Let’s Not Burn Down the House 🔥 (Intro from Jake)
Hey there, home hero. You finally got your hands on a new furnace—maybe from the furnaces collection at The Furnace Outlet—and you’re either planning a solo install or supervising one closely. Good call. Because let me tell you, installing a furnace isn’t rocket science… but it ain’t LEGO either.
There are 10 big, fat mistakes I’ve seen time and again on job sites, in DIY forums, and (yes) from the pros who should know better. Some of them will leave you chilly. Some could be dangerous. All are avoidable.
Let’s break 'em down. 👇
1. Skipping the Load Calculation ❄️🔥
The Mistake:
Buying a furnace without sizing it for your home.
Why It’s a Problem:
Too small? It’ll run nonstop. Too big? It’ll short-cycle and stress itself out. Either way, you’re wasting energy and money.
Dodge It:
Do a Manual J load calculation. You can hire someone or use a reputable tool. Energy Vanguard breaks down why Manual J matters.
Rule of thumb? About 30–60 BTUs per square foot, depending on climate zone, insulation, and windows. But guessing? That’s what gets you into trouble.
2. Bad Furnace Placement 🧱🧯
The Mistake:
Cramming the furnace in a tight closet or placing it too close to flammable materials.
Why It’s a Problem:
Lack of clearance leads to overheating, fire hazards, and nightmare maintenance access.
Dodge It:
Follow the clearance specs in the install manual. Most furnaces need 1–6 inches on each side for airflow and service. And keep it on a level surface. No one wants to service a furnace lying sideways in a crawlspace.
Bonus: NFPA 54 (National Fuel Gas Code) has clearance and combustion guidelines worth a look if you’re playing with gas.
3. Improper Duct Sizing or Sealing 🌬️
The Mistake:
Connecting your new furnace to old, undersized, or leaky ducts.
Why It’s a Problem:
It chokes the airflow, overworks the blower motor, and ruins your efficiency.
Dodge It:
Make sure the ducts match the furnace’s output. For a 100,000 BTU unit, you need a return duct of about 20" and supply of at least 8" round per room, give or take.
Seal all joints with foil tape, not duct tape. Family Handyman has a legit guide on doing this right.
4. Forgetting the Condensate Drain (on High-Eff Units) 💧
The Mistake:
Not installing or improperly sloping the condensate drain line.
Why It’s a Problem:
Water backs up, clogs the unit, or causes mold.
Dodge It:
Use ¾-inch PVC and make sure it slopes downward. If you're draining to a pump, install a check valve. Clean it with vinegar every season to avoid gunk.
Carrier’s install manuals offer diagrams that can help if you're visual.
5. Lazy Thermostat Wiring 😬
The Mistake:
Crossing thermostat wires or using the wrong voltage.
Why It’s a Problem:
Your system won’t heat—or worse, you could fry the control board.
Dodge It:
Follow color codes:
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R = Power
-
W = Heat
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G = Fan
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C = Common
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Y = Cooling
Snap a pic of the old wiring before you disconnect anything. If you’re adding a smart thermostat, DoItYourself’s guide explains how to adapt wiring safely.
6. No Combustion Air Supply 🚫🌫️
The Mistake:
Installing a gas furnace in a sealed room with no fresh air access.
Why It’s a Problem:
Poor combustion = carbon monoxide, poor efficiency, and a furnace that shuts off mid-cycle.
Dodge It:
High-efficiency furnaces usually have sealed combustion (direct vent). If yours doesn't, make sure there’s an air intake from outside or a vented louver in the door.
Check out HVAC.com's combustion air guide for visual examples.
7. Not Leveling the Furnace 📏
The Mistake:
Installing the furnace crooked, especially on sloped basement floors.
Why It’s a Problem:
Condensate won’t drain properly. Burners may fire unevenly. And your blower motor will wear unevenly.
Dodge It:
Use shims or a furnace pad. Make sure it’s level side-to-side and front-to-back. A simple torpedo level works wonders here.
8. Ignoring the Flue Pipe Rules 🔥
The Mistake:
Using the wrong type or length of vent pipe—or worse, skipping the pitch.
Why It’s a Problem:
Poor venting = CO poisoning or tripped safeties.
Dodge It:
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Type B gas furnaces need metal flue pipe with 1/4" rise per foot
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Condensing furnaces need PVC venting with specific length limits
Always slope your vent away from the furnace for metal, or toward the drain for PVC. WM Henderson has a great breakdown of venting types.
9. Skipping the Startup Sequence Checklist 🚨
The Mistake:
Turning it on and walking away.
Why It’s a Problem:
You miss warning signs—like no ignition, weird smells, or flame rollout.
Dodge It:
Watch the startup sequence:
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Thermostat sends call
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Draft fan kicks on
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Ignitor glows
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Gas opens
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Burner fires
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Blower starts
If that’s not happening in order? Something’s wrong.
10. Not Registering the Warranty 📬
The Mistake:
Leaving that warranty card in the box.
Why It’s a Problem:
Many HVAC brands—like Goodman—offer 10-year parts warranties, but only if registered within 60 days.
Dodge It:
Register online. Keep your invoice and serial number handy. Without it, you’re stuck with a basic 5-year warranty. Don’t lose out.
Wrap-Up: Avoid These Mistakes and You’re Already Ahead 🧤💪
So there it is. Ten of the most common furnace install mistakes—most of them made by rushed techs, distracted DIYers, or people who thought they could skip the manual. Avoid these traps, and your install is smoother, safer, and ready to bring the heat.
And hey—if you haven’t picked up your system yet, check out the lineup at The Furnace Outlet. Good prices, good gear, and they don’t try to upsell you on stuff you don’t need. You just might see your new furnace show up in a box big enough to live in. (Not that I’ve tried. 😅)
Need to know how to diagnose furnace startup failures? Visit my guide: From Ignition to Blower.
Stay warm out there—and if it smells like burnt toast during your test run, it’s probably just dust. Probably.
Until next time,
- Jake, your comfort loving tech
