Installation Guide: What to Know Before You Replace Your Old Furnace

👋 Introduction — Mark’s Furnace Replacement Story

When Mark’s 20-year-old gas furnace started making metallic clanks and barely pushed out warm air last winter, he knew it was time for a replacement. But before calling his HVAC installer, Mark wanted to understand the process — not just the product.

“How hard is it to replace an old furnace?”
“What’s involved beyond just swapping units?”
“Do I need to upgrade my ducts, vents, or gas lines too?”

If you’re asking the same questions, this guide will walk you through everything you need to know before replacing your old furnace — whether you’re planning to upgrade to a 92% AFUE Goodman GR9S920603BN or any modern, high-efficiency model.

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🧰 1️⃣ Understanding the Furnace Replacement Process

Replacing a furnace isn’t a simple plug-and-play job. Your system is part of a larger ecosystem that includes ductwork, venting, gas supply, thermostat controls, and even your home’s insulation.

Here’s what a typical professional replacement involves:

1. Home evaluation and load calculation (Manual J).
   The installer calculates the exact heating requirements of your home.

2. Equipment selection.
   The right size and efficiency rating (AFUE) are chosen based on the load results.

3. Old system removal.
   The technician disconnects gas, power, and duct connections and removes the old furnace.

4. New unit preparation.
   Proper placement, leveling, and adjustments to venting or drainage lines.

5. Connection & wiring.
   Gas, electrical, and thermostat connections are secured and tested.

6. Startup & testing.
   The tech verifies temperature rise, combustion safety, and airflow.

7. Cleanup and homeowner walkthrough.

According to the U.S. Department of Energy, a properly installed high-efficiency furnace can save homeowners 15–20% on heating costs compared to older units — but only when installed and sized correctly.

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🧮 2️⃣ Step 1: Size Your Furnace the Right Way (Manual J Load Calculation)

The first mistake many homeowners make? Assuming their old furnace size is still correct.

Over the years, home efficiency may have changed — through new windows, added insulation, or renovated rooms.

A Manual J load calculation, developed by the Air Conditioning Contractors of America (ACCA), determines the exact heating load (in BTUs) your home needs based on:

• Square footage

• Insulation quality

• Window type and placement

• Ceiling height

• Air infiltration

• Orientation to sunlight

For example, if Mark’s old 80,000 BTU furnace was installed in the early 2000s, but he since added insulation and sealed leaks, his new load might only require 60,000 BTUs — making the Goodman GR9S920603BN (92% AFUE, 60k BTU) an ideal upgrade.

You can read more about ACCA’s Manual J method here:
👉 ACCA Manual J Residential Load Calculation.

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🏗️ 3️⃣ Step 2: Choose the Right Efficiency and Fuel Type

Before you replace, you’ll need to decide what kind of furnace best fits your goals.

AFUE (Annual Fuel Utilization Efficiency) indicates how much heat is converted from the fuel.

• 92% AFUE = 92¢ of every $1 heats your home

• 8¢ is lost through venting

If you live in a moderate or northern region, upgrading to a 90%+ AFUE furnace provides excellent comfort and energy savings.
See Energy.gov’s Furnace Efficiency Guide for AFUE rating comparisons.

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🧱 4️⃣ Step 3: Assess Your Venting System

Modern high-efficiency furnaces often use PVC venting instead of metal flues. This is because they produce cooler exhaust gases that can safely exit through plastic pipes.

Older furnaces vent through metal chimneys, which can cause condensation issues if reused.

Checklist before installation:

• ✅ Confirm vent diameter (usually 2" or 3" PVC).

• ✅ Verify proper slope for condensate drainage.

• ✅ Ensure termination meets code (minimum clearance from windows).

• ✅ Seal joints for airtightness.

If you’re upgrading from an 80% to a 92% model, your installer will confirm whether the existing flue can be reused or must be converted.

Reference: HVAC.com Venting & Flue Guide

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⚙️ 5️⃣ Step 4: Evaluate Ductwork Compatibility

Even the most efficient furnace won’t perform well if your ductwork is leaking or improperly sized.

