We recently moved into our home in Waxhaw, NC. We want to replace the HVAC filters but aren't certain which kind to get. We currently have MPR 1900 (12x12x1 & 14x25x1). We've been told everything from: use the cheapest throw-away filters / replace them monthly, to nothing better than MPR 1500 as it would put too much strain on the blower motor. Help?