I buy my Furnace Filters at Home Depot. They have their own Rating System.
The Filter Company says to change Filters every 3 Months.
I know I was wasting money every year even if I used them for 6 Months.
Last Fall, I did some research and you may want to change the Filter every 3 Months
IF
you smoke, have a fire place, burn candles or have a dog / I have none of these.
What I ended up doing is installing a
Differential Pressure Gage to the Furnace.
http://www.dwyer-inst.com/Product/Pressure/DifferentialPressure/Gages/Series2000
1.) The Furnace Company shows (confirmed by sticker inside furnace) a maximum
Static Pressure of .50
inch of water column.
2.) Install a new Filter and gage will read approx .30
3.) Change Filter when gage reads .50
The Bracket I bought to mount gage to PVC pipe is
Dwyer A-610.
I mounted (2) of these gages / with 2, I don't question the accuracy.
Dwyer Model: 2001 ...... 0-1 Inch of water Column
and
Dwyer Model: 2002 ...... 0-2 Inch of water Column.
Bought them on E-Bay for about 30.00 each.
Time will tell, but it looks like I may get 18 Months out of a Filter.
I may be able to get a more restrictive filter and have it last 12 Months.
As long as I don't go over the .50 inch of water column.