What is the normal draft in a heater and why?

• A. Slightly negative. This is so the hot gases stay in.
• B. Slightly positive. This is so the hot gases stay out.
• C. Neutral. This prevents leaks.
• D. Highly negative to draw combustion air in

Answer: (A)

Draft is the small pressure difference that moves combustion gases up the flue. In a heater, a slight negative draft is kept so the pressure inside the vent is a bit lower than the surrounding air. That suction makes the hot combustion gases flow upward and stay in the venting path, rather than leaking into the living space. If the draft were neutral, movement of gases would be unreliable and leaks could occur; if it were positive, fumes could push into the room. A highly negative draft would pull in too much outdoor air, causing drafts and energy loss, so the normal condition is a slight negative draft to keep the gases contained and vented properly.