HVACR Time Control Basics

Learn the basics of how time controllers are used in the HVACR industry to control heating and cooling.


Learn the basics of how time controllers are used in the HVACR industry to control heating and cooling.

Scroll to the bottom to watch the YouTube tutorial.

Time control allows systems and components to turn on and off at certain times as well as for certain duration of time. Or we can delay something turning on or off or a certain amount of time. For example, on a cold morning we want the heating to turn on before we wake up so that the house is nice and warm. A simple time control uses a cam to push contacts together and complete a circuit. As the time dial rotates it will force the cam to open and close the contacts to turn the heating on and off.

Timers used to be all mechanical however these days timers are usually electronic and typically use relays to open and close circuits.

By the way we have covered how relays work in our previous article, click HERE to read.

Electronic controllers are compact and much easier to use. They have far more functions but usually at least a 7 day time schedule with multiple on off times per day. This is useful for example in a small office which is closed on weekends. So during the winter the heating is scheduled to turn off on weekends and then it starts slightly earlier on a Monday morning because it will take longer to warm the building up. A more sophisticated system typically used in much larger buildings will use an optimizer and for this example we have it connected to a programmable logic controller. This will check with the clock should the heating on today and if so at what time will the building be occupied. The clock replies yes, the scheduled occupancy time is 9am, the PLC checks the  current temperature of the room and calculates the difference between this temperature and the desired temperature. It then checks the outdoor temperature to calculate how long it will take to heat the building because on a cold day it will take longer so it will need to start earlier.


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