When the first air conditioning systems came in, they were simple mechanical “machines”, with very few electrical components, just for making it work. There were no options for external control/integration. The only way of controlling them was by “cutting” off the HVAC unit power. Therefore, the only way to integrate HVAC systems was by adding a controllable (on/off) relay on the power supply line to the HVAC unit.
Later on, small split systems that entered the markets brought in a new control capability – the IR. Similar to TVs, Home Automation controllers “learned” the codes and started controlling those small split systems by their own IR transmitters, as an alternative to the original HVAC remote. Of course, it wasn’t ideal, since it was “one-way” integration, but it was much better than nothing those days.
The communicating thermostats that became very popular provided additional integration options, but they were mainly used as a stand alone solution.
How do we do it today? What is expected in the Future?
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Wth the penetration of the inverter type HVAC systems to the world markets, the situation has dramatically changed: The existing solutions (power line cut off and communicating thermostats) became irrelevant as an integration option.