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. Regardless of that, a new filter should go in after carpeting has
been changed out or after a dusty home improvement...so yuck doesn't sneak by
and into the HVAC system or your air.

I also found that high MERV ratings didn't effect my efficiency at all either.
So, I went with 3M's Ultra Allergen filters, even though they're almost twice
the MERV rating warned against by the HVAC manufacturer. They've kept my
system like new and dustless for a full 10-years now.

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