Your heating and cooling systems account for about half of all your energy costs. Therefore, you want your HVAC system to run as efficiently and effectively as possible for as long as possible, naturally. But it can only do so if it is professionally installed and serviced.
For example, an inexperienced HVAC contractor may not know how to appropriately size a new air conditioner for your home, and as a result you could wind up with an underperforming system, or a cooling system that cycles on and off rapidly (short-cycling), resulting in inefficient performance.