HVAC Control System - the old-fashioned way

In the example above a hypothetical thermostat is either connected by wire or by wireless RF to an HVAC Controller module that regulates a fan. If an RF connection is used then the protocol is likely to be proprietary.

Note the following:

• The system is logically “close-coupled” – i.e. the thermostat has a dedicated connection to its associated HVAC Controller in a one-to-one relationship. The two units are said to be “aware” of each other because they co-exist as master and slave. You set the temperature at the thermostat and it controls the fan.
• The system is a “closed” architecture since it does not readily lend itself to expansion. For example, you can’t easily add another HVAC Controller and fan and have it controlled by the same thermostat if you find that a single fan cannot handle the load requirement. Likewise, you can’t easily add another thermostat if perhaps you want the fan to respond to temperature changes in more than one locality.
• There’s an inherent lack of dynamic flexibility in this implementation. If you decide that you want, say, other machinery in the local area to turn on or off if the temperature reaches a pre-defined limit, or for the HVAC system to respond to external events (from other devices or remote human control or even a time-schedule) then that can’t easily be done. The system is controlled at the thermostat and that’s pretty well it.

HVAC Control System - the BeePlex way

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