Types of Pipes Used for Furnace Exhaust: A Complete Guide

The choice of pipe for furnace exhaust affects safety, efficiency, and code compliance. This guide explains common materials, installation considerations, sizing, and maintenance so homeowners and HVAC professionals can pick the right venting solution for gas and high-efficiency furnaces.

Pipe Type	Best For	Key Advantages	Limitations
PVC/CPVC	High-efficiency (condensing) furnaces	Corrosion-resistant, inexpensive, easy to install	Not for high-temperature flue gas
Stainless Steel (Class A, B-vent, Lined)	Mid- to high-efficiency, masonry chimneys, outdoors	Durable, heat-resistant, code-accepted for many appliances	Higher cost, heavier
Aluminum/Vent Pipe	Older furnaces and some venting applications	Lightweight, economical	Prone to corrosion with acidic condensate
Galvanized Steel	Some vent connectors	Low cost, rigid	Not suitable for long-term exposure to acidic condensate
Flexible Vent (Aluminum or Stainless)	Tight spaces, short connectors	Flexible routing, easy to install	Higher friction, not for long runs

How Furnace Exhaust Works And Why Pipe Choice Matters

Furnace exhaust, also called flue or venting, removes combustion byproducts such as carbon monoxide, water vapor, and combustion gases from a home to the outside. The composition and temperature of the exhaust vary with furnace type and efficiency.

Higher-efficiency condensing furnaces produce cooler, moisture-laden exhaust that is acidic and requires corrosion-resistant venting such as PVC, CPVC, or stainless steel. Conventional high-temperature furnaces need heat-resistant materials like stainless or listed vent pipe.

Common Pipe Materials For Furnace Exhaust

PVC And CPVC

PVC (polyvinyl chloride) and CPVC (chlorinated polyvinyl chloride) are common for modern high-efficiency furnaces that vent at low temperatures. They are lightweight, inexpensive, and resist corrosion from acidic condensate.

Codes typically permit PVC/CPVC for appliances with exhaust temperatures below the material’s rated limit, often used for direct vent and sidewall terminations. CPVC offers a slightly higher temperature rating than standard PVC.

Stainless Steel

Stainless steel is a durable option for many furnace exhaust applications. It withstands high temperatures and corrosive condensate and is commonly used as liner material inside masonry chimneys or as rigid vent runs.

304 and 316 grades are common; 316 provides greater corrosion resistance, which is beneficial in coastal or highly corrosive environments.

Aluminum And Galvanized Steel

Aluminum and galvanized steel ducts have been used historically for furnace exhaust and connectors. They are inexpensive and light but can corrode when exposed to acidic condensate from condensing furnaces.

Galvanized steel is often used for short internal connectors but is not recommended for long-term exhaust exposure where condensation occurs.

Flexible Venting (Flex)

Flexible vent pipe, made from aluminum or stainless steel, is useful for short connectors or routing through tight spaces. It reduces installation complexity but increases friction losses and should not be used for long vertical or horizontal exhaust runs.

Flexible liner is available for masonry chimneys and can be a cost-effective restoration method when matched to appliance requirements.

Codes, Standards, And Manufacturer Requirements

Venting requirements are governed by local building codes, the International Residential Code (IRC), NFPA 54 (National Fuel Gas Code), and furnace manufacturer instructions. The appliance’s installation manual typically specifies acceptable vent materials.

Important rule: The manufacturer’s venting instructions must be followed. Even if a material is code-approved, using a non-listed pipe against manufacturer guidance can void warranties and violate codes.

High-Efficiency Condensing Furnaces: Why PVC And CPVC Are Common

Condensing furnaces extract more heat by condensing water vapor in the exhaust, producing cooler, acidic condensate. This low-temperature flue gas allows the use of plastic venting like PVC and CPVC, which offer corrosion resistance and low cost.

Typical installation details include sealed horizontal runs to a sidewall termination or concentric pipe systems. A condensate drain must be provided and sloped to prevent pooling.

