Gas furnaces are the most popular type of furnace installed in residential homes. They are equipped with many controls to make sure safe and efficient operation. It is important to check the furnace specifications and manuals to properly understand how the controls work. Here are five major furnace controls found on a residential gas furnace.
Gas Pressure Regulator
Because the gas from the main line is subject to different pressures, a gas pressure regulator ensures a consistent gas pressure into the burner manifold. This is usually located in between the supply tank and the main gas valve. This ensures your furnace receives the right amount of gas to burn; not too weak and not too much.
Main Gas Valve
Just like a gas pressure regulator is needed so is a main gas valve. Its main function is to control the flow of gas from the outside supply line or storage tank to the gas burner. They come in many different shapes and sizes and usually located behind the front panel of the furnace. The valves will include a safety shutoff valve that allows you to turn off the main gas line and the pilot gas. In a modern furnace that does not have a standing-pilot light, these main gas valves are called electronic ignition gas valves.
Mercury Flame Sensor
You will find a mercury flame sensor in a newer furnace with an electronic ignition system. It is made up of a mercury-filled sense end, a capillary tube and an SPDT switch assembly. The sensor end is filled with mercury and heats up from the direct burner flame. When heated, the mercury liquid triggers the switch and opens the gas valve.
A room thermostat is the main control of when a furnace operates based on the temperature of the room. A thermostat senses changes in air temperature that is being heated and send an electrical signal to open or close the automatic gas valve of your furnace. Most thermostats are wired with the pilot safety valve, limit control and the automatic main gas valve.
A thermocouple is the safety device used to detect whether the pilot light is is lit before the main gas valve is opened to supply gas to the burners. A Thermocouple uses a heat-sensing device that senses the heat from the pilot light and then converts it into a electric current allowing gas into your furnace to ignite.
Sometimes called a thermopile generator or millivolt generator, a thermopile is another safety device for the main gas line and pilot light. Just like a thermocouple, the heat-sensing device is used to detect the pilot light is on. A thermopile is larger and delivers more electricity than a thermocouple. The advantage of a thermopile is it does not require a transformer.