Oil to gas furnace conversion: what diameter chimney flue liner is needed for an 80k BTU unit?
Short answer
An 80,000 BTU gas furnace conversion requires a 4-inch or 5-inch diameter stainless steel chimney liner based on chimney height and shared appliances. A dedicated unit venting through a 15 to 30-foot chimney needs a 4-inch liner, while heights under 15 feet or systems shared with a standard water heater require a 5-inch liner to maintain proper draft velocity and prevent acidic condensation.
An 80,000 BTU Category I gas furnace conversion requires a 4-inch or 5-inch diameter UL 1777 stainless steel chimney liner, depending on total chimney height, connector rise, and whether a water heater shares the flue.
Switching from oil to natural gas leaves an oversized 7-inch to 8-inch clay flue that cools exhaust gases too quickly, causing acidic condensation that destroys mortar joints. Contractors replacing oil equipment under NFPA 54 (National Fuel Gas Code) standards must downsize the vent pathway to maintain thermal draft velocity and protect building masonry.
If you only do one thing: Match the liner diameter to NFPA 54 vent sizing tables using your exact chimney height and fan-assisted appliance rating rather than reusing the existing unlined masonry opening.
- Dedicated 80% AFUE furnace: A standalone 80,000 BTU fan-assisted Category I unit—measuring Annual Fuel Utilization Efficiency—requires a 4-inch diameter liner for chimney heights between 15 and 30 feet, or a 5-inch liner for heights below 15 feet.
- Common venting with a water heater: Pairing the furnace with a standard 40,000 BTU atmospheric draft water heater increases total input to 120,000 BTU, requiring a 5-inch diameter liner across standard two-story residential chimney heights.
- Liner material specification: Install a UL 1777 listed corrugated or smooth-wall stainless steel liner (Underwriters Laboratories standard for chimney relining systems) with an insulated blanket wrap when installed in an exterior chimney chase to prevent flue gas chilling.
- Condensing furnace alternative (90%+ AFUE): High-efficiency condensing gas furnaces do not use a chimney liner; they vent low-temperature exhaust horizontally through 2-inch or 3-inch schedule 40 PVC or polypropylene pipe directly through a side wall.
- Connector rise and pitch: The vent connector pipe from the furnace collar to the chimney must maintain a minimum upward slope of 1/4 inch per linear foot, with total horizontal run not exceeding 75% of vertical chimney height.
- Watch out for: Leftover oil soot reacting with water vapor in gas exhaust to form sulfuric acid, which rapidly destroys new liners if the chimney flue is not professionally swept prior to installation.
- Watch out for: Flue liner oversizing (such as leaving a 6-inch or larger liner on a standalone 80k BTU unit), which causes flue gases to stall, condense, and spill carbon monoxide back into the mechanical room.
- Watch out for: Failing to install a rigid chimney top plate and storm collar, allowing rain intrusion that degrades insulation and corrodes the furnace draft inducer housing.
Measure the total vertical chimney height from the appliance breech to the crown, and call ARS/Rescue Rooter at 866-399-2885 to schedule a gas conversion and venting inspection backed by an Exceptional Service Guarantee.