Using Setback Heating/Cooling Thermostats to Control Conditioning Heating Units

If a space is occupied on a predictable schedule, you can use either timeclocks or setback thermostats to turn the conditioning units on and off. A setback thermostat is a heating thermostat/ cooling thermostat with an internal timeclock that changes the space temperature settings at different times.

A setback thermostat normally operates through the thermostat controller circuit of the conditioning heating units. A timeclock is normally installed to control the power to the entire conditioning heating units. However, you could install a setback thermostat so that it controls the power to the entire unit. Or, you could install a timeclock to interrupt the thermostat controller circuit alone.

Where to Use Timeclocks and Setback Heating/Cooling Thermostats

If the space served by a conditioning heating units 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 energy management system. This inevitably leads to dissatisfaction.

Don’t use temperature setback if the thermostat controller 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 cooling/ heating thermostat were used with this type of system, switching to the lower night time temperature would consume energy to cool the space to the night time temperature, and to maintain that temperature until the setback cooling – heating thermostat switched back to the daytime temperature.

If the conditioning heating 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 and heating 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 Cooling/ Heating 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 heating-cooling thermostat is that it restarts the conditioning heating units 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 heating units 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.