When it comes to the efficiency of your HVAC system leaking seals at ducting connections can be costing you big money.
As home inspectors we see a lot of seal failure at ducting connections due to either poor initial fastening, improper use of sealing materials ( a little know fact is that actual "duct tape" is the worse thing you can use), and simple age deterioration of sealing tape.
Even if the proper materials were used at ducting connections that material breaks down over time. (average life span for these materials is around 10 years)
In the photo below the sealing tape at the main HVAC plenum box has failed allowing conditioned air to leak into the attic space of this 30 year old home.
When buying a home we recommend an HVAC professional not only clean and service your system but also check the integrity of the seals.