We’d need a few more details.
A virtual switch is just something that SmartThings will recognize and treat like a regular physical switch even though it only exists in the software. But because it’s not a physical device, it can’t really do anything on its own. There has to be something else that triggers it.
In the case of an HVAC system, that means there has to be some way for SmartThings to know that the HVAC system has changed state. So the HVAC System itself has to communicate with SmartThings. So the first thing we would need to know is the brand and model of the HVAC system.
If you just asking how to create a virtual switch, here’s the FAQ for that.
But as far as associating the virtual switch with the HVAC, The first step is to determine how the HVAC unit will communicate with smartthings independent of the existence of the virtual switch. In some cases this might be through an intermediary like IFTTT, it might be through a direct connection such as with a Z wave thermostat, or it might be through a cloud to cloud connection like the ecobee.
Once both the real HVAC and the virtual switch are known to smartthings, you can set up rules that will associate them, either through smart lights, through rule machine, or through one of the other smart apps.