Friday, August 6, 2010

Communicating Control Systems

The standard air conditioning control system uses relay logic or an electronic representation of relay logic. Things are either off or on. The controls work something like a light switch – when the switch is on the light operates, when it is off the light is off. Thermostats are basically switches that are controlled by temperature. The thermostat closes a set of contacts to complete a circuit to a relay coil, the relay coil then closes the relay contacts to complete a circuit to a motor. When the thermostat is satisfied its contacts open, breaking the circuit to the relay coil. The relay opens its contacts, breaking the circuit to the motor. Everything works based on the presence or absence of control voltage. One advantage of this system is that it is easy to understand, and it has been the basis for HVAC/R controls for many years. However, this control system needs a separate control wire for each function. Some split system heat pumps require 12 control wires running between the indoor and outdoor section. Even with twelve wires, the range of control is still somewhat limited.

A communicating control system is more like a computer network. The system components communicate over a serial network. Each part has its own unique electronic signature or address, allowing the controller to recognize all the parts and coordinate their operation. Most residential communicating systems use four wires between all components – two for power and two for communication. It does not matter if the component is a furnace, air handler, air conditioner, heat pump, or zone control – everything uses the same four wire connection. This works because the units respond to commands sent over the communication network, not the presence or absence of control voltage. Commands for different components can be sent over the same network. Further, communication is not just one way. Communication between components allows them to know what the other components are doing and adjust accordingly. For example, Carrier’s Infinity zone control knows what the blower CFM is and what the CFM in each zone is. Better yet, it can report this to the service technician. The system airflow can be ramped up or down to match system capacity. Staged furnaces, air conditioners, and heat pumps give the system the ability to modulate system capacity and airflow as the house load requires, improving efficiency and reducing energy use. Just a few years ago Carrier was the only game in town if you wanted a residential communicating system. Some folks were not paying too much attention because these systems were only offered in high end equipment made by one equipment manufacturer. Today it seems like everyone is offering a communicating system. Equipment manufacturers advertising communicating systems now include Carrier, Bryant, Trane, American Standard, Lennox, York, Rheem, Ruud, Amana, and Goodman. I apologize in advance for leaving anyone off. The point is that communicating controls are booming. Communicating systems used to be proprietary, now there is an open standard called ClimateTalk. The ClimateTalk will encourage the development of equipment that plays nice across different brands. I don’t believe we are at the point of interoperability yet, but that day is coming. Besides the proprietary systems built exclusively for a particular brand, two major control manufacturers are producing communicating systems – Emerson and Johnson Controls. The availability of “standard” components and an open protocol means that smaller manufacturers can use communicating controls, opening the market up even more. In the very near future, every technician will need to work with communicating controls regardless of the brand equipment their company sells and services.

For more details regarding communicating control systems check out Unit 52 Heat Pump Installation, and Unit 59 Zone Control Systems in Fundamentals of HVAC/R. Many manufacturers also have some information online regarding their communicating systems, including


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  2. Will a communicating thermostat make a multi-speed single stage furnace like ICP N8mxl work any more efficiently? Or are the different blower speeds controlled only by the furnace CPU board.

  3. angle valves in piping systems hence experiencing a number of changes in flow direction of fluid. window air conditioner