Nah, anything driven by relays and or pneumatic controls can be replaced with modern, low cost PLC hardware. It is common for old chemical plants to have their control systems replaced while still operating. Control is handed over from the old system to the new system (hot cutover) one control loop at a time. All you need is adequate planning.
Source: I literally just lead a project to upgrade the PLC controls on a few air handler units and build entire server and network infrastructure. We had to run Ethernet to each of these from the network closets because they previously had went to a Windows XP box in the maintenance guys office. There were many challenges, one being getting the configuration off a couple of the old PLCs to make the new ones even talk to them.
>but HVAC systems aren't just basic relays and pneumatics.
Old ones are.
Feel free to withdraw your overly aggressive comment any time you'd like.
The person you’re replying to was saying that the old method of HVAC/process control, prior to digital controls, was pneumatics and relays. Old HVAC control systems were indeed pneumatic systems, I’ve seen some in the wild in the last few years in the US.