Tony here. Let me say this upfront:
Most furnace problems don’t start at start-up — they start before the furnace ever gets set in place.
I’ve seen brand-new, high-efficiency furnaces blamed for noise, short-cycling, lockouts, and high gas bills when the real issue was sloppy prep. The Goodman 96% AFUE 100,000 BTU two-stage furnace is a solid machine — but it only performs as well as the system it’s installed into.
This guide is your pre-installation checklist. Not the manufacturer’s bare-bones version — the real-world one installers use when they don’t want callbacks.
If you get this right, start-up becomes boring. And boring is good.
📦 Step 1: Confirm You Bought the Right Furnace (Before the Truck Arrives)
Before you touch a wrench, verify the equipment is actually correct for the job.
The Goodman 96% AFUE 100,000 BTU two-stage furnace in a 21-inch cabinet is designed for medium-to-large homes, tighter envelopes, and modern duct systems.
Double-check:
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BTU output matches the home’s heat load (not the old furnace size)
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Cabinet width fits your mechanical space
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Orientation (upflow or horizontal) matches the install plan
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Gas type is natural gas (not propane unless converted)
👉 Product reference:
If the furnace doesn’t physically fit or match the application, no amount of start-up magic fixes that.
📐 Step 2: Measure the Space Like a Pro (Clearances Matter)
High-efficiency furnaces need room to breathe, drain, and be serviced.
Minimum checks before install:
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Front clearance for blower removal
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Side clearance for gas valve and wiring access
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Top clearance for vent connections
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Floor support (especially in horizontal installs)
I’ve pulled furnaces back out because someone “eyeballed” clearances instead of measuring. Don’t be that guy.
Tony rule:
If a tech can’t remove the blower without uninstalling the furnace, the install is already wrong.
🌬️ Step 3: Inspect the Ductwork Before You Blame the Furnace
Two-stage furnaces expose bad ductwork fast.
Before install, inspect:
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Return size vs furnace airflow requirements
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Supply plenum size and transitions
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Existing restrictions (old dampers, crushed flex, undersized returns)
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Filter location and size
This furnace uses a 9-speed ECM blower. That blower will try to move air — but if the duct system is choking it, you’ll get noise, limit trips, and comfort complaints.
Checklist item:
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Verify return duct can handle low-stage and high-stage airflow without exceeding static pressure limits
If you don’t own a manometer, stop here and get one.
🔥 Step 4: Verify the Gas Supply Before You Set the Furnace
This is where most installs quietly fail.
Before install:
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Confirm gas line size supports 100,000 BTUs
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Measure available gas pressure
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Inspect shutoff valve placement
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Check sediment trap (drip leg) location
A furnace starved for gas won’t trip immediately — it’ll run just badly enough to cause nuisance issues later.
Tony truth:
A perfect furnace on a bad gas line is still a bad system.
🧱 Step 5: Plan the Venting System — Don’t Reuse Old Mistakes
This Goodman is a condensing furnace. That means PVC venting, correct pitch, and proper termination.
Pre-install vent checklist:
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Verify allowed vent pipe material (PVC, CPVC, or polypropylene)
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Measure total equivalent vent length
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Plan proper slope back to furnace
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Confirm intake and exhaust termination locations
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Check local code for clearances from windows and doors
Never assume the old venting is acceptable just because the old furnace “worked.”
👉 Manufacturer reference:
https://www.goodmanmfg.com/products/gas-furnaces
💧 Step 6: Condensate Drain Planning (Where Most People Get Lazy)
Condensing furnaces make water. That water must leave — every time, in every condition.
Before install:
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Identify a gravity drain or pump location
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Plan trap installation
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Ensure freeze protection in cold spaces
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Verify drain slope (¼″ per foot minimum)
If the drain is an afterthought, the service call is guaranteed.
Tony rule:
No drain plan = no start-up.
⚡ Step 7: Electrical and Control Prep
This furnace is smart — but only if it’s wired correctly.
Pre-install electrical checks:
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Dedicated circuit available
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Correct breaker size
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Proper grounding
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Clear routing for thermostat wire
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Thermostat compatibility with two-stage heat
Don’t cheap out on the thermostat. Two-stage furnaces shine when staging is controlled properly.
🧰 Step 8: Tools You Should Have Before Installation Day
If you don’t have these on site, you’re not ready for start-up.
Minimum tool list:
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Combustion analyzer
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Digital manometer
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Multimeter
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Level (yes, really)
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Drill with proper vent bits
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PVC cement rated for flue gas temps
Borrowed tools lead to skipped steps. Skipped steps lead to callbacks.
📋 Step 9: Pre-Start Documentation and Setup
Before the furnace ever fires:
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Register the warranty
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Review installation manual (yes, actually read it)
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Verify conversion kits if applicable
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Document baseline measurements
This takes 15 minutes. It saves hours later.
✅ Step 10: The “Ready for Start-Up” Final Walkthrough
Before first ignition, walk the system:
✔ Furnace is level
✔ Gas line tested
✔ Venting glued, pitched, and supported
✔ Drain trapped and tested
✔ Duct connections sealed
✔ Filter installed
✔ Thermostat wired correctly
If every box is checked, start-up becomes simple.
🔚 Final Tony Takeaway
The Goodman 96% AFUE two-stage furnace doesn’t need babysitting — it needs preparation.
Most “problem furnaces” are victims of rushed installs, ignored ductwork, and lazy planning. Do the work before the install, and start-up becomes the easiest part of the job.
In HVAC, the system doesn’t forgive shortcuts — it invoices them.
🔗 External References
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Goodman Gas Furnace Product Line
https://www.goodmanmfg.com/products/gas-furnaces - U.S. Department of Energy – Furnaces & Boilers
https://www.energy.gov/energysaver/furnaces-and-boilers
