Furnace Flue Pipe Types: B-Vent, PVC, and CPVC Explained for Minnesota Homes

One of the most overlooked aspects of furnace replacement is the venting system. When you replace a furnace—especially when moving from an 80% AFUE unit to a 96% unit—the venting requirements change fundamentally. Using the wrong type of pipe is a safety hazard and a code violation. This guide explains the three main types of furnace flue pipe, when each is used, and what Minnesota homeowners need to know.

Why Venting Type Matters

Your furnace burns natural gas or propane, producing combustion gases including carbon dioxide, water vapor, and small amounts of carbon monoxide. These gases must be safely exhausted to the outside. The type of pipe required depends on the temperature and chemistry of those exhaust gases, which varies by furnace efficiency:

• 80% AFUE furnaces produce hot, dry exhaust gases (400°F+)—requires metal pipe that can handle high heat
• 96% AFUE furnaces produce cooler, acidic condensate (100–150°F exhaust)—requires plastic pipe that can handle acidic moisture

This is the fundamental reason you can't simply connect a new 96% furnace to old B-vent metal flue pipe, and why upgrading to high efficiency sometimes requires more extensive installation work.

Type 1: B-Vent (Double-Wall Metal Pipe)

B-vent is the double-wall metal pipe used to vent traditional 80% AFUE "natural draft" furnaces and some mid-efficiency units. It consists of an inner and outer metal wall with an air gap for insulation.

• Used with: Natural draft 80% AFUE furnaces; some 80-90% AFUE mid-efficiency units
• Heat resistance: Rated for high exhaust temperatures (up to 400°F+)
• Installation: Terminates through the roof or chimney; relies on natural draft (hot exhaust rising)
• Cost: Moderate; standard sizes are 4", 5", or 6" diameter

Minnesota concern: B-vent terminating on the roof faces extreme conditions in Minnesota winters—ice formation, snow blockage, and extreme cold can affect natural draft performance. Keep termination caps clear of snow accumulation.

Can you keep existing B-vent when upgrading to 96%? No. A 96% AFUE furnace cannot use B-vent. The cooler exhaust from a high-efficiency furnace won't rise naturally up a B-vent chimney, and the acidic condensate will corrode metal pipe. New PVC venting is required.

Type 2: PVC (Polyvinyl Chloride)

Schedule 40 PVC is the standard venting material for 90%+ AFUE condensing furnaces. It's the white plastic pipe you see at home improvement stores, made for corrosion-resistant drainage and low-temperature pressure piping.

• Used with: 90–96% AFUE condensing furnaces (the vast majority of new residential furnaces)
• Temperature rating: Up to approximately 140°F—safe for condensing furnace exhaust
• Installation: Typically exits through the side of the home (sidewall termination) or upward through the roof; requires both an exhaust pipe and a combustion air intake pipe
• Diameter: 2" or 3" (verify with furnace specs—BTU output affects required diameter)
• Cost: Inexpensive; PVC is a commodity material

Minnesota-specific consideration: Sidewall terminations are preferred in Minnesota because rooftop PVC vents can ice over or get blocked by snow drifting on the roof. Sidewall terminations should be located away from windows, doors, walkways, and meters—local code specifies minimum clearances. In extreme cold (-20°F+), some high-efficiency furnaces can have condensate lines freeze at the sidewall termination; proper installation angle and sometimes heat tape prevent this.

PVC pipe should not be confused with CPVC (below)—they look similar but have different temperature ratings and are not interchangeable for furnace venting.

Type 3: CPVC (Chlorinated Polyvinyl Chloride)

CPVC is a higher-temperature variant of PVC used in some specific furnace venting applications. It's cream or yellow-orange in color vs. white PVC.

• Used with: Certain furnaces with higher exhaust temperatures that exceed PVC's limits—primarily some 80%+ AFUE direct-vent furnaces or specific high-static applications
• Temperature rating: Up to 200°F
• Cost: More expensive than PVC

For most residential Goodman furnaces operating at 96% AFUE, standard Schedule 40 PVC is appropriate. Always verify your specific furnace's installation manual for approved venting materials—this is not something to guess at.

The Two-Pipe System: Exhaust and Combustion Air

Modern condensing furnaces (90%+) use a direct-vent or sealed-combustion design—they pull combustion air directly from outside rather than from your living space. This requires two pipes:

1. Exhaust pipe: Carries combustion gases out
2. Intake pipe: Brings fresh combustion air in from outside

This two-pipe design offers several advantages over older single-pipe systems:

• No combustion air is drawn from inside the home, so the furnace doesn't depressurize the house
• Carbon monoxide risk from backdrafting is eliminated
• The furnace is not affected by exhaust fans, kitchen hoods, or other depressurization sources

Venting a New Furnace in a Home With an Existing Chimney

Many older Minnesota homes have masonry chimneys that previously vented the old furnace. When replacing with a 96% unit, you have several options:

• New sidewall PVC termination: The most common approach; runs PVC out through an exterior wall, abandons the old chimney for furnace use
• PVC liner in existing chimney: A PVC liner can be inserted in the chimney and used to vent the new furnace up through the existing chimney—adds cost but useful when sidewall penetration is impractical
• Repurposing the chimney: If your water heater still uses the chimney, it may remain in service for that appliance. However, if you also replace a natural-draft water heater with a power-vent or tankless unit, the chimney becomes unused

What Goes Wrong With Improper Venting

Furnace venting errors are a leading cause of carbon monoxide incidents. Common problems include:

• Using wrong pipe material (PVC where CPVC is required, or vice versa)
• Improper pitch on the exhaust pipe (condensate must drain back to the furnace—incorrect pitch causes pooling)
• Inadequate clearances from windows, doors, or the gas meter
• Incomplete sealing at pipe joints (leaking combustion gases into living spaces)
• Snow or ice blockage at terminations not properly protected

Always have furnace venting installed by a licensed Minnesota HVAC contractor—see our contractor licensing guide—and ensure the installation is permitted and inspected. See our carbon monoxide safety guide for detector placement and CO risks.

Furnace Installation Day and Venting

When your installer arrives to replace your furnace, venting is one of the first things they'll assess. Make sure they:

• Identify the correct pipe size and material for your specific unit
• Plan the routing before cutting any holes
• Check clearances from windows, doors, and gas meter
• Properly support and pitch the pipe
• Install appropriate termination caps

Our furnace installation day guide walks through the full process. Ready to replace? Browse our factory-direct Goodman furnaces and have your installer quote the complete job including any venting modifications.