Intro from Mark
Alright, friends—it’s Mark here, back with another furnace deep dive. Grab your coffee, because this one’s going to be both practical and a little bit fun. We’re tackling the Goodman GR9T801205DN, a 120,000 BTU natural gas furnace with two-stage heating and a nine-speed blower. Sounds fancy, but the magic really comes down to how you install it.
Now, Goodman designed this bad boy to be flexible—you can run it upflow (air blowing upward) or horizontal (air blowing sideways). Which setup works best? That depends on your home, your ductwork, and how much headroom you’ve got. Today, I’ll break down the step-by-step installation process for both options, compare the pros and cons, and share tips to avoid the mistakes I see way too often.
So let’s roll—because a properly installed furnace is the difference between cozy comfort and winter misery.
Understanding the GR9T801205DN
Before we start tossing wrenches around, let’s make sure we know the star of the show.
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Capacity: 120,000 BTUs—plenty of heat for large homes
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Efficiency: 80% AFUE (not ENERGY STAR, but strong value)
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Blower: 9-speed ECM motor (quiet, efficient, adaptable)
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Ignition: Silicon nitride igniter (reliable, long-lasting)
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Orientation: Upflow or horizontal only (not downflow)
This furnace isn’t a “one size fits all” box—it’s built for versatility. That’s why the install orientation matters so much.
If you want the manufacturer’s technical scoop, Goodman’s own product documentation lays it out in detail. But don’t worry—I’ll keep this guide homeowner-friendly.
Tools and Prep Work
Let’s be real—you can’t just plop a furnace in and call it a day. Here’s what you’ll need:
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Tape measure (for clearances)
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Level (to make sure the furnace sits straight)
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Drill and sheet metal screws
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Gas line tools (pipe wrench, thread sealant)
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Electrical tools (wire stripper, multimeter)
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PVC or metal vent materials (depending on vent type)
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Safety gear (gloves, eye protection)
And before you even unbox that shiny Goodman, make sure your space meets code for clearances and venting. The Air Conditioning Contractors of America has excellent resources on installation standards—worth a read if you’re DIYing.
Step 1: Choosing Orientation—Upflow vs. Horizontal
Here’s where you decide how this furnace fits your home.
Upflow
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Air comes in from the bottom, out through the top.
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Best for basements or closets with ductwork above.
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Easy to service, great airflow.
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Requires headroom (watch out for low ceilings).
Horizontal
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Air comes in one side, blows out the other.
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Perfect for attics or crawlspaces.
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Saves vertical space.
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Harder to service (tight squeeze).
Think about your duct layout. If your ducts run overhead, upflow is natural. If they snake through the attic, horizontal makes more sense.
The Energy.gov guide on heating systems highlights how matching equipment orientation with your ductwork improves both performance and efficiency.
Step 2: Setting the Furnace in Place
For upflow, place the furnace upright on a level platform or pad.
For horizontal, set it on its side, supported evenly to prevent sagging.
Clearances matter:
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24" front for service access
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1" sides and top
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0" back (flush mount okay)
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6" from vent pipes to combustibles
Use a level. An unlevel furnace = blower noise and drainage issues.
Step 3: Connecting the Ductwork
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Upflow: Connect supply plenum on top, return on bottom.
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Horizontal: Supply connects to one side, return to the other.
Seal joints with mastic or UL-listed foil tape. Don’t skip this—leaky ducts waste money. The HVAC School blog has a whole section on duct sealing mistakes to avoid.
Step 4: Gas Line Hookup
This is where my inner safety cop comes out. If you’re not 100% confident, hire a licensed gas fitter. Natural gas isn’t forgiving.
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Use black iron or approved flex connectors.
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Apply thread sealant (not Teflon tape).
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Test with a manometer—11–14 inches water column is typical for natural gas.
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Check for leaks with soapy water.
Never light up the furnace until you know it’s leak-free.
Step 5: Venting and Combustion Air
Since this is an 80% AFUE Category I furnace, you’ll need metal venting (B-vent or single-wall, depending on clearance). Slope the vent upward 1/4" per foot. Terminate it outside per local codes.
Don’t forget combustion air. Furnaces need oxygen. Provide fresh air openings or ducts into the mechanical room. The International Code Council outlines the combustion air requirements if you want to dive deep.
Step 6: Electrical Connections
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Connect 120V power supply with a service switch nearby.
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Wire in the thermostat (R, W, G, Y, C terminals).
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Double-check polarity and grounding.
This furnace has a smart control board that won’t run right if wired incorrectly. Always match the wiring diagram provided in the furnace manual.
Step 7: Startup and Testing
Here’s the fun part—fire it up.
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Turn on the power and gas.
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Set thermostat to heat.
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Watch for ignition.
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Check flame quality (steady blue, not yellow).
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Verify blower operation on both heating and cooling speeds.
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Monitor for error codes on the LED board.
If the furnace flashes codes, reference the manual. The self-diagnostic system makes troubleshooting much easier than old-school guessing.
Common Mistakes and How to Avoid Them
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Forgetting to level the furnace—causes condensate pooling.
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Undersized return air—chokes airflow and overheats the unit.
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Improper vent slope—leads to condensation and rust.
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Ignoring combustion air—creates unsafe conditions.
Mark’s tip: Keep a log of your install—gas pressure, temperature rise, static pressure. If a problem pops up later, you’ll thank yourself for the notes.
Outro from Mark
And there you have it—your step-by-step guide to installing the Goodman GR9T801205DN in either upflow or horizontal orientation. I know, it’s a lot of detail, but trust me—doing it right the first time saves you a world of headaches later.
If you’re sizing up a furnace for your own home and want the nitty-gritty specs, check out the Goodman GR9T801205DN at The Furnace Outlet. That’s the same workhorse we’ve been talking about all along.
Need more troubleshooting tips for this unit? Visit my guide: Decoding Error Codes.
So whether you’re dropping this in a basement in upflow mode or squeezing it sideways into an attic, keep it level, sealed, and properly vented. Do that, and this furnace will keep you as cozy as a cat napping on a sunbeam. Until next time, I’ll be here with my coffee, keeping it real and keeping it warm 🔥☕.
- Mark, your go-to HVAC tech!