Can I just replace my outdoor unit and nothing else? What does Matched System mean?

• Matched systems are now required by the energy code and you cannot replace the outdoor unit only. A matched system is necessary for proper performance and to avoid premature failures.
  b. Matched Systems:

• A matched system refers to a combination of components that were tested and approved by the manufacturer for use together. A matched system doesn’t necessarily mean that a 3-ton outdoor condensing unit must have a 3-ton indoor evaporator coil. While this combination is usually acceptable, it may not be the best combination for you. Now we come to the third major air conditioning component, the furnace or blower unit. Proper total airflow is essential. Here are a list of tips that your contractor should use when determining what to recommend for your home.
• An indoor evaporator coil, furnace or blower is rated in tons. This refers to the maximum size of outdoor unit that should be used. For example, a 3 or 3½-ton outdoor unit may be approved for use with a 4-ton coil, often with improved capacity and efficiency. Trane also manufactures High Efficiency coils that provide the maximum performance available. Sometimes a larger sized, standard efficiency coil is approved that also gives improved efficiency. While the use of these larger, more efficient coils may be prohibited due to limited space it is usually a good purchase decision.
• Above are examples of new, larger indoor coils. Never reverse this by installing a larger outdoor unit with a smaller indoor evaporator coil or blower.
• New outdoor units with old indoor coils can rob your system’s capacity and efficiency by 10 to 30%, preventing it from keeping you cool on the hottest days. In a recent test a Trane outdoor unit achieved 13.65 SEER with a new Trane High Efficiency Coil but only 10.50 SEER when using the old coil which was cleaned prior to testing. It makes good sense and good “cents” to change the indoor coil.
• Never “wing it” by assuming a coil will work with a certain outdoor unit. Trane has tested and approved combinations that work as intended. (They have also tested combinations that don’t work well, suffering from poor humidity removal, unreliable operation or shortened system life. For your protection, these combinations are not approved.) Reliability, Durability and Efficiency — the three most important attributes that customers look for when purchasing a new air conditioning system. The unfortunate thing is they probably won’t get any of the three when dealers don’t sell a matching indoor evaporator coil or air handler for the new outdoor unit. Always use an indoor coil and outdoor unit combination that has been tested and approved by the manufacturer.