Environmental conditions such as temperature or humidity are generally controlled or affected in buildings by a single environmental control unit, such as a furnace or an air conditioner. The operation of the environmental control unit is regulated through a closed-loop feedback control system in which a sensory device senses the environmental condition in a particular area, compares the environmental condition to a preset value, and energizes the environmental control device when the sensed environmental condition is less than the reference value. For simplicity of explanation, the preferred embodiment is explained with reference to the environmental condition being temperature. However, it will be understood that the principal of the invention is applicable to any other environmental condition.
In a conventional heating or cooling system, a thermostat is mounted in a room which is heated and/or cooled by a central furnace or air conditioner. A system of this type is capable of accurately regulating the temperature in the room in which the thermostat is located. However, it is incapable of accurately regulating the temperature in other rooms that are heated and/or cooled by the furnace or air conditioner since these other rooms may be subject to different heat losses or gains. Also, the coupling from the furnace or air conditioner to each room may vary so that operation of the furnace or air conditioner to maintain the temperature constant in the room in which the thermostat is located may have greater or lesser effects in other rooms.
The inability to adequately regulate the temperature in rooms other than the rooms in which the thermostat is located presents the greatest problem when the coupling to the room in which the thermostat is located varies. In the most serious case, one may wish to prevent the heating or cooling device from heating or cooling the room in which the thermostat is located when such room is not being used. However, doing so virtually breaks the feedback path controlling the operation of the heating or cooling system. This is because the heating or cooling device will have little, if any, effect on the temperature in the room in which the thermostat is located.
The key to solving the above-described problem is to reposition the thermostat from room to room, depending upon which rooms are either occupied or closed off from time to time. However, existing thermostats are connected to the heating or cooling device by wires, thereby precluding easy portability. Although the use of a radio link as a substitute for the wires may appear possible, problems associated with the expense of a radio link, the difficulty in providing a link which does not interfere with other such radio links, and the high power consumption of a radio link appear formidable.
Another, somewhat different problem associated with thermostats for heating and cooling devices stems from the need to supply power to some thermostats through lines which, of necessity, are shunted when the heating and cooling device is energized. For example, many thermostats include an internal timer for disabling the heating or cooling system during certain portions of the day or evening. These timers are conventionally powered by the AC signal on the control lines when the lines are not shunted. Most gas, electric and oil furnaces as well as central air conditioners are controlled through a pair of terminals which receive a relatively low-voltage AC signal. An energizing relay is connected in series with the terminals so that it is actuated by shunting the terminals with a relatively low-impedance circuit element, such as a switch. Shunting these terminals reduces the voltage thereacross to zero so that the thermostat is incapable of receiving power through the terminals when it is energizing the heating or cooling device. Consequently, existing thermostats are powered by either a separate power supply or, most commonly, an energy storage device, such as a battery, charged by the AC signal when the terminals are not shunted. The energy storage then supplies power to the thermostat when it is shunting the terminals to energize the heating or cooling device. This latter approach is satisfactory as long as the energy storage device is capable of storing sufficient energy during the "off" time of the heating or cooling device to last through the "on" times. With a relatively high ratio of "on" to "off" time, this is not always possible. Moreover, the problem cannot be solved by merely storing energy at a faster rate since the increased current flow into the energy storage device might energize the relay controlling the heating and cooling device.