How do you know which way the air flows?
Similarly, how do I know which way the air flows in my AC?
If you are looking at a furnace filter, find the arrow that indicates the proper air flow direction. That arrow must always face toward the furnace and away from the return duct that carries the air in need of heating or cooling.
Beside above, what if I put my furnace filter in backwards? If you put your filter in backwards, the usually collective end of the device will not face the air supply. Your filter will aid in keeping debris in the air. This results in a clogged filter and inaccurately cleaned air getting into your lungs.
Moreover, which way does air filter go in?
Luckily, most filters are made so they won't fit right unless they're inserted in the correct direction. There should be airflow arrows printed on the side of the air filter to indicate what direction the filter should be inserted. When you insert your new filter, the arrow should point toward the furnace.
Is MERV 11 too restrictive?
A decade ago, it used to be that if you had an HVAC system that used panel filters rated 6 through 9 MERV, putting in a MERV 11 or higher filter would restrict airflow (known as filter pressure drop). In turn, this would lengthen the system's run-time, adding to your heating and cooling costs.