I've been running an airgradient sensor for a year now and as a result I am seriously considering adding a MERV to my HVAC system. Ideally I'd like to get two more sensors (one outside, one in the HVAC ducts) and setup home assistant to govern air exchanges.

Tip: avoid the cheap 1" filters that only have a few pleats. Fewer pleats = less surface area = more flow resistance.

Many HVAC people will recommend thicker filters (3-6" thick) if using a higher MERV, but the 1" 3M 1900 often has more actual surface area if laid flat.

Testing by Project Farm and others showed the 3M 1900 filter has the lowest airflow resistance of any 1" high-MERV filter. If your HVAC only accepts a 1" filter, that's the only brand I'd use. Change the filter ideally every 3 months, but to avoid costly HVAC repairs don't exceed 6 months maximum.

You can damage/destroy your furnace if you put in a too restrictive filter. Better to follow manufacturer recommendations and add circulating air filters in your house.