Heil makes gas furnaces which work by burning gas, typically propane or natural gas, to heat air in a heat exchanger. Once the furnace heats the air, it is spread throughout the home through a forced-air heating distribution system.
Furnaces like those manufactured by Heil typically have a pilot light in the combustion chamber that always remains lit due to a steady but small flow of gas. When the thermostat detects that the temperature in the home has dropped below the minimum, gas flows into the combustion chamber and ignites, and the blower, the device that pushes air through ducts and into living spaces, turns on.
Heil produces furnaces with single-stage, two-stage and modulating operation. In a single-stage furnace, the heat is either on or off, while a two-stage furnace has high and low heat settings.
Two-stage operation is more energy-efficient because it allows the furnace to respond incrementally to heat needs. Modulating operation is even more efficient. In this design, the furnace has numerous heating stages and runs nearly constantly during cold months. This has the added bonus of circulating air through the furnace's air filter constantly, leaving the home with cleaner air. Additionally, some Heil models have variable speed blower motors, which also improve efficiency.