Can a gas-fired appliance and a liquid-fuel-fired appliance be vented into a common chimney, and if so, how are the vents arranged?

Can a gas-fired appliance and a liquid-fuel-fired appliance be vented into a common chimney, and if so, how are the vents arranged?

• A. Gas vent above liquid-fuel vent
• B. Gas vent and liquid-fuel vent are both on the same level in the chimney
• C. Both vented to separate chimneys
• D. Liquid-fuel vent above gas vent

Answer: (D)

When two different-fuel appliances share a chimney, the exhausts must be arranged to prevent crossflow of combustion products. Put the liquid-fuel exhaust uppermost in the chimney, with the gas-fired exhaust below it. This vertical ordering helps prevent oil-fired flue gases, which can carry soot and condensate and may be more prone to draft-related backflow, from entering the gas appliance. If the gas vent were above the oil vent or if both vents were at the same level, downdrafts or draft changes could push oils’ combustion products into the gas appliance. Venting the oil side above the gas side keeps the flow separation intact within a common chimney; if that arrangement isn’t possible, separate chimneys are required.

When two different-fuel appliances share a chimney, the exhausts must be arranged to prevent crossflow of combustion products. Put the liquid-fuel exhaust uppermost in the chimney, with the gas-fired exhaust below it. This vertical ordering helps prevent oil-fired flue gases, which can carry soot and condensate and may be more prone to draft-related backflow, from entering the gas appliance. If the gas vent were above the oil vent or if both vents were at the same level, downdrafts or draft changes could push oils’ combustion products into the gas appliance. Venting the oil side above the gas side keeps the flow separation intact within a common chimney; if that arrangement isn’t possible, separate chimneys are required.