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Where the gas goes: seven checkpoints on every furnace.

A gas furnace burns fuel a few inches from the air you breathe. Here is the path that combustion takes — and the seven places we check that it stays on the right side of the wall.

The combustion path

Gas in, flue gases out, clean air to your rooms.

The combustion path through a gas furnace, with seven numbered safety checkpoints Gas comes from the meter through a valve to the burners. The flames heat a sealed heat exchanger. An inducer fan pulls the flue gases out through a vent to the outdoors, while combustion air comes in from outside. House air passes around the outside of the heat exchanger and goes to the rooms. A carbon monoxide alarm sits in the living space. Numbers mark the gas valve, burners, heat exchanger, inducer, vent, combustion air and alarm. Gas meter Combustion air Heat exchanger Burners Vent to outdoors Warm air to rooms House air returns Living space CO 1 2 3 4 5 6 7
Simplified drawing of a typical gas furnace. Older floor furnaces and wall heaters have the same checkpoints in a different shape.

  1. Gas supply and valve

    Gas pressure set to the furnace's rating, and every connection leak-tested.

  2. Burners and flame

    A clean, steady, mostly blue flame — and a combustion analyzer to confirm what's in the flue, not a guess.

  3. Heat exchanger

    The wall between burning gas and your air. A crack lets flue gases mix in, so it's inspected at every tune-up.

  4. Inducer and draft

    The fan or draft hood that pulls flue gases up and out. A pressure switch proves it's working before the burners light.

  5. Vent

    Intact, sloped the right way, clear of nests, snow and debris, and ending outdoors where it should.

  6. Combustion air

    A furnace needs air to burn cleanly. A closet sealed too tight or a blocked intake starves it.

  7. Carbon monoxide alarm

    The last line of defence, near sleeping areas. Colorado requires CO alarms in many homes with fuel-burning appliances or attached garages.

General guidance, not an inspection. If you smell gas, leave the building and call your gas utility or 911 from outside.

A technician holding a combustion analyzer probe in the flue pipe of a gas furnace in a basement
On every heating visit

Measured, not eyeballed.

A flame can look fine and still be making too much carbon monoxide. So every heating tune-up includes a combustion analysis in the flue, a look at the heat exchanger, and a check of the vent from the furnace to where it leaves the building.

You get the readings, not just "looks good".

Older Pueblo houses

Floor furnaces and wall heaters need the same care.

Many older bungalows here still heat with a floor furnace under a grate or a wall heater in the hallway. They have the same seven checkpoints — often with older venting that deserves a closer look.

The metal floor grate of an older gas floor furnace in the hallway of a Pueblo bungalow, wood floors around it
A furnace vent cap on the roof of a house against a clear blue sky, the chimney flashing visible
Businesses

Rooftop units and unit heaters burn gas too.

A gas-fired rooftop unit, a shop heater or a church furnace has the same combustion path. In buildings full of people, the checks matter more, not less.

Homes

Test the alarm when you change the clocks.

Press the test button on your CO alarms each spring and fall, and replace alarms at the age the maker specifies — it's printed on the back.

Before the cold

Book a heating check.

All seven checkpoints, with the readings — for a house or a building.

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