During a combustion safety inspection, a technician uses a draft gauge at the draft hood of a natural-draft gas furnace and measures a positive pressure reading instead of the expected negative draft. What does this most likely indicate?
- AThe gas valve is undersized for the burner orifice
- BThe thermostat anticipator is set too low, causing short cycling
- CThe furnace is operating at peak efficiency with proper venting
- DFlue gases, including carbon monoxide, are spilling into the living space rather than venting outdoors
Show answer & explanationAnswer & explanation
Correct answer: D. Flue gases, including carbon monoxide, are spilling into the living space rather than venting outdoors
A properly venting natural-draft appliance should show a slightly negative draft pressure (commonly around -0.02 in. w.c.) pulling flue gases up and out of the chimney. A positive pressure reading at the draft hood indicates backdrafting or spillage, meaning combustion byproducts including CO are entering the living space instead of venting to the outdoors—a serious safety hazard requiring immediate corrective action.
Why the other options are wrong
- A. Gas valve/orifice sizing affects burner performance, not draft pressure readings.
- B. Thermostat anticipator settings affect cycling behavior, not combustion venting draft.
- C. Positive pressure indicates a venting failure, not efficient operation.
Draft Pressure and Spillage
Natural-draft gas appliances should maintain a slightly negative pressure at the draft hood to safely vent combustion gases; positive pressure indicates spillage/backdrafting and CO hazard.
- Normal draft: approximately -0.02 in. w.c.
- Positive reading = spillage into living space
- Common causes: blocked flue, negative house pressure, undersized chimney
Memory trick: Negative draft pulls danger up and out; positive pushes it back in.