A basic, off-the-shelf, thermostat employs one wire to command each power, fan, heating and cooling operation in a HVAC system. It is an inexpensive, low-tech way to put comfort control on a customer’s wall.
But the problem with a basic, direct-wired thermostat is that up to 18 separate wiring connections may be necessary in order to reap the full benefits of the heat pumps, furnaces and air handlers deployed in a home. Installers must spend a lot of time to make sure each connection is properly wired and the system is properly configured. And after making all those connections, the customer is still left with a thermostat that is only capable of turning components on and off. On-off cycling is an inefficient way to control HVAC equipment — even if done with a programmable thermostat that gives the customer numerous scheduling and setback options.