Furnace Buying Guide: Why Condensing Furnaces Save More Long-Term

The moment you start wondering if your old furnace is costing you

If your 80% AFUE furnace is pushing out warm air and a big gas bill, you’re not imagining things. Standard furnaces send a lot of heat up the chimney. Condensing furnaces (95%+ AFUE) squeeze that extra heat out instead, which is why homeowners see 15–20% lower fuel use in real life. In this guide, we’ll walk through what’s different drainage, venting, and the money math so you can decide with confidence. I’ll explain it like I would for a neighbor: simple, clear, and based on installs we do every season. If you want to browse equipment while you read, our full furnace selection are handy starting points. No fluff here, just what matters, what it costs, and how to avoid rookie mistakes that cause leaks, lockouts, or code issues later.

What a condensing furnace does (and why it makes water)

A condensing furnace uses a secondary heat exchanger to pull more heat from the exhaust. Cooling those gases below the dew point turns water vapor into liquid that’s the “condensate” you’ll see draining from the unit. Standard furnaces never get exhaust that cool, so they don’t make this liquid. That one design change is why condensing models can safely use PVC venting (exhaust around 100–150°F), and why they need proper drains. Think of it like wringing out a wet towel: you’re getting more heat into your home, and what’s left is cooler exhaust plus water.

Quick wins:

• Expect water. It’s normal evidence the furnace is actually condensing.

• Plan a drain path during the quote, not after install.

• Check slope on any drain or vent piping they’re part of the system, not decoration.

Need help matching options? The Help Center has plain-English answers.

Condensate 101: how much and how to think about it

High-efficiency furnaces can produce a surprising amount of liquid during long heating cycles. That condensate is simply water + combustion byproducts after heat has been reclaimed. It’s one reason these furnaces are so efficient: they grab the extra heat before the vapor leaves your house. The byproduct is a steady drip that must be controlled, sloped, and safely routed. If your old system never had a drain tube near the furnace, don’t panic that’s expected when moving to condensing.

Watch-outs from real installs:

• Long, flat runs of drain tubing cause gurgling and backups.

• Improper traps let exhaust smell into the space or stall the pressure switch.

• Switching from chimney venting to PVC often shifts where we can place the furnace.

Curious about layout options? Our Design Center can help you think through routing in tight basements or utility closets.

The condensate is acidic (pH ~3–5) what that means for your home

Condensing-furnace condensate is mildly acidic (about pH 3–5) because it contains dissolved carbon dioxide and trace byproducts. That acidity matters for materials. We use PVC drain lines because traditional metal components (including old galvanized drains) can corrode. The drain must tie into a floor drain, utility sink, or standpipe as local code allows. If gravity can’t get you there, plan on a condensate pump.

Non-negotiables:

• Use PVC for drain lines and fittings that touch condensate.

• Install a trap specifically rated for high-efficiency furnaces it’s essential to keep exhaust out of the drain.

• Keep service access clear; traps and pumps need periodic cleaning or replacement.

The drainage parts you’ll actually see (and typical costs)

A good condensing install includes a few specialized parts you might not have seen on your old furnace:

• Condensate trap: Mandatory to block flue gases from entering the drain.

• Primary drain line: PVC, sloped at least 1/4 inch per foot back toward the furnace so water flows without pooling.

• Condensate pump (if needed): Used when gravity can’t carry condensate to a legal drain point. Replacement cost typically $250–$440 when it ages out.

• Floor drain/utility sink/standpipe: Code-approved termination points.

Keep the run simple and accessible. Fewer fittings = fewer clogs. During installation, ask your tech to label the pump’s power plug and test the float switch with you; this five-minute walkthrough prevents panicked calls later.

Stocking up? See accessories we commonly use.

Venting: why PVC replaces your metal flue on high-efficiency models

Because condensing furnaces cool exhaust to roughly 100–150°F, we can vent with PVC or CPVC instead of metal. That’s a big change from standard furnaces (exhaust 300–500°F) that require metal flues or chimneys. PVC venting is safer at these lower temps and often routes out a sidewall, which can free you from a tired chimney.

Key benefits:

• Lower-cost materials than metal venting.

• Flexible routing (often shorter runs).

• Direct-vent options that bring in outside air for combustion—great for tight homes.

If your laundry room or water heater shares a chimney, converting the furnace to PVC may leave that chimney “orphaned.” Ask us to evaluate other appliances and venting to keep everything to code. Our Help Center covers common venting questions.

Two venting styles: direct vent (two-pipe) vs. single-pipe

Most condensing installs use Direct Vent (two-pipe): one PVC pipe brings in outside air, the other exhausts. This keeps indoor air out of the combustion process, helps efficiency, and avoids pressure issues in tight homes. A single-pipe setup is sometimes used, drawing combustion air from indoors while exhausting outside through PVC.

Choosing between them:

• Two-pipe is the most common and best for sealed, efficient homes.

• Single-pipes can work in older, leakier homes with plenty of indoor air—but it’s less ideal if you’re air-sealing.

Check your outdoor terminations in winter. Two pipes close together can drift snowwise; we place the exhaust 12 inches higher than intake to prevent re-entrainment. If drifts are common, a short snorkel termination helps. Want help comparing layouts? Start with a photo quote.

Install rules we follow (so your system runs quietly for years)

Small details make or break a condensing furnace. Here are the ones we never skip:

• Pipe sizing: Usually 2–3 inch PVC, based on BTU and run length.

• Exhaust slope: Minimum 1/4 inch per foot back to the furnace so condensate flows home.

• Clearances: Keep at least 2 feet around sidewall terminations in all directions.

• Termination heights: Exhaust typically at least 12 inches above the intake.

• Clean routing: Fewer elbows = less pressure drop = happier pressure switches.

• Drain routing: Short, sloped, and easy to service.

Before we leave, we run a condensate and vent checklist with you: trap primed, pump tested, intake/exhaust clear, drain slope verified. Make this your annual habit and you’ll prevent 80% of nuisance shutdowns. For deeper planning, see our Design Center.

Simple upkeep: 10-minute habits that prevent most service calls

You don’t need to be a tech to keep a condensing furnace happy. Once a month in heating season:

1. Look at the trap and tubing. Any kinks, sags, or algae?

2. Listen to the pump. It should cycle briefly, not run constantly.

3. Check outside terminations. Clear leaves, lint, or snow.

4. Swap or clean filters on schedule restricted airflow causes lockouts.

Every fall, add:

• Flush the trap and drain with a little warm water.

• Confirm slope on visible PVC runs.

• Visually inspect intake/exhaust for insect screens or nests.

If you see water under the furnace, unplug the condensate pump first (if present) and lift the float if it doesn’t run, that’s likely the fix. Our HVAC Tips blog covers quick diagnostics.

The money math: a quick, honest savings check

Upgrading from 80% to 95% AFUE often trims ~15–20% off fuel use. If your average heating spend is $150/month, that’s about $22.50/month in-season savings. Multiply by your heating months to estimate annual savings. Keep it simple:

• Your numbers:

• Last season cost: $________

• Efficiency lift: ~15–20%

• Estimated annual savings: last season × 0.15 to 0.20

For many homes, that lands around $200–$400 per year. That’s before rebates. Over 15 years, steady savings add up; for example, $300/year at a 5% discount rate has a present value around $3,140. Want a second opinion on your inputs? We’re happy to help—just reach out via Contact Us.