13 is better obv, but merv-8 is a great drop in for most systems, and it makes a huge difference. my system runs the fan for an hour every evening, and that also has been an improvement.
replyi have to do a major upgrade next month (30 year old gas furnace!) and i will need to do some research, but i would like to even ascend to true HEPA if it's feasible
Mismatched merv levels between some heating systems (ie., forced air) and what they were designed for can also create strain on the system and premature failures.
replyAlso, in new homes, beware of undersized heating systems that will fail prematurely.