if filter is new. you should check the CFM on all floor register. Dose it have strong air pressure come out form it? Again, you have to make sure the air is flow freely., do you have air conditioner in the house ? if you have, also check the "Addon coil" on the top of the blower fan or top of the heat exchanger too. you may have a plugged coil which also limits the air flow. View the picture you will see where the "addon coil" is. the above picture have some info. good luck Usefull link for all home owner. http://oee.nrcan.gc.ca/publications/infosource/pub/energy_use/air-conditioning-home2004/centralac.cfm