Because an air conditioner moves heat between the indoor coil and the outdoor coil, both must be kept clean. This means that, in addition to replacing the air filter at the evaporator coil, it is also necessary to regularly clean the condenser coil. Failure to keep the condenser clean will eventually result in harm to the compressor, because the condenser coil is responsible for discharging both the indoor heat (as picked up by the evaporator) and the heat generated by the electric motor driving the compressor.

We bought 2 top of the line Lennox HAC for $29,000 less about $3,500 rebates, thinking they would at least last 10 years. Well, after 6 years, the system leaked. We are paying $1,700 to fix that leak. We thought at least it is still under warranty - yes BUT - the warranty covered $15 & the rest we were up for ,even though they advertise 10 year warranty. We are trying to sell & move out of high cost southern Calif & have looked at 20+ places in Arizona. Most of the houses we looked at were built from 1998 to 2005. We asked if the HAC was original & in most case it was - even though the highs were 115 to 120 & the lows 90F - versus where we live the highs run 85 to 95 & the lows are in the 60s. I asked the repairman WHY ours went out at 6 years. He replied that they just do not build them the same anymore. In Arizona, I did not see any Lennox - mostly Trane or Carrier. We had Fujistu in Australia which were fantastic - nothing went wrong in the 9 years we had them. Also, they had full induction motors not like the energy consuming 2 stage motors in Lennox. Maybe the Japanese built Fujusita still builds quality & the warranty is meaningful?
Typically speaking, the lower SEER, more basic models are also more reliable.  Reliability is big with us, so we steer clear of features with reliability issues, and it’s as simple as that. When you factor in the price of repairs and the extra expense of the 21-SEER, you may actually lose money! 16 SEER is the sweet-spot, and it is key to reducing your HVAC installation cost.
The performance of vapor compression refrigeration cycles[26] is limited by thermodynamics. These air conditioning and heat pump devices move heat rather than convert it from one form to another, so thermal efficiencies do not appropriately describe the performance of these devices. The Coefficient-of-Performance (COP) measures performance, but this dimensionless measure has not been adopted. Instead, the Energy Efficiency Ratio (EER) has traditionally been used to characterize the performance of many HVAC systems. EER is the Energy Efficiency Ratio based on a 35 °C (95 °F) outdoor temperature. To more accurately describe the performance of air conditioning equipment over a typical cooling season a modified version of the EER, the Seasonal Energy Efficiency Ratio (SEER), or in Europe the ESEER, is used. SEER ratings are based on seasonal temperature averages instead of a constant 35 °C (95 °F) outdoor temperature. The current industry minimum SEER rating is 14 SEER.[27][dead link]

Cost Factors: The size of the unit, its efficiency and it’s single-stage, two-stage or variable-capacity are the top cost factors. Features like communicating technology and improved dehumidification performance also affect the price. Learn more about communicating technology here including the pros and cons, before being agreeing to a communicating system.
Before opening the electrical cover on the A/C unit, be sure to shut off all power to the compressor unit and the indoor furnace or air handler, and verify that it is off. Note that this may mean shutting off a circuit breaker that serves the furnace and air handler, and then, near the compressor, pulling out the disconnect block (or shutting off the 220-volt power) to the outdoor compressor.