A gas furnace is a type of forced air system to warm your home in the winter. Several parts carry out the process in a sealed system. There are two airflow directions to complete the cycle repeatedly in order to produce warm air.
A gas furnace has a metal exterior casing that encloses all of the working components. There is the main gas supply line that leads to the ignition system, which may be a pilot light or an electronic ignition. These light the burner and create heat within the heat exchanger through the air movement. When the air is warm, it enters a plenum or air duct exhaust and travels through the damper to the rooms of your home where it exits through the heat registers.
Airflow in and out of your gas furnace allows it to operate. The beginning of the heating cycle starts as air is pulled out of each room containing a return air duct or vent. The air then travels through a sealed air system of duct work to the furnace to begin the warming process and repeats itself until the air is warm enough to switch off the thermostat.
The duct work for both entering and exiting the gas furnace is an integral part of proper functioning. Ducts are metal with a covering of insulation or flexible plastic insulation to retain temperatures of the air within them. Any leaks in the duct work will draw cooler air into the heating system, which will take more energy to effectively warm the space and therefore increase your electricity usage.
It is important that the size of a gas furnace match the size of the space you are heating. If it is too large for the area if will short-cycle on and off repeatedly and can lead to spikes in electricity as well as wear on internal furnace components. A system that is too small will run constantly in cold weather and may not be able to produce enough heat to warm your home.
There are websites that can help you determine what size gas furnace to purchase. The determining factors are the size of area to warm, your location and the efficiency of your home's insulation. A well-insulated home requires a smaller unit because heat can be lost through leaky windows and doors.