I have a furnace/air-conditioner that can support filters of various thicknesses: 1", 2", or 4". Filters are available in various MERV ratings. How should I decide which filter to use?
Depends a bit on your furnace. If you have someone that services your furnace, ask them.
I used to use a pleated filter with a high MERV value. The second time my furnace broke when it was well below 0 degrees, I spent some time talking with the service man. Apparently the type of furnace I have does not draw air through these filters as well as it should. This causes this part to burn out.
I now use the filters that are fiberglass screens and no longer have the problem.
if you refer to the "Microparticle Performance Rating", such as the difference between red Filtrete and purple Filtrete (and there are cheaper or more expensive then these 2), then I would say that it depends on: 1. do you have pets? 2. does anyone in your family get allergies, especially when you turn the heat/ac on?
If yes to both, go with higher filter. If no, go with a lower one.
One thing I do is in the winter, I start with the cheapest (about $3, 1") filter. Since in the first month, most of the big junk will get into the filter (junk waiting all summer to get sucked into the vent). After the first month of heavy use, maybe 2, I switch to a slightly more expensive one (usually the Red). I have no pets.