When new, your air conditioner or heat pump was comprised of 5 properly matched components:

1. Indoor evaporator coil
2. Outdoor condensing coil
3. Correct sized refrigerant tubing
4. Properly sized circuit breaker and fuses
5. Properly sized electrical wiring

 
When properly assembled, they will produce a specific amount of cooling at a specific rated efficiency. Capacity and efficiency ratings are produced by the manufacturer under strict testing conditions. Any substitution of any of the components will produce a variation in either capacity or efficiency (or both).

 
 
 
 
 
When an old outdoor condensing unit fails, it is very important that the indoor evaporator coil be matched and replaced at the same time for a number of reasons. Some of them are:

1. Impossibility of attaining capacity and efficiency ratings of the new condensing unit if the evaporator coil is not changed.
2. The emphasis on "high efficiency" during the past few years has brought about some startling technological changes in equipment. Installing a new high efficiency condensing unit with even a 5 year old evaporator is inviting trouble.
3. It is very common to have compressor failure in a new unit after just one or two years when an old evaporator is used. Manufacturers of air conditioners and compressors alike are unanimous in stating that over 63% of the failures of outdoor condensing units are caused by a restriction or refrigerant leak in the indoor evaporator coil.
4. The warranty does not protect you from compressor failure due to an old evaporator. Manufacturers test bad compressors and can tell the cause of failure. If due to a bad evaporator coil, they may disallow and void the warranty.