To be more specific. The contractor is responsible for his work and the equipment and if there is a probelm with performance, then he is the one to properly troublshoot and repair it. If the unit is not performing there is something wrong with either the installation or a componant in the system and if so, it is the contractors responsibility to make it right. How he makes a claim if something is wrong with the equipment part is between he and the distributor or mfg.
There are contractors who do gaurentee savings but it is clearly spelled out in their agreement that they make the system right and at the first sign of hesitence by the owner over work that needs done such as new ducts, duct sealing, proper refrigerant lines, insulation, clearnaces around the outdoor section (the list goes on)...or they come to where the customer doesnt want to spend the money to have it corrected, the deal is off. These guys aren cheap and dont even get into the bidding game, they dictate how it will be done and if everyone agrees, the deal is on.