"Don Y" wrote in message
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We buy the cheap(est) -- DIRT cheapest! -- filters and try to replace them
monthly. We tend to have a fair bit of dust/dirt from the climate in
which we live so they get "visibly dirty" pretty quickly.

Our thinking is that it's easier to remember to do something (trivial)
every month than to keep track of N month intervals.


As there is just the wife and I here and we never open the windows we do
about the same thing. Buy the ones that come in a pack of 4 or 5 for around
$ 5. That is only about $ 10 to $ 15 every year. We change them on the odd
months and they never look dirty. Had the coils cleaned after about 8 years
and the man said the inside ones did not need it, but did it anyway as it
was part of the required job.

This is with a heat pump in the middle of NC so it
runs most of the year, heating or cooling.