"Tony Hwang" wrote in message
...
You never turn the fan on for like circulating air or freshen up the stale
indoor air specially when cooking, etc.? Filter gets dirty not only
depending on how long fan ran, it also depend how much dust is
generated indoor if there is pets, plants... I'd think under replacing it
may be a bad idea. I'd rather over replace. My filter size is 16x25x5. I
replace twice a year. They don't look too bad when pulled.
But It gives me peace of mind. Programmable saves energy depending on
how you use it. I use it in my house and out at cabin.

With just me and the wife at home and no pets and we keep the windows closed
there is not much dust and dirt. I use the least expensive 20x20x1 filters
and change them every 2 months on my heat pump. Most of the time they seem
to be just about as clean as the new ones. At less than $ 10 per year it is
not worth me worring about. Just change them on the odd numbered months at
the first of the month. Some months it runs a lot and some not too much,
but I just keep on schedule. Not worth the trouble to keep up with how
dirty they get.