Programmable thermostat

A programmable thermostat is a thermostat which is designed to adjust the temperature according to a series of programmed settings that take effect at different times of the day. Programmable thermostats are also known as setback thermostats or clock thermostats.

Benefits

Heating and cooling losses from a building (or any other container) become greater as the difference in temperature increases. A programmable thermostat allows reduction of these losses by allowing the temperature difference to be reduced at times when the reduced amount of heating or cooling would not be objectionable. For example, during cooling season, a programmable thermostat used in a home may be set to allow the temperature in the house to rise during the workday when no one will be at home. It may then be set to turn on the air conditioning before the arrival of occupants, allowing the house to be cool upon the arrival of the occupants while still having saved air conditioning energy during the peak outdoor temperatures. The reduced cooling required during the day also decreases the demands placed upon the electrical supply grid. Conversely, during the heating season, the programmable thermostat may be set to allow the temperature in the house to drop when the house is unoccupied during the day and also at night after all occupants have gone to bed, re-heating the house prior to the occupants arriving home in the evening or waking up in the morning. Since (as a matter of sleep hygiene) people sleep better when the bedroom is cool, and furthermore the temperature differential between the interior and exterior of a building is the greatest on a cold winter night, this reduces energy loss. Similar scenarios are available in commercial buildings, with due consideration of the building's occupancy patterns. According to Consumer Reports magazine, programmable thermostats can reduce energy bills by about $180 a year.

Controversy While programmable thermostats may be able to save energy when used correctly, little or no average energy savings has been demonstrated in residential field studies. Difficulty with usability in residential environments appears to lead to lack of persistence of energy savings in homes. According to the US EPA regarding residential programmable thermostat, "Available studies indicate no savings from programmable thermostat (PT) installation. Some studies indicate slight increased consumption." This is supported with studies by Nevius and Pigg, Cross and Judd and others and Peffer et al. has a recent review of the topic. In addition to potential increased energy consumption, digital programmable thermostats have been criticised for their poor usability. Several studies have found that digital programmable thermostats are difficult for users to programme and older people in particular can struggle to use them (see Combe et al.). It has been noted that the use of programmable thermostats is hampered by misconception about the setback feature, reducing the amount of heating or cooling in a building needs for a short time (e.g. at night or when it is unoccupied). The belief is that if the building is allowed to change temperature, its heating or cooling system has to "work harder" to bring it back to a comfortable temperature, counteracting or even exceeding the energy saved during reduced heating or cooling. If set up correctly the setback and recovery feature can result in energy savings of five to fifteen percent as the heat transfer between a structure and its environment is proportional to the temperature difference between the inside and outside of the structure.

Construction and features

Clock thermostats

The most basic clock thermostats may only implement one program with two periods (a hotter period and a colder period), and the same program is run day after day. More sophisticated clock thermostats may allow four or more hot and cold periods to be set per day. Usually, only two distinct temperatures (a hotter temperature and a colder temperature) can be set, even if multiple periods are permitted. The hotter and colder temperatures are usually established simply by sliding two levers along an analogue temperature scale, much the same as in a conventional (non-clock) thermostat. This design, while simple to manufacture and relatively easy to program, sacrifices comfort on weekends since the program is repeated each of the seven days of the week with no variation. To overcome this deficit, a push-button is sometimes provided to allow the user to explicitly switch (once) the current period from hot period to a cold period or vice versa; the usual use of this button is to over-ride a "set back" that takes place during the workday when the home is normally unoccupied. The clock mechanism is electrical. Two methods have commonly been used to operate it: [1] A separate, continuous source of 24 volts alternating current (24 VAC) is provided to the thermostat. [2] A rechargeable battery in the thermostat operates the clock. This battery charges when the thermostat is not calling for heat and 24 VAC is available to it. It discharges to operate the clock when the thermostat is set for heating or cooling.

Digital thermostats

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