Hi, are all HVAC units the from the same manufacturer?
If so have you checked with them to see if they have a ready plug in network system that will do what you ask.
The local health establishment where I live, about 15 years ago went through a major rebuild/refubishment of building and HVAC systems, over 3 separate major locations around the city.
I know the designer that was involved in implementing the entire HVAC system, he used the ethernet system and PLC's.
About 5 years ago he emigrated to the USA to work for that PLC company on there hotline and teaching department.
So the job is not a little one.
Tom..... 