6 Major Furnace Controls

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.

Aviation Weather: Wind

Wind plays an important role in aviation; the movement of an aircraft is greatly affected by the wind and its severity. The weather is also dependent largely on the wind and its circulation in the atmosphere. Wind movement is caused by pressure differentials between different geographical regions over the Earth, and also due to the Earth’s own rotation. In simplest terms, the horizontal movement of the air over the surface of the Earth is called the wind. It is expressed in terms of the direction from where it is blowing, along with its speed or strength. The direction coupled with the speed is known as the Wind Velocity.

Measurement of the wind

Near the surface of the Earth, the wind is affected by friction with the surface and other obstructions or ground features. The Anemometer is the instrument that is used to measure the surface wind velocity; for getting accurate readings, it is placed in open areas along with wind wanes near the airports and runways. The most common types of wind measuring Anemometers are Cup Anemometers; however, Pressure Tube Anemometers are also used to calculate winds and they work on the same principle as the Pitot tube in an aircraft. Both these types of instruments have certain limitations and lack complete accuracy in some conditions. The wind direction is always mentioned with reference to True North; whereas the wind speed or velocity is expressed in knots. The upper air winds are measured through wind balloons and other methods.

Layers of Winds

Aviators generally differentiate winds into three basic categories as:

  • Winds below 2000 feet above ground level
  • Winds at 2000 feet above ground level
  • Winds above 2000 feet above ground level

Winds at 2000 feet is taken as the differentiating criteria since it is the level where the winds are free from the effect of surface friction of the Earth. 2000 feet is also the approximate level for the universal pressure distribution unaffected by the geography of the landscape below. At and above 2000 feet the Geostrophic Force and the Pressure Gradient Force affect the wind. The difference in pressure between areas cause the Pressure Gradient Force, and the Earth’s rotation causes the Geostrophic Force. The Geostrophic Force deflects the objects moving over the surface of the Earth, and is due to Coriolis Force.

Coriolis Force is a resultant force caused due to the rotation of the Earth on its axis. The Coriolis Force has no effect near the equator, but tends to deflect objects at higher latitudes, to the right in the northern hemisphere and to the left in the Southern Hemisphere; when viewed from the equator.

Wind Considerations during Flying

A pilot should be aware of the affects of head wind, tail wind, and the cross wind on an aircraft during all phases of the flight; however, takeoff and landing are the most critical phases for considering the wind. All Takeoffs and landings are planned into the headwind, the tolerance for a tailwind takeoff or landing in most aircraft is minimal. Similarly, each aircraft is limited by a certain severity of the cross wind factor that it cannot exceed during takeoff and landings, otherwise the safety of the aircraft may be impaired.