According to ENERGY STAR, up to 30% of heated air can escape through duct leaks in attics or crawlspaces.

During replacement, ask your contractor to:

• Inspect for disconnected or undersized runs.

• Seal joints with mastic (not duct tape).

• Add insulation to ducts in unconditioned spaces.

• Check static pressure and airflow balance.

Goodman’s 9-speed blower motor (in the GR9S920603BN) is designed to adjust airflow smoothly — but it still relies on proper duct design to deliver comfort evenly across rooms.

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💡 6️⃣ Step 5: Verify Electrical and Gas Line Requirements

Your furnace needs more than just fuel — it needs safe electrical and gas connections.

Electrical checks:

• Confirm circuit breaker size (usually 15–20 amps).

• Ensure grounding and GFCI protection where required.

• Inspect thermostat wiring (especially if upgrading to a smart thermostat).

Gas line checks:

• Confirm adequate gas supply pressure.

• Replace flexible connectors if older than 10 years.

• Test for leaks after installation (using a manometer and soap solution).

A professional installer will ensure everything meets local code and manufacturer specifications.
For detailed safety standards, refer to NFPA 54: National Fuel Gas Code.

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🧰 7️⃣ Step 6: Understand Condensate Drainage (for 90%+ Units)

If you’re installing a high-efficiency condensing furnace, it will produce water as part of the heat-extraction process. That’s the “condensate.”

Installers must ensure:

• A dedicated PVC condensate line drains properly (gravity or pump-assisted).

• Lines are sloped downward with no sags.

• Trap and vent configurations prevent suction or backflow.

• The condensate drains into a floor drain or pump.

Ignoring drainage design can cause corrosion or short cycling — two common issues in DIY installations.

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🧾 8️⃣ Step 7: Check Clearance and Airflow Requirements

Furnaces need breathing room — both for combustion and maintenance.

Typical clearance recommendations:

• Front (access side): 24 inches

• Top: 1 inch

• Sides: 3 inches

• Back: 0 inch (if noncombustible wall)

Never block return air pathways with insulation or stored items.
Proper airflow ensures stable temperature rise (usually 35–65°F).

You can verify specifications in Goodman’s Installation Instructions for GR9S92 Series.

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🧠 9️⃣ Step 8: Plan for a Thermostat Upgrade

Modern furnaces can communicate more efficiently with digital or programmable thermostats.

If your old thermostat is non-programmable or mercury-style, this is the perfect time to upgrade.

Recommended thermostat types:

• Programmable: Set heating schedules for automatic savings.

• Smart Wi-Fi: Learn your patterns and optimize runtime.

• Zoned systems: Manage comfort by floor or room.

ENERGY STAR estimates programmable thermostats save 8–10% annually on heating costs (ENERGY STAR Smart Thermostats).

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🔧 1️⃣0️⃣ Step 9: Budget for Professional Installation

The total furnace replacement cost depends on unit size, efficiency, and installation complexity.

While DIY installation may sound tempting, gas and vent systems are not DIY-friendly — improper installation can void warranties or cause safety hazards.

Always hire a licensed HVAC professional with manufacturer certification.

The Air Conditioning Contractors of America (ACCA) Directory lists qualified local installers.

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🔥 1️⃣1️⃣ Step 10: Understand the Startup and Testing Process

After installation, your technician performs several tests to verify safe and efficient operation.

Standard commissioning checklist:

• ✅ Gas pressure test

• ✅ Combustion analysis (CO, O₂, efficiency)

• ✅ Flame sensor and igniter test

• ✅ Temperature rise measurement

• ✅ Blower motor speed adjustment

• ✅ Safety switch verification

They’ll also record the model and serial number for warranty registration — something Goodman requires within 60 days (Goodman Warranty Registration).

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🌡️ 1️⃣2️⃣ Step 11: Register Your Warranty and Save Your Documentation

After the install, complete your warranty registration within the timeframe specified.

