My evaporator unit was replaced by a A/C Pro. He installed a 17.5 inch box on top of my 15.5 Furnace. Can this be done? Is this the right thing to do? There is a lot of silver tape. The unit is off center and I am concerned that the air flow and cooling is reduced. The new evaporator is marked for the same tonnage as the old one was. The temperature at the registers is lower by 5 or 6 degrees.