1.) Filters And Ductwork
In order for that nice warm to reach you it has to go through your air filter(s) and ductwork for your unit. A dirty filter will clog the airflow of your system which will slow the air, causing your system to work longer and harder. Ductwork is vital in any home HVAC system and if there are leaks, blockage, or maybe even a whole piece of the ductwork sliced, air is going to escape the ductwork and not reach you. Imagine a scenario where there is a lot of air leakage in the ductwork and a dirty filter, you’re going to be losing out on efficiency. You might just need to change your air filter and/or have the ducts sealed up.
Filters Can Also Affect The Heat Exchanger
The heat exchanger is necessary to transfer heat to the air in your home. A dirty filter could have caused some kind of dirt or soot build up on the exchanger that is preventing any heat transfer. This problem will require a technician.