For Goodman furnaces:

• Heat Exchanger: Lifetime limited warranty

• Parts: 10 years (with registration)

• Labor: Typically covered by the contractor

Keep these documents:

• Purchase receipt

• Installation invoice

• Warranty registration confirmation

These will protect your investment if future repairs are needed.

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🧩 1️⃣3️⃣ Step 12: Schedule Regular Maintenance

Once your furnace is installed, maintaining it is key to preserving efficiency and performance.

Mark’s Annual Furnace Care Plan:

1. Replace filters every 1–3 months.

2. Have a licensed technician tune up the furnace each fall.

3. Check for condensate leaks and clean drain traps.

4. Inspect vent pipe for corrosion.

5. Test carbon monoxide detectors annually.

Following a maintenance schedule keeps your furnace at peak 92%+ efficiency.
For a detailed guide, see Energy.gov’s HVAC Maintenance Tips.

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💡 1️⃣4️⃣ Step 13: Plan for Energy Efficiency Upgrades

If you’re already replacing your furnace, it’s the perfect time to tackle other energy upgrades:

• Add attic insulation (R-38 or higher).

• Seal door and window drafts.

• Upgrade to ENERGY STAR appliances.

• Insulate exposed ducts.

• Install a programmable thermostat.

These improvements can reduce your heating load and extend furnace life.
See ENERGY STAR’s Home Upgrade Guide for project ideas.

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🧾 1️⃣5️⃣ Step 14: Check Local Permits and Code Requirements

In most regions, furnace replacement requires a mechanical permit to ensure safety and compliance.

Your contractor should:

• Pull the appropriate city or county permit.

• Schedule an inspection after installation.

• Provide a certificate of completion.

Permitting ensures the system meets local building codes, gas line standards, and venting safety requirements.
Reference: International Residential Code (IRC) Mechanical Systems.

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🏠 1️⃣6️⃣ Step 15: Learn the Warning Signs of a Bad Install

Even professional jobs can go wrong if not carefully checked.

Watch for these red flags after installation:

• 🔴 Furnace cycles on/off every few minutes (short cycling).

• 🔴 Whistling or rattling ducts (air leaks).

• 🔴 Cold spots or weak airflow in rooms.

• 🔴 Persistent odor from vents (exhaust leaks).

• 🔴 Condensate puddles near unit.

If you notice any of these, contact your installer immediately for inspection.

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🧠 1️⃣7️⃣ Bonus: Should You Replace the Furnace and AC Together?

If your air conditioner is also over 15 years old, it might be wise to replace both systems at once.

Reasons:

• Shared blower and ductwork improve efficiency when matched.

• Dual installation saves on labor costs.

• New paired systems ensure balanced airflow.

Goodman offers furnace + AC bundles with matched coil systems for consistent performance.

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🌎 1️⃣8️⃣ Environmental Impact — Why Replacing Matters

Older furnaces waste more fuel and release more CO₂.

Upgrading from an 80% to a 92% model reduces gas use by about 13% — cutting 1.4 metric tons of CO₂ annually (the equivalent of 16 trees planted each year).

Calculate your emissions savings using the EPA Carbon Footprint Calculator.

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🧾 1️⃣9️⃣ Cost Recovery and ROI Timeline

Let’s estimate Mark’s real-world payback:

Mark’s moderate Ohio climate made 92% the best balance — short payback, long comfort.

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🔍 2️⃣0️⃣ Final Checklist Before You Replace

✅ Get a Manual J load calculation.
✅ Verify venting type and clearance.
✅ Inspect and seal ductwork.
✅ Confirm electrical and gas line readiness.
✅ Budget for thermostat upgrade.
✅ Hire a licensed professional.
✅ Register your warranty.
✅ Schedule yearly maintenance.

Replacing a furnace isn’t just a purchase — it’s a performance upgrade for your whole home.
The Goodman GR9S920603BN gives you high-efficiency comfort, reliable heat, and long-term savings — all backed by one of the best warranties in the business.

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In the next topic we will know more about: Upflow vs. Horizontal Installation: Which Configuration Fits Your Home Layout?