A Hands-On Story from the Field — and the Basement
When Tony decided to replace the aging furnace in his own home, he could’ve picked any brand. After 25 years in the HVAC trade, he’s installed them all — Trane, Lennox, Rheem, Carrier. But this time, he chose something familiar, reliable, and built to handle real-world use:
The Goodman 80% AFUE 120,000 BTU Two-Stage Natural Gas Furnace (Model GR9T801205DN).
This wasn’t just another installation for a customer. This was Tony’s own system — the one that would heat his home, his family, and his workshop for years to come.
Below, Tony shares a detailed walkthrough of his install process, the tools he used, what went smoothly, what didn’t, and his best professional advice for anyone planning to install (or have installed) a Goodman 120,000 BTU furnace.
🏠 Why Tony Chose the Goodman GR9T801205DN
Tony’s previous furnace — a 100,000 BTU single-stage unit from the early 2000s — was still kicking, but its blower was noisy, and it short-cycled on milder days.
“I wanted two things: quieter comfort and longer run cycles,” Tony says. “That’s why I went with a two-stage Goodman. It gives me the power when I need it, but it doesn’t hammer on and off all day.”
Here’s what sold him:
-
Two-stage heating: smoother comfort and less stress on parts.
-
9-speed ECM blower: efficient, quiet, and self-adjusting.
-
Aluminized steel heat exchanger: durable, rust-resistant design.
-
Compact 24.5” cabinet: perfect fit for his utility space.
🔩 Step 1: Prepping the Space
Tony always says, “The install starts before the furnace even leaves the box.”
His first task was prepping his utility room:
-
Cleared 36 inches of space in front of the furnace for service access.
-
Checked supply and return duct alignment to ensure a straight connection.
-
Verified floor levelness using a small torpedo level — just ⅛" of slope can affect blower balance over time.
-
Made sure the existing flue was solid, with no cracks or corrosion.
He also turned off power to the old system, shut off the gas valve, and tagged the line for safety.
“You’d be surprised how many installs go bad because people rush this part,” Tony says. “Good prep saves hours later.”
⚙️ Step 2: Removing the Old Furnace
With the new Goodman ready, Tony started carefully removing the old single-stage unit.
Disconnection Steps:
-
Shut off gas supply and capped the line temporarily.
-
Labeled all thermostat wires before disconnecting — red (R), white (W), green (G), yellow (Y), and common (C).
-
Unbolted the return and supply plenums, cutting the old sheet metal cleanly for reuse.
-
Disconnected the exhaust flue and electrical whip.
Tony inspected the old furnace interior for rust or soot buildup. The burner chamber had a faint white residue — a sign of moisture exposure.
“That’s usually from an aging flue,” Tony notes. “When you see that, it’s time for a vent upgrade too.”
🧰 Step 3: Setting the Goodman Furnace in Place
The GR9T801205DN is versatile — it can be installed upflow or horizontal — so Tony chose an upflow setup to match his home’s duct configuration.
He placed a rubber isolation pad beneath the base to absorb vibration and noise. Then he verified:
-
1” side clearance
-
3” clearance from combustibles around the vent
-
Solid floor contact with no tilt
He reattached the return and supply ducts using flexible canvas connectors, which help reduce sound transfer between the metal ducts and furnace cabinet.
“That’s one of my favorite tricks,” Tony says. “You get noticeably quieter airflow and less vibration.”
🔥 Step 4: Gas Line & Electrical Connections
Next came the heart of the install — gas and power hookups.
Gas Line Steps:
-
Installed a manual shutoff valve and drip leg per code.
-
Applied pipe dope rated for natural gas (never Teflon tape) on threads.
-
Checked line pressure using a manometer — Goodman recommends 3.5” WC for natural gas.
Electrical Setup:
-
Ran 120V power to the disconnect box and furnace junction.
-
Connected ground wire securely to cabinet.
-
Hooked up low-voltage wiring (R, C, W1, W2, G) from the two-stage thermostat.
Tony’s advice:
“Never guess gas pressure. Measure it. I’ve seen too many systems running at 2 inches when they need 3.5 — and that kills performance.”
🔗 Reference: Energy.gov – Gas Furnace Safety Tips
🌬️ Step 5: Venting & Air Intake Setup
Since this is an 80% AFUE furnace, Tony kept the existing B-vent metal chimney but made key upgrades:
-
Replaced all elbows with new corrosion-resistant fittings.
-
Ensured ¼” rise per foot on the horizontal vent run.
-
Used foil HVAC tape, not duct tape, on joints for airtight sealing.
-
Verified draft strength with a smoke test — ensuring the exhaust pulled up the chimney properly.
“A good vent draft is your first safety check,” Tony explains. “If the smoke hangs or rolls out, stop everything.”
He also checked the combustion air openings were unobstructed — one of the most overlooked steps in gas furnace installs.
⚡ Step 6: Thermostat Wiring & Control Board Setup
Tony installed a two-stage programmable thermostat to take full advantage of the Goodman’s capabilities.
