How do you control hvac system from the computer?

Answer 1

First write a simple document that describes your goal; what do you want the control system to do. In a large scale computer control system that would be called the "functional specification".

next you will need sensors and computer controlled relays to control your AC from your computer.

Several manufacturers provide RS-232 Digital relays, and although there are, in fact many manufacturers, I have used the relays and serial data acquisition modules from B&B electronics and have found them to be of good quality and relatively inexpensive. Other manufacturers provide industrial rated devices that are much more expensive.

After selecting the digital I/O control equipment that you will be using, then you can create the computer program that communicates between the different devices and provides the functionality that you want.

After selecting the hardware, it's a good idea to resurrect the functional specification and start to fill in the specifics, for example, if you wrote in the functional specification that you want to turn on the fan motor, that's when you would determine the sequence that you would send to the data acquisition module to turn on the fan. This step saves a lot of trouble later so that you don't need to re-write segments of code because of a surprise that you encountered when reading about how to turn on a relay. In a commercial system, that document would be called a "detailed system design".

Once you've gone that far, just connect the equipment and write the code.

Have fun.