The home owner's cooling system was pushing twenty years old and just wasn't performing the way it used to. On the hottest days the system used to keep the home at a nice, chilly 72 degrees, but now it was struggling to hold 78.
A call to his service company revealed the problem; the system was old and "inefficient". The solution (according to the contractor) was a new system.
A new system was installed. A new condenser, new furnace, new line set, and a nice new programmable thermostat. The technician fired the system up, took operational readings, and told the home owner it was running like a champ.
And everything was grand, until the next day. The new system couldn't pull the home down below 78 degrees.