Hi, I have a sister who's on a pretty tight budget. She has a "newer" furnace and I keep on her to change her filters routinely. Here's the issue. Her housing for the filter fits a 16x25x4" filter. These babies run about $30+ per filter (at the "box" stores). What's up with that? My unit..and most I've seen use a 1" thick filter which costs a fraction of the 4" filter. I don't see anything else really different about her unit than mine or others. What's the rational for a 4" filter? Can I change the filter housing or retrofit something to allow her to use a 1" filter? I notice that the 4" filter mentions on the package that it may only have to be changed once every 6 or 12 months. So, does it have a longer interval between changes? She has 4 kids and 2 dogs in a small townhouse. She usually requires frequent changes. Thanks for any input.