Service called. Upon arrival customer stated the thermostat would not reach desired temperature and that the middle floor was unbalanced compared to the heat upstairs. Customer does only have one thermostat. Went ahead to the look at the system. System ran for about two minutes while I was there, until high limit was tripped And the system was throwing out seven flashes which indicated that. Went to head and checked out. The filter was extremely dirty and the cars are tripping on the limit. Went ahead at this point and advise customer to run maintenance. Customer excepted. Checked inducer motor within range. Checked flame sensor and cleaned. Checked igniter within range. Checked carbon monoxide levels. Well within range. Checked blower motor and wheel, wheel was extremely dirty. Checked heat, exchanger, and burner assembly, heat exchanger has a lot of damage to it. Some extreme hotspots due to overheating. Also checked static pressure which was not within range. Nick, the field superv