If the space served by a conditioning unit operates on a regular schedule, a timeclock or setback thermostat is usually the best method of automatically starting and stopping conditioning. For greater control flexibility, you can combine timeclocks and setback thermostats with controls that respond to other conditions, such as outdoor temperature, occupancy, etc.

Timeclocks and setback thermostats are not appropriate for spaces that operate on irregular schedules, such as conference rooms, auditoriums, and surgical suites. Unfortunately, facility managers sometimes try to schedule specific times for conditioning such spaces, most commonly through an energ management system.

This inevitably leads to dissatisfaction.

Don’t use temperature setback if the thermostatic control system would use energy to force the setback temperature. This limitation exists in units that provide both heating and cooling, where neither can be turned off.

For example, four-pipe fan-coil units with pneumatic controls are sometimes installed with a single thermostatic element controlling both the heating and cooling coils. These are installed in some luxury hotels so that guests do not have to select heating or cooling.

If a setback thermostat were used with this type of system, switching to the lower nighttime temperature would consume energy to cool the space to the nighttime temperature, and to maintain that temperature until the setback thermostat switched back to the daytime temperature.

If the conditioning unit uses reheat, study the unit’s controls before using temperature setback. Setback while reheating actually increases energy consumption.  (Reheat is rarely used in room conditioning units. When it is used, the purpose is usually to control humidity.

To minimize energy consumption for dehumidification, the reheat should be controlled by a humidistat in the space.)

Comparison of Timeclocks and Setback Thermostats
In most cases, but not in all, a setback thermostat is preferable to a timeclock. Setback thermostats behave in the same way as timeclocks, until the temperature falls (or rises) to the temperature limit that is set. The main advantage of a setback thermostat is that it restarts the conditioning unit if the temperature drops to the lower setting.

This feature keeps the space warm enough for a quick warm-up, or you can use a setback thermostat
as a safety feature to protect against excessively low (or high) temperature.

If the conditioning unit has a continuously running fan, using a timeclock to control the power to the unit saves fan energy, whereas a setback thermostat allows the fan to keep running. In some cases, it is fairly easy to install a setback thermostat so that it controls the operation of the entire unit.

Setback thermostats cost more than timeclocks, but the labor cost of installing a setback thermostat in the thermostat circuit usually is less than the cost of installing a timeclock in the main power circuit. On the other hand, you could install a timeclock in the thermostat circuit.

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