Traditional Air Conditioning Systems

The first individual air conditioning system was invented by H.H. Schultz and J.Q. Sherman in 1931. It was a window air conditioner unit with the control interface built in.

Later on, especially when the first split air conditioners came out, the thermostat became the most important part of an air conditioning control system. Basic electrical controls required for the operation of air conditioning components (such as fans and compressors) were mounted inside the air conditioner. All the operational logic, energy-efficiency algorithms, operational schedules, etc., were implemented in the thermostat, which had become the "brain" of the systems.

The first thermostats were simple mechanical ones, based on bimetals. They provided basic functionality, such as turning the system on and off, setting temperatures, choosing an operation mode (cool, heat, etc.), and adjusting fan speed. This was accomplished using simple buttons, toggles, and control "wheels".  The rapid demand for more sophisticated control and scheduling functions spurred the development of digital thermostats of various kinds, offering wide functionality, touchscreen capabilities, scheduling, and more.

These days, market demand expectations have changed once again, with many customers expecting a solution for connecting residential air conditioners to home automation systems. This led to the development of communicating thermostats that integrate with home automation systems through wired or wireless communication.

Thankfully, traditional air conditioning systems can be upgraded to meet customers' demands by simply adding a smart thermostat.

Modern Air Conditioning Systems

HVAC Communication Thermostat Control Wiring conceptual diagram

The increasing demand for saving energy has led to the development of new technologies in the HVAC industry, such as inverter-based compressors. This technology allows the compressor (the "heart" of the air conditioning system) to operate proportionally, regulating the amount of refrigerant used, resulting in more accurate climate control and, eventually, reduced power consumption.

Inverter-based compressors require more sophisticated control, involving frequency modulation, DC conversion, and more. To support these capabilities, larger electronic PCBs (Printed Circuit Boards) that must include all relevant electronic components are now mounted inside HVAC units (indoor, outdoor, or both).  The control flow is now completely different.

The electronic PCB becomes an active controller, gathering data from various inputs (temperature and pressure from the built-in sensors, outdoor temperature, and user input). Based on these inputs, the controller defines how the HVAC system should operate to meet demand.

The traditional thermostat, as a concept, has now become a simple control panel that is only required to fulfill the user's climate demands: operation state (On/Off), operation mode (cool/heat/dry), set point temperature, and desired fan speed.

 

The user input can be communicated to the HVAC unit in several ways:

  1. IRAir Conditioning Control Wiring Diagram
  2. Wired Remote
  3. Wireless (non-IR): Wi-Fi, Bluetooth, Zigbee etc. This trend is relatively new and is expected to become more widespread in the next few years.
The important part is that the user input interface (IR receiver and transmitter, or a wired remote) is provided by the same HVAC manufacturer, as an integral part of the unit or as an optional accessory. Traditional third-party thermostats cannot be used here, as each HVAC manufacturer has its own closed communication interface.

 

As HVAC manufacturers' initial philosophy was to meet all customers' needs for control, integrating these air conditioning units with home automation and building management systems (BMSs) became a more complex task.

The variety of HVAC manufacturers and models has created a situation in which each requires a specific approach and a different integration process. As this isn't a priority for most HVAC manufacturers, it is clear that 3rd-party integration platforms are needed to enable these HVAC units to communicate with home automation/building management systems in a satisfactory manner.

 

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Final Thoughts

In the near future, there is still room and a need for smart communicating thermostats as traditional residential HVAC systems are still manufactured and sold. There is also a growing retrofit market they can fit into.

However, in the long run, as HVAC manufacturers increasingly adopt inverter technology, demand for these thermostats will decrease, as manufacturers will provide their own control panels, and HVAC operation logic will be built into HVAC control systems. The control of HVAC systems from different brands and their integration into automation systems, however, is a challenge that is set to become increasingly prominent.

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