Wiring Details:
-
R – Power
-
C – Common
-
W1/W2 – Stage 1 & Stage 2 heat
-
G – Blower
-
Y – Cooling (for future A/C pairing)
Inside the furnace, he configured the control board’s DIP switches to “Auto” stage mode — letting the board decide when to jump from low to high fire.
“Auto-staging is the sweet spot. The furnace stays efficient most of the time and only goes full tilt when it’s truly cold.”
🔗 Guide: HVAC.com – Two-Stage Furnace Wiring
🧪 Step 7: Testing and Fine-Tuning
The big moment came: Tony flipped the switch.
First Fire Checklist:
✅ Power on
✅ Thermostat call for heat
✅ Igniter glow
✅ Gas valve click
✅ Blue, stable flame
He ran the system through both stages, measuring:
-
Gas pressure: 3.5” WC steady
-
Temperature rise: 52°F (Goodman spec range: 45–75°F)
-
Static pressure: 0.6” WC (well within range)
Tony used a digital thermometer at the supply and return ducts to ensure even heating. The ECM motor ramped smoothly, with no vibration or noise.
“That’s when you know you nailed the install,” Tony says. “When the blower’s quiet and the flame’s steady, you can breathe easy.”
🧯 Step 8: Safety & Final Checks
Before calling it done, Tony performed every inspection he’d expect from a pro job:
-
✅ Gas leak test with soap solution
-
✅ Verified vent clearance outside
-
✅ Checked for carbon monoxide at the register (0 ppm reading)
-
✅ Confirmed thermostat cycle timing
He also installed a CO detector within 10 feet of the furnace — a must-have in any gas system home.
“I’ve seen perfect installs go bad because nobody checked CO levels after year one. That detector is a $25 life-saver.”
🧼 Step 9: Post-Installation Maintenance Setup
Tony built a maintenance schedule for himself — and every homeowner should too.
Seasonal Checklist:
-
Replace air filter every 3 months
-
Vacuum burner compartment annually
-
Inspect vent pipe for rust or leaks
-
Check flame sensor once a year
-
Oil blower bearings if non-sealed
His chosen filter: MERV 8 pleated 16x25x4 — high enough for air quality, low enough for airflow efficiency.
🔗 Maintenance Resource: EnergyStar – Furnace Care Tips
💡 Lessons Tony Learned from This Install
After decades of installs, Tony still finds new lessons each time.
1. Preparation Saves the Day
“The hardest work happens before you ever lift the furnace. Lining up ducts and checking vents takes time, but it makes the rest easy.”
2. Gas Pressure Is Everything
Even a slight deviation affects performance. Tony’s manometer is his most trusted tool.
3. Label Everything
“I’ve opened plenty of panels where wires were just guessed. Label first, wire later — always.”
4. Balance Airflow Properly
Tony spent 30 minutes adjusting dampers to ensure even temperatures between floors.
“The quiet efficiency of a two-stage unit only shines if the airflow’s balanced.”
5. Register That Warranty
Goodman’s 10-year parts warranty and lifetime heat exchanger warranty are incredible value — but they only count if registered.
📊 Time, Cost & Tools Summary
| Category | Details |
|---|---|
| Total Time | 7.5 hours (including removal & test) |
| Tools Used | Multimeter, manometer, drill, level, tin snips, Magnehelic gauge |
| DIY Material Cost | ~$3,000 (including vent pipe, sealant, wiring) |
| Professional Install Estimate | $4,500–$5,000 |
| Tony’s Verdict | “If you have experience, it’s a clean, straightforward install. Goodman’s design makes it easy — everything’s accessible and labeled.” |
🧠 Pro Tips from Tony
-
Tip 1: Use metal foil tape, not duct tape, on flue joints.
-
Tip 2: Check thermostat staging after 24 hours — ensure it’s switching smoothly between W1 and W2.
-
Tip 3: Write down gas pressure and temperature rise on the furnace panel for future reference.
-
Tip 4: Keep a spare igniter handy — they’re inexpensive and easy to replace.
-
Tip 5: Add a surge protector for electronics.
💬 Tony’s Takeaway: “Preparation Makes Perfection”
“Installing a Goodman isn’t hard if you respect the details. This 120k two-stage furnace runs quieter, smoother, and steadier than anything I’ve had before. Every step I double-checked paid off the first time I heard that soft blower hum and felt even heat throughout the house. That’s when you know you did it right.”
✅ Final Word
The Goodman GR9T801205DN isn’t just a high-performing furnace — it’s an installer’s dream. Everything from its cabinet design to wiring layout shows that Goodman understands the realities of installation.
For Tony, that meant less frustration, faster setup, and the satisfaction of knowing his home now runs on a system built for long-term comfort.
“It’s not the fanciest unit out there,” Tony says, “but it’s solid, efficient, and honest. That’s the kind of furnace you can rely on.”
In the next topic we will know more about: How Much Does a 120,000 BTU Furnace Installation Cost in 2025?