Natural Draft And Power Vent Furnaces: Metal Options

Older natural draft furnaces and some power vent systems produce hotter flue gases, requiring metal venting such as stainless steel or listed B-vent. These materials handle higher temperatures and maintain structural integrity under heat stress.

Power venters and induced draft blowers allow more flexible routing but still require compatible, listed vent material rated for temperature and internal pressure conditions.

Sizing, Slope, And Vent Configuration

Proper vent size and slope are essential for safe exhaust flow. Sizing is determined by appliance input rating, type of venting, and configuration according to code tables or manufacturer charts.

Horizontal PVC/CPVC runs typically require a slight slope (1/4 inch per foot) toward the furnace to allow condensate to drain into the condensate trap. Vertical sections should be plumb and supported at code-specified intervals.

Terminations And Outdoor Considerations

Exhaust terminations must prevent water ingress, animal entry, and blockage. Termination caps, screens, and clearances to windows, doors, and intakes are specified by code and the appliance manual.

Material selection for exterior exposures must consider UV resistance and temperature extremes. Stainless or painted metal may be preferred for exposed vertical stacks in freezing climates.

Common Installation Mistakes To Avoid

• Using PVC or CPVC with a non-condensing high-temperature furnace — unsafe and noncompliant.
• Improper slope on horizontal condensate lines causing pooling and corrosion.
• Mixing incompatible materials without proper adapters or transition fittings.
• Exceeding maximum allowable vent length or number of elbows specified by the manufacturer.

Inspection, Maintenance, And Lifespan

Regular inspection helps detect corrosion, loose joints, blockages, and condensate leaks. Homeowners should schedule annual HVAC inspections and promptly address any vent damage or unusual furnace behavior.

Material lifespans vary: PVC/CPVC can last decades in appropriate applications; stainless steel often lasts longer under harsh conditions. Aluminum and galvanized components may corrode sooner when exposed to acidic condensate.

Choosing The Right Pipe For A Specific Furnace

Identify the furnace type (condensing vs non-condensing), review the installation manual, and consult local codes. Match pipe material to exhaust temperature and condensate characteristics.

When in doubt, consult a licensed HVAC professional for load calculations, vent sizing, and code interpretation. A professional can recommend transition components, proper supports, and termination options to ensure safe, efficient venting.

Cost Considerations And Tradeoffs

PVC/CPVC systems typically have the lowest material and labor cost for condensing furnaces. Stainless steel provides durability and is more expensive up front but may reduce long-term repair needs.

Flexible venting can cut labor in tight spaces but may reduce efficiency due to increased friction. Consider lifecycle costs, warranty implications, and repair accessibility when comparing options.

Frequently Asked Questions

Can PVC Be Used For All Gas Furnaces?

PVC is suitable only for condensing furnaces that produce low-temperature exhaust per manufacturer and code limits. Using PVC on older high-temperature furnaces is unsafe and noncompliant.

Is Stainless Steel Always Required For Chimneys?

Stainless steel liners are often required when relining masonry chimneys or when flue gas conditions demand corrosion resistance and heat tolerance. The specific grade and thickness depend on application and local codes.

How Often Should Furnace Exhaust Be Inspected?

Annual inspections are recommended for vent systems, with immediate checks after unusual furnace operation, visible leaks, or odors. Inspections should include joint integrity, slope, terminations, and condensate management.

Practical Checklist For Safe Furnace Venting

• Verify furnace type and review manufacturer venting instructions.
• Select pipe material rated for exhaust temperature and condensate properties.
• Follow code clearance and termination requirements for safety.
• Ensure proper slope and secure supports for horizontal and vertical runs.
• Install condensate drains/traps where required and route to approved disposal.
• Schedule annual professional inspections and prompt repairs.

Key Takeaway: The correct furnace exhaust pipe depends on furnace efficiency and exhaust temperature. PVC/CPVC is commonly used for condensing furnaces, while stainless steel or listed metal venting is required for higher-temperature applications. Always follow the furnace manufacturer’s instructions and local code to ensure safe and compliant venting.