Our 17 year old system went kaput last year during 110 degree days in July. We got a new Trane system (both heat and air) 16 seer with special filter (that my wife insisted on) and it has both lowered our bills considerably AND increased our comfort level by extreme magnitudes of order. Eye-wateringly expensive but worth every penny. Other than the pipe that carries the sewer AWAY from your house its the single most important expenditure you may make. Don't go cheap.