The primary purpose of zoning ducted air conditioners, heat pumps,
and furnaces is to improve comfort.

Increased comfort is attained by having the capacity of the HVAC system (cooling or heating delivered) follow the shift in load as it changes across the house. For example, it is common for two-story homes to be too hot on the second floor in both summer and winter.

Providing the most agreeable temperature to all the zones is comfortable, but it carries with it the distinct possibility of increased energy consumption.

There are two primary methods by which the common multi-zoned dampered system lowers the capacity and efficiency of an air conditioner. These are:

• Lower airflow due to the additional restriction of zoning dampers, and
• Recirculation through the air conditioner due to the use of a bypass duct.

Both of these items lower the evaporator coil temperature, which lowers the capacity and efficiency of the unit.

The reverse Carnot cycle establishes a theoretical Coefficient of Performance (COP_C) of a vapor compression air conditioner. The Carnot cycle is a clearly unattainable ideal, but it make two things perfectly clear:

• Higher condenser temperatures reduce the efficiency of the air conditioner.
• Lower evaporator temperatures reduce the efficiency of the air conditioner.

Figure 12 graphs the COP_C of a unit with a condenser temperature of 95°F and varying evaporator temperatures.

EER = COP * 3.414

Air Conditioner Efficiency Falls with a Colder Evaporator Coil
Low evaporator coil temperatures are produced when the airflow is low and when cold air is introduced into the return plenum.

Demonstration with numbers:
Why bypasses are dumb with John Proctor P.E.