Thermostats for HVAC (heat, ventilation, air conditioning)

August 9, 2009

A thermostat is an electronic switch much like a light switch, i.e., the switch is either ON (contacts closed) or OFF (contacts open). In a thermostat this is done several ways.

1. The old mercury thermostats use a vile of mercury. When the vile is tipped one way, the mercury closes two contacts at that end by touching them both. When the vile is tipped the other way, it opens those two contacts. The vile of mercury is connected to a bi metal spring that reacts to temperature tipping the vile back and forth. The use of mercury is no longer acceptable, but there are many still in use.

2. Dispose of mercury very carefully as it is extremely dangerous. Call your local recycler to find a disposal station.

3. Some mechanical thermostats use a bi metal device that causes the two contacts to touch or open depending on temperature. Modern electronic thermostats use what is called a magnetic latching relay. This is a device that has two contacts that close when a coil on the relay is pulsed with a voltage. To open it, a second coil is pulsed and the contacts open. The voltage pulses are generated by the electronics inside the thermostat. The electronics is what makes thermostats different from each other. The purpose of most thermostats is to close two contacts to turn on an HVAC function and to open them to turn off that function.

There are 3 main power sources used for switching HVAC functions on and off, Gas
Millivolt (750mv), 24VAC and line voltages of 115VAC or 230VAC.

1. The Gas millivolt system requires the thermostats to switch only about 750Mv (3/4 volt) to turn on and off the gas heat. Any 24VAC thermostat can be used.

2. The 24VAC systems require the thermostat to switch 24VAC. All low voltage thermostats will easily switch these two systems on and off. This is the most popular. These cannot be use for line voltage heating or cooling.

3. The 115VAC or 230VAC is used for electric baseboard, wall heaters, or ceiling heat; this takes a special “LINE VOLTAGE” Thermostat.

If the old thermostat has 2 or 4 large, stiff wires, coming out of a plastic or metal box in the wall (J box), it is likely the system is 115/230VAC, and must use a line voltage thermostat. Connecting line voltage to a 24VAC thermostat is dangerous and will severely damage the thermostat. Always shut off power before replacing any thermostat. Standards for identifying low voltage thermostat wiring are set by the National Electrical Manufacturers Association (NEMA) Standards Publication DC 3-2003. These standards set down what letters to be used for what function in thermostats. Though many thermostats adhere to those standards, some thermostats do not. Below is a wiring chart to help compare wiring letters in most systems. As we discuss various HVAC systems, we will be using the NEMA standard letters for our hermostats. Refer to this chart when replacing old thermostats that do not have the same letters as the Ritetemp thermostats. There is no standard for wire colors.

Line Volage Single Pole Heating (2 Large Wires)

The Single Pole type will only have two large wires coming out of the thermostat.

Single Pole: Just one side of the power goes through the thermostat the other side of the power goes directly to the heating element


Max Rating at 230VAC at 16 AMPS (about 3680 Watts)
Max Rating at 115VAC at 16 AMPS (about 1840 Watts)


One Response to “Thermostats for HVAC (heat, ventilation, air conditioning)”

  1. husle Says:

    Nice description. Programmable thermostats have taken the HVAC community by a storm with their ability to significantly reduce energy use when the air conditioning blower is not needed. The next big step in thermostat technology is zone control, where you can not only program the system to turn on and off, but you can set what areas turn on and off and what areas need what kind of ventilation.

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