Abstract: An HVAC system is provided with control communication over a serial data bus. In this manner, the hard wired controls of the prior art are eliminated. A thermostat includes a central control microprocessor that communicates control signals to and from a microprocessor at an indoor unit. The indoor unit may be a furnace or a fan/heater combination. The microprocessor on the indoor unit is operable to receive signals from the central control microprocessor and control the indoor unit accordingly. Moreover, the microprocessor at the indoor unit is operable to pass control signals on to an outdoor unit such as an air conditioner or heat pump. Most preferably, this outdoor unit is provided with its own microprocessor. Further, other peripheral units may be incorporated to be controlled over the same data bus from the thermostat. Installation and updating of HVAC systems is greatly simplified by this control arrangement.

Abstract: An electric heater is provided with an identifying resistor associated with a connection between the heater and a fan control. When the electric heater is connected to the control, the identifying resistor provides information with regard to the capacity of the particular electric heater that is being attached to the control. The control then utilizes this particular capacity to control both the electric heater and an associated fan motor.

Abstract: A system for self-configuring complex HVAC systems has each of several units provided with a microprocessor control. The several units are each available in various optional styles, sizes, etc. The microprocessors provide information to a central control of the particular characteristic of the particular unit. Thus, when the HVAC system is initially assembled, the microprocessors associated with the individual units report these characteristics to the control. The control then determines the characteristics of each of the individual units, and accesses control strategies for the combination of individual units that are being utilized in the particular HVAC system.

Abstract: An electric heater is provided with an identifying resistor associated with a connection between the heater and a fan control. When the electric heater is connected to the control, the identifying resistor provides information with regard to the capacity of the particular electric heater that is being attached to the control. The control then utilizes this particular capacity to control both the electric heater and an associated fan motor.

Abstract: A blower control circuit for a furnace includes a blower relay having normally-open contacts connected in the power circuit to the blower. Energization of the relay via a drive circuit serves to close the contacts and thereby operate the blower. Control of the drive circuit resulting in relay energization is effected by either a time-delay circuit which responds to a space thermostat or a safety sense circuit which responds to a high temperature limit thermostat.