I have a 3.5 ton Heil in a 2100sf ranch. It does not cool properly and the monthly cost is too high. I did the HVAC-Calc and it calls for a 3.5 ton. The current system runs almost a 100% of the time on a hot day. Is there a rule of thumb as to how much a unit should run if properly sized? I am thinking about a 4 ton 13 seer replacement. Would that make much of a difference or would you reccommend a 5 ton? Also, would it be an advantage to change out the furnace at the same time? Finally, any comments on the Lennox Merit system. Thanks is advance.