I would recommend checking the filter EVERY month and change it when it
gets dirty. I have seen houses that need filter changes every 2 weeks
and some that could 6 months and everything in between. In my area
about a month seems to be the most common. As for duct cleaning, if
you have unlined metal ducts it may be OK in some cases. If you do it
make sure the contractor is NADC certified. I will not do it in my
HVAC business because I do not believe it is worth it. Most ducts in
my area are duct board and flex that are dificult to clean and can be
damaged by the process. Use caution if you proceed. With 50 year old
ducts, most likely they are unlined sheet metal.


Stretch