16 MERV House Air Filters

This is a common sense good answer.

Part of that stack could also be electrostatic filters which have a metal media. They can be washed out and reused.

Also on my list a MERV rating of 16 is as high as it goes .3 to 1 microns. Unless you have someone sick that needs the air to be filter to that extent at it is a bit of an over kill. The only other reason I can think of for that filter rating is a lengthy warranty on the HVAC. To keep any contamination from build up inside which may cause failure. Also it's easy way for the manufacurer to get out of the warranty. Cost of the filter being as much as it is many people would change to a lower MERV rated filter.

Buy 5 cheap 1" filters, stack them and tape the edges together if you really want to save money.

I had thought about either making up either a 1" spacer piece since 4" filters are readily available or a 4" spacer piece in the interest of frugality. I see no shame in not wasting money.

The average HVAC filter service interval for residential setting is dependant on the level of hygiene of the occupants. Household dust is mostly dander so bathe often and extend filter life

What about HEPA filters

http://www.tset.org.uk/jsmallfib_top/upload/documents/DOP%20test%20protocol%20final%20version%20April%2008.pdf

HEPA filters are around 12.

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MERV RATING CHART

Unless you have a pack of pets or live in a dust bowl that filter is good for about one year, due to the over all surface area. You can cheap out and stack several thiner filters together, but you probably be reducing your return air flow.

You might want to locate a filter manufacturer and purchase a box of exact replacements. Your unit cost will be less, but there is a bigger up front hit. The good news is that you are set for several years.