re: BR HVAC Recommendations

Other things to consider:

I assume you have one dedicated return (where your filter is). If you leave doors closed, you can starve the return duct of air flow. The return will then pull air from the attic or from outside (thru leaks) to make up for the loss. I’ve tested air velocity leaving my supply registers and there’s a substantial reduction when doors are closed.

I’ve also seen poorly installed ductwork. Flex duct that is pinched, duct exiting the end of the plenum, etc.