuss how to get rid of the Dirty Sock Syndrome. They have found great results using special coil cleaners that I have pictured below.

dirty Sock Syndrome Cleaner solutions
dirty Sock Syndrome Cleaner solutions

If you want to learn how the defrost on a heat pump works then we have an excellent YouTube video below where AC Service Tech LLC explains how a heat pump defrost works. Thanks to AC Service Tech LLC for making this excellent video! I hope this information helps you understand what causes Dirty Sock Syndrome and how to get rid of Dirty Sock Syndrome! If you have any questions please comment below or email us anytime at arnoldservice@gmail.com We would love to try and help you out! 

Comstar Dirty Sock Spray
Comstar Dirty Sock Spray


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