On Monday, September 4, 2017 at 5:14:10 PM UTC-4, Iggy wrote:
replying to Jonny R, Iggy wrote:
Just so any passersby have a little more answer. In my own testing I found no
difference on my electric bill ("efficiency" is only based on the blower motor
or air movement, in this case) when I went to annual and now even every
2-years of a change out...I used to smoke and no longer do.

I would've expected 10%, but I saw nothing when I had to wait 2-months after
moving-in for a custom filter size. The old filter's corrugations were
completely filled and the filter face was flat smooth with dust. I put the new
filter in and saw no difference on my bill and heard no relief from the blower
motor nor ductwork.

My findings agreed with the basic recommendation. If you can't see through the
filter when held up to the Sun or bright light, then and only then it should
be changed.

That test would seem to depend on what kind of filter you have, if at all.
Filters range from the cheap fiberglass ones that are less than an inch
thick to 5" thick pleated ones that have a high MERV rating. The latter,
you can't see through when they are brand new.

I agree that with any filter you'd have to block it up a substantial amount, raise
the pressure drop significantly for it to have any significant effect
on efficiency.