Key Takeaways
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Condensing furnaces: 90%+ need sealed combustion for clean indoor air.
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Venting: PVC/CPVC with slope + drains handles cool, moist exhaust.
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Two-pipe/concentric: One in, one out—stable, efficient, code-ready.
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Layout: Diameter, length, elbows affect safety/performance.
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Guides: Use checklists + manufacturer tables to avoid errors.
Why combustion air and venting matter
A 90%+ furnace burns gas, turns most of that energy into heat, and sends the leftovers outside. To do that safely, it needs the right air to burn and the right path to exhaust. With sealed combustion, the furnace pulls air from outdoors through dedicated piping, not from your living room. That means cleaner indoor air, a steadier flame, and fewer drafts. Because the exhaust is cooler and contains water vapor, these systems use PVC or CPVC pipe and need a gentle slope plus a drain. When homeowners skip details like pipe size or termination clearances, the furnace can lock out or short-cycle. This guide explains your options—two-pipe, direct-vent, and concentric—so you can choose a setup that matches your house, your climate, and your local code. If you’re still shopping, compare high-efficiency furnaces and keep this guide handy during installation.
Problem customers face (and why it matters)
Common complaints with condensing furnaces sound like this: “It runs, then stops,” “I smell exhaust,” “There’s water near the unit,” or “Rooms feel stuffy.” Most of these trace back to air and vent mistakes—pipes too long or too small, too many elbows, blocked terminations, or a missing condensate trap. Tightly sealed homes can also starve older appliances for air, raising safety risks. The good news: sealed combustion and correct venting fix these issues by separating indoor air from the burn process and giving exhaust a clean, sloped path outside. Getting this right protects your family from carbon monoxide, keeps humidity where it belongs, and helps your furnace reach its rated efficiency.
How sealed combustion works in 90%+ furnaces
In a condensing furnace, the burner sits inside a sealed chamber. A small fan pulls outdoor air into that chamber through an intake pipe, mixes it with gas, and lights the flame. Hot gases move through a primary and secondary heat exchanger, squeezing out extra heat. That extra cooling makes water vapor condense, which is why you’ll see a drain line. Another fan pushes the cooled exhaust through a PVC vent to the outdoors. Because both intake and exhaust are piped, the house’s indoor air pressure doesn’t push the flame around. That means steadier combustion, fewer odors, and better efficiency—often 95–98% AFUE under the right conditions.
Venting options: two-pipe, direct-vent, and concentric
Most homes pick one of three layouts. Two-pipe (preferred): one pipe brings in outdoor air, the other expels exhaust. It’s stable, quiet, and isolates the burn from indoor air—great for tight homes. Direct-vent single-pipe: the furnace uses indoor air and only vents exhaust outside; it’s simple but not ideal for sealed homes. Concentric vent: intake and exhaust share one wall opening using a “pipe-in-pipe” kit—clean look, fewer penetrations, but follow spacing rules carefully. No matter the style, manufacturers publish tables for maximum length and elbows. Exceeding those limits can cause pressure switches to trip, error codes, or water buildup. Terminate the exhaust a safe distance from doors, windows, soffits, and grade, and keep clear of snow lines in cold climates. For practical details on slope, termination height, and materials, this venting safety guide is a solid reference.
Pipe material, diameter, length, and layout
For 90%+ furnaces, PVC is standard because the exhaust is cool and wet. CPVC or polypropylene may be used for longer runs or higher temperatures—follow the furnace manual. Size the pipe using the manufacturer’s chart; a pipe that’s too small chokes airflow, and one that’s too big can slow velocity so condensate pools. Count every elbow (each adds “equivalent length”), keep runs as straight as possible, and support pipes to prevent sagging. Horizontal exhaust usually slopes ¼ inch per foot back toward the furnace so water drains to a trap, while concentric kits have their own slope rules. Plan the termination location before drilling—respect clearances from vents, meters, and walkways. If you want a deeper dive, see “Vent Pipe Sizing: A Practical Guide” for rules of thumb and tables you can show your installer. (You can also browse our HVAC Tips for installation challenges and fixes.
