Understanding Forced-air Systems
About forced air systems
If you have air vent, which deliver warm or cool air throughout your home, then you have a forced-air system. These systems can use an oil or gas furnace to heat the air, which is then sent throughout the house with the use of a small fan. In addition to the oil, or gas furnace, and fan there are other controls, which work in harmony so that your forced-air system will deliver the appropriate amount of warm or cool air.
The first component is the thermostat, which when the temperature of the room falls below the set temperature, the thermostat communicates with the furnace so that more heat is sent into the room. Generally, the thermostat operates on 24-volts and is connected to one side of the blower relay switch. The blower relay is the switch that turns the fan on or off. The fan is connected to the other side of the blower relay switch and uses 110-volts of electricity.
The next component is the fan-and-limit control, which prevents the fan from starting until the prescribed temperature of the firebox is reached. This is usually about 115oF, or 46oC. Once this temperature has been reached and the air begins to circulate then the fan forces the air into a large central vent known as the plenum. If you look at your forced-air systems, the plenum would be the vent or box where all of the vents throughout your home are attached. The heated air is then forced throughout your home, as the cooled air is returned to the furnace through the cold-air return vents.