I have a roughly 2 year old hot tub that has a musty odor much of the time.  We take very good care of it, but must confess that there may go a week or more when the tub isn't opened.  Especially in winter. 

I have been told that the musty smell is coming from the cover, and not the tub, or condition of the water.  Was told to open it often, even if we don't intend to use it, and let the cover dry out.  I have done that - just about every day.  No change.  Treating the water with chlorine helps, but I fear I'm only masking the smell with another. 

I've also been told that the smell may be locked between the plastic lined styrofoam and the cover (the area that does not get a chance to dry out.  This seems plausible.  I have just taken the styrofoam pads out of the cover, and although they smell musty as well, the side that gets no air was in fact dry.  Also should note that earlier this spring, we did replace the black plastic liner to the styrofoam, as it was starting to break down.  At that time, we cleaned everything and allowed it to air dry.  That didn't solve the problem either.  I'm going to wash everything now with a chlorine solution and let it air dry again, but thought I'd throw my problem out there and see if I'm perhaps going down the wrong path. 

I hope someout out there can provide me some direction.  I would be eternally grateful.