If there doesn't seem to be enough airflow or it is less than it used to be, there are some things to check before calling for service.
Keep in mind that many of today's high efficiency and electrostatic air filters do restrict more air. They must really be kept clean or they can cause airflow problems.
Below is a list of possible causes and things to check. Items in blue usually require a service call. Items in red however can be addressed, even fixed by the homeowner.
Faulty indoor blower motor
Loose, worn fan belt
Incorrect pulley sizes
Indoor coil frozen due to malfunction, restricting airflow
Fan speed set too low, possibly boosting to higher speed
Blower wheel and/or coil could be dirty
Ductwork not properly sized or installed
Debri, obstruction, or closed damper in ductwork
Dirty filter - the cleaner the filter the better the air-flow
Make sure vents are open, including return vents
Make sure dampers are opened - handle should be in-line with duct. Turning handle 90 degrees closes damper
The bottom causes in red are common problems and can be addressed by the homeowner. Try to check for these conditions first before calling for service.
Hope this helps, remember - these are just rough guidelines and not all possible situations are covered.
Hannabery HVAC is registered with the Commonwealth of Pennsylvania's Office of Attorney General: PA#8215
