Commercial installations often have two-stage thermostats that automatically change over from heating to cooling. Typically there ’s a 10 or 15°F range that is fixed into the controls, so that if the space temperature drops 15° below the cooling setpoint, the unit switches to the heating mode, and vice versa for cooling.
Many spaces can have a greater range of the changeover controls, such as storerooms, mechanical rooms, shops, and equipment rooms. This is not possible with a regular two-stage thermostat, which can’t adjust the range between the two setpoints. This adjustment can, however, be obtained with a programmable thermostat. The space temperature is set to vary between the setpoints, and less energy is used to maintain space temperature.
This device permits a greater range of flexibility, and enhanced energy savings, in other two-stage applications. The switchover of a two-speed air-handler fan can be delayed to take advantage of thermal mass of a space, which emits radiant energy that cannot be measured by a thermostat. Recent studies have suggested that radiant heat makes a significant contribution to comfort. Lower space temperatures are acceptable if they coincide with an increase in radiant energy, such as from warmed exterior walls or windows. In such circumstances, the changeover can be adjusted seasonally for a more comfortable thermal environment, in most cases for energy savings too.
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Excerpt from Energy Conservation Projects.