My repairman says that I need to replace all my ductwork and the lined plenums (I have two units – upstairs and downstairs and it will cost $4500) with duct that is lined on the outside – not the inside.  Here’s my question.  I read the note about duct liner but it didn’t say anything about a smell.  Have you heard of eroding duct liner smelling bad?

  So, my story is, last spring I replaced the upstairs electric heat pump.  This winter I had a stinky smell blowing out with the warm air!  It was musty and chemically all at once, and I would have to cover my nose!  But, there was no consistency in the timing, temp, or duration of the smell.  My repairman has concluded that the 30 year old liner inside my cold air return ducts and plenums is deteriorating and bits hit the heat strips and burn…and that causes the stink.  He also says that mold was present on the liner.  I can see inside the cold air returns and the black paper-like liner (glued and stapled to the metal duct) looks dusty, dirty, and there are bits of something hanging off of it.  Should I do the repair?  If so, do duct systems that are lined on the outside produce more air noise and allow more room to room noise?-Rebecca, North Carolina 

            Generally, duct liners have little odor associated with them.  They typically consist of a three-eighth inch layer of fiberglass with a thin coating of adhesive that attaches it to the duct and a thin coating of latex on the airstream side.  The fiberglass should not be a source of odor and in old duct liner material neither should the adhesive nor the latex coating. 

            Ductliner can catch a variety of particles overtime and those particles that are organic can serve as a source of food for microorganisms such as fungi (mold) if the humidity is high or there is liquid water present.  Given the relatively high humidity in your region in the summer months, it is quite possible that mold growing on the dust accumulated on the duct liner could produce musty odors that would be sensed when the heating/cooling system comes on. 

            Your description of the inside of the return air ducts indicates that such a problem exists.  It also indicates that the duct liner may have deteriorated to the point where pieces of it break off and potentially are carried toward the blower and heating/cooling coils.  These pieces are usually quite large and should be captured by your furnace filter before they have a chance to deposit on any mechanical equipment.  If your filter is really poor, there is certainly the possibility that large particles from duct liner could get through. 

            If pieces of the duct liner were to be deposited on a hot heat exchanger, they should produce some type of burned odor (such as hot/burning rubber or even paper if as you say the liner is paper-like).  

            It is notable that the odor which you characterized as a musty/chemically odor appeared after you replaced the heat pump.  Many heat pumps are insulated on the inside with duct liner that is glued to the inside of the metal housing of the unit.  In the past, I knew of heat pump units that produced chemical odors every time the heat came on; the colder outside, the more chemical odor was present.  Therefore, I would recommend some experimentation with that heat pump by turning the heat off and on.  I suspect that the chemical odor that you describe is associated with the heat pump.  Heat pumps can also produce musty odors if organic dust accumulates in the condensate drip pan during cooling season operation. 

            I don’t recommend the use of duct liner on the inside of ducts since the latex layer often deteriorates to produce a “black dust” problem and of course can even be physically detached because of the effect of air velocity over time.  Duct systems lined on the outside to my knowledge work relatively well in reducing noise levels associated with the operation of blower fans.

 

March 3, 2006