A furnace fueled by Natural Gas or Propane typically uses the forced air style of heating. This means the air is heated in the heat exchanger of the furnace itself and is then forced through the attached duct-work to the vents that feed the home.
A climate control device (ie. Thermostat) regulates and controls the usage of the furnace. A digital thermostat can be programmed to activate the system at predetermined times. This helps to conserve energy and expenses making the system more efficiency.
Older types of gas-fired furnaces lose significant amounts of energy in the hot waste gases exhausted up the chimney. More modern high-efficiency furnaces condense the water vapor emitted as one of the products of consumption. This increases the efficiency of the system to over 90%.
The two most popular types of residential gas furnace are Natural Gas and Propane. While Natural gas is typically delivered to the home by means of a main gas line submerged a few feet beneath the earth, propane is typically stored onsite in regulated tanks ranging in size from 100lb to 1000lb in larger applications and is delivered by truck.
Also consider that heat pump add on options are available for both types of gas-fired furnaces. They are simple to install and can improve the efficiency of your home by reducing the amount of gas you consume. When a heat pump is added to these systems the gas portion then becomes an auxiliary or back up heat and will only operate when the heat pump cannot absorb enough heat from the outside air to properly heat the home, this normally occurs at temperatures below -15C. Another added benefit to a heat pump add on is air conditioning which normally would not be a feature associated with a gas-fired furnace.