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.
It is vital that your return vents are located in each room so that proper
ventilation can be maintained. Otherwise, the rooms would become pressurized,
thus preventing any more warm air from entering the room. This causes the
furnace to over heat, consume more energy than what is required and lowers the
efficiency of the system.
Once the temperature of the room has reached its optimal setting, the thermostat
will then turn off the heat, the fan-and-limit control will still monitor the
firebox. Once most of the residual heat has been forced into the home, the
control lets the fan shut off. Then when the temperature of the room falls below
the set temperature, the cycle begins again.
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