The thermostat doesn&#x27;t actually control the plant directly.  It requests heat or AC by closing a connection.  The HVAC controller responds to those requests as it is able to as per its programmed parameters.<p>You&#x27;re absolutely right that furnaces aren&#x27;t designed to run 24&#x2F;7.  But your thermostat can request heat 24&#x2F;7 and unless the furnace control board is broken, it&#x27;ll the cycle the system as necessary to keep up with the requests.<p>Most people never learn this because the systems are designed for peak needs and most days don&#x27;t get anywhere near peak.