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One of the most common ways to heat a home or other type of building, is with a gas-fired furnace. In contrast to an electric furnace, a gas fired furnace converts the stored chemical energy in methane rich natural gas to produce heat. While natural gas is cleaner than other fossil fuels, it still emits carbon when consumed, and so it is a greenhouse gas emitter. Its chief benefit is that it is often a cheaper alternative to expensive electric heating bills.

Natural gas is used to provide space heating by heating either air, in a forced-air system, or water, in a hydronic heating system. Both systems rely on either pumps or fans to push either the heated air or water throughout the building.

Basic Principle

A gas fired forced-air furnace operates on a similar principle as a central electric furnace. Both require a control circuit to sense the temperature of the space to be heated, and a fan to circulate air throughout that space. The furnace is usually located in the basement or crawl space of a house.

If the room temperature falls too low, the main thermostat clicks on and opens the main gas valve which allows the combustible gas to fill the heat exchanger. The inflowing gas is ignited via a pilot light, direct spark ignition, or an electrically heated hot-surface igniter similar to a diesel engine glow plug.  Once the temperature in the heat exchanger has risen to a sufficient level, a local thermostat or timer switch engages the blower fan.

An upper-limit safety thermostat is installed in the plenum chamber to disengage the main gas valve should the blower fan fail to engage, and the temperature rise too high.

The blower fan pushes the heated air through the ducting system and the building, usually via floor registers. Since hot air rises, it is always more efficient to install heat registers as low as possible and near places of high ambient temperature loss, such as windows and exterior walls or doors. Return-air registers draw cooled air back down to the furnace and the heat exchanger to be warmed again.

As the blower fan draws the return air, it pushes it through a filter before heating it again. This helps maintain healthy air quality levels in the building, and filters should be replaced regularly.

If a hydronic heating system is used instead of a forced-air system, a pump will push the heated water to radiators throughout the building to dissipate their thermal energy.

One of the by-products of combusting gasses to produce heat, is smoke, which needs to be vented safely away from the house. The heat exchanger keeps the toxic byproducts of combustion separate from the air that is circulated throughout the building. Most of the heat is transferred to the circulating air, but some still lost through exhaust. Older heat exchangers had an efficiency of roughly 50%, meaning half the heat produced was wasted up the chimney. Modern designs have improved efficiency to approximately 70% to 80%, and recent high-efficiency furnaces can achieve heat distribution efficiencies above 90%.