Running a central air system, also known as an HVAC system, without gas occurs, although it's not how the system is designed to be used. HVAC systems incorporate a dual system design that, depending on the temperature desired, shifts between putting out heat or cool air. Having no gas flow essentially makes one-half of the system unusable (the heater).
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When Gas is Used
Natural gas in an HVAC system kicks in when the heater side of the system begins to be used. Electrical sensors turn the system on and off. When the temperature setting falls below a certain setting, the gas flow will be turned on in the furnace generating flames, which in turn creates heat.
What Happens With No Gas
A central air system can run without natural gas, but when the heat needs to be turned on it won't work. The heater side of the system requires gas to generate the flames and subsequent hot air to blow through the system. Without the gas, the system will simply blow cold or existing air without heat.
Worrying With Gas But No Flame
A more worrisome problem occurs when the gas does flow but the furnace in the HVAC system does not turn on. This means the pilot light is likely extinguished and needs to be relit. However, if the air in the home is not first vented, lighting a flame with built-up gas could cause an explosion.