1. Field of the Invention
The present invention relates to thermostats. More specifically, the present invention is directed to a plural zone control system using respective zone thermostats.
2. Description of the Prior Art
Conventional heating and cooling systems for residential buildings are typically controlled with a single thermostat. Thus, only one temperature can be specified by the homeowner. The temperature in the vicinity of the thermostat will be controlled to the desired level, but in other parts of the house the temperature can vary widely due to inadequate air distribution, solar radiation entering through south facing windows, outdoor wind, heat generated by people and appliances and many other factors. In response to these problems and the desire for greater comfort and flexibility, so-called "zoned" systems have been developed. A zoned system allows different parts of a residence to be controlled at different temperatures or at equal temperatures. This is accomplished by a separate thermostat in each temperature zone and a means for controlling the flow of heat or cool to that zone. The most common heating/cooling system is the so-called forced warm air system. In a zoned system of this type, dampers are placed in the ductwork to allow the airflow to each zone to be turned on or off or to be modulated.
Although the zoned heating/cooling system described above offers the advantages of greater comfort through the more accurate control of temperature, greater flexibility by allowing the homeowner to set different temperatures through the house and the potential for energy savings by discontinuing the heating or cooling of unoccupied areas of the home, these systems also have some disadvantages. In its most common form, a zoned system allows each individual thermostat to turn on the heating/cooling plant (furnace, air conditioner, etc.) and simultaneously open the corresponding zone damper while closing the dampers to other zones not requiring heating/cooling. While this approach is quite simple and easy to implement in practice, it creates the following problems. With several individual thermostats independently turning the heating/cooling plant on and off, this appliance will experience significantly more cycles than normal and the service life of the plant will therefore be shortened. For example, without coordinating the calls for heating or cooling, the potential exists for short-cycling the plant, e.g., thermostat A might shut off the furnace, but thermostat B turns it back on 15 seconds later). These short cycles can damage the heating/cooling plant.
Further, when only one zone thermostat requests or calls for heat, the entire output of the heating plant (which is normally capable of heating the entire house) will be supplied to that zone causing large fluctuations in air temperature. When only one zone thermostat calls for heat, the air flow rate through the heating plant may be reduced because the ductwork to that zone cannot conduct the full flow capacity of the blower. This may lead to excessive temperatures in the plant and may cause the high temperature safety switch to trip repeatedly. On the other hand, during cooling operation, the reduced airflow can lead to ice formation on the cooling heat exchanger and similar safety and reliability problems. The present invention is directed to a zone control system that addresses the problems discussed above by allowing multiple thermostat requests to be coordinated such that the air flow through the system is maximized, and the on/off cycles of the plant are minimized.