Combustion air intake: placement, screens, and noise
Intake air must be clean, dry, and free of pollutants. Place the intake away from dryer vents, exhaust hoods, and areas where vehicles idle. Keep clear of roof valleys where snow drifts, and use a bird-screen or manufacturer-approved cap to block debris without restricting flow. On side-wall terminations, separate intake and exhaust per the manual so fumes aren’t pulled back in. In windy zones, a snorkel or elbowed termination can reduce nuisance trips. Inside the mechanical room, keep other fuel-burning appliances properly vented so negative pressure doesn’t affect them. If you’re upgrading from a non-condensing furnace, don’t reuse old metal flues for condensing exhaust; acidic condensate can corrode them. For a homeowner-friendly view of vent materials and clearances, this furnace venting guide breaks it down with photos and diagrams you can copy into your install plan.
Details pros never skip: slope, sealing, and condensate
Small details keep big problems away. Seal every joint with the approved primer/cement and support long runs so joints don’t twist. Slope horizontal exhaust back to the unit to drain water, and verify a proper trap on the condensate line (filled with water at startup). Route the drain to an approved receptor or condensate pump; protect against freezing if it passes through unconditioned space. Outside, label the pipes and keep at least 12 in. above grade to avoid snow clogging. Add a cleanout tee where the manual recommends. After start-up, verify pressure switch readings and check for CO at the cabinet and registers (should be zero). If you like checklists, our blog’s venting requirements article lists the common field mistakes and how to avoid them handy for DIYers and pros alike.
Pro install checklist (print this)
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Confirm model-specific pipe size and max length
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Count elbows; recalc equivalent length
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Plan terminations and clearances
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Set exhaust slope and trap routing
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Secure, label, and test under load
Decision framework: pick the right setup for your house
Use this quick framework:
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Home tightness: If your home is well-sealed or recently encapsulated, choose two-pipe sealed combustion.
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Vent path: Short, straight runs are best. If you need one hole in siding, consider a concentric kit.
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Climate and snow: Side-wall vents should clear drifts; roof vents need snow standoffs.
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Noise and placement: Keep terminations off patios and near-ear level windows.
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Codes & manuals: Local code + manufacturer limits always win.
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Serviceability: Add cleanouts and labels so future techs work faster.
Want a sanity check on brand and model matches? Visit our Sizing Guide, then request a Quote by Photo
Bundles, accessories, and smart controls that help
Matched parts make installs smoother. Many furnaces ship with or accept vent kits (standard or concentric), condensate traps, and mounting hardware. Add a good condensate pump if gravity won’t do the job. A modulating or communicating thermostat lets high-efficiency furnaces run longer at low fire, which keeps temperatures even and saves fuel. Stock a few extras accessories, spare PVC elbows, and a small tube of cement—so you’re not stuck mid-project. If you’re building out a new system, compare air handlers and package units for add-on spaces or future upgrades. Finally, confirm rebates with your utility and keep the install checklist with your records. For deeper reading, skim our plain-English explainers on vent materials, slopes, and sizing before you cut the first hole
Frequently Asked Questions (FAQ)
What’s the simplest way to tell if I need two pipes?
If your furnace is 90%+ AFUE and your home is tightly sealed or recently air-sealed, plan on a two-pipe sealed-combustion setup. It’s quieter and more stable than pulling air from indoors.
Why does the exhaust pipe slope back toward the furnace?
Condensing exhaust contains water. A slight slope lets that water drain to the trap instead of pooling in the pipe, which can cause shutdowns.
Can I reuse my old metal chimney?
Not for a condensing furnace. Acidic condensate will corrode it. Use PVC/CPVC or approved polypropylene sized per the manual.
How far from doors and windows should terminations be?
Follow the manufacturer table and local code. As a rule, keep terminations well away from openings and at least 12 in. above expected snow lines.
What if my vent run is long?
Larger diameter pipe, fewer elbows, or a different route may be required. Check the model’s maximum equivalent length chart.
Do I need a special thermostat?
A modulating or communicating thermostat helps a high-efficiency furnace run at lower fire more often, improving comfort and fuel use.
Where can I learn more?
Browse our HVAC Tips for furnace venting, sizing, and maintenance walk-through
