Gas Furnace Installation Red Flags: What Every DIYer Should Watch For

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Alright my HVAC DIYers—Tony Here.

Let’s talk about installing 3.5-ton R-32 air conditioner and gas furnace systems. These units hit that sweet spot for mid-sized homes: not too big, not too small—but they pack a punch, and they’re not forgiving if you mess up.

These aren’t plug-and-play Amazon specials. These are full-on, 208/230V, multi-stage, pressure-balanced machines that rely on solid ductwork, clean gas lines, correct venting, and proper commissioning.

You thinking about installing one yourself? Cool—I respect it. But before you start wrenching away, you better know what red flags to watch out for.

Red Flag #1: Undersized or Existing Ductwork Isn't Rated for 3.5 Tons 😤

A 3.5-ton system moves about 1,400 CFM. That’s a whole lot of air. But too often, I see folks trying to jam that through 12" returns or choked-off supply trunks that were built for a 2-ton unit.

Here's what happens:

Static pressure skyrockets
The blower overheats or ramps up constantly (if variable speed)
You get hot and cold rooms
And the gas furnace starts short cycling

Minimum duct size recommendations for 3.5-ton systems?

Return trunk: at least 18" round or 20"x12" rectangular
Supply trunk: same deal—or split into balanced zones

Still not sure if your ductwork is cutting it? HVAC Design Guide has a helpful online tool to run quick duct sizing checks.

Red Flag #2: Furnace Gas Line Isn't Sized or Pressured Correctly 🛑💨

A 3.5-ton system typically comes with an 80,000–100,000 BTU gas furnace, and that kind of firepower demands proper fuel delivery.

Here’s the mistake I see all the time:

Homeowner taps into a ½" line that already feeds a water heater
Then wonders why the furnace is starving for gas and short cycling

You want a steady 3.5–5 inches WC of manifold pressure. Use a manometer—don’t guess. If you don’t have one, stop now. You’re playing with live gas.

For more on proper gas sizing, this gas line sizing table is a great reference.

And always:

Use pipe dope for gas, not PTFE tape
Pressure test with air—not with the furnace connected
Check local codes on flex connectors (some areas prohibit them for furnace installs)

Red Flag #3: No Fresh Air Intake or Improper Combustion Setup 🫁🔥

That 96% AFUE Goodman furnace you're installing? It needs combustion air, especially if it's tucked into a sealed attic or closet.

What can go wrong:

No outside air = poor combustion
Poor combustion = soot buildup
Soot buildup = heat exchanger stress
Heat exchanger stress = cracked exchanger
Cracked exchanger = carbon monoxide risk

Don’t be that guy.

Check this Building America guide on combustion air if you’re unsure whether your space needs sealed intake/exhaust.

Bonus tip: Use schedule 40 PVC venting, and slope it properly back to the furnace to prevent condensation in the pipe.

Red Flag #4: You Didn’t Clock the Meter or Do a Combustion Test ⏱️🧪

You wouldn’t drive a car without checking the oil, right? Same idea here.

After installing the gas furnace:

Clock the gas meter to ensure it’s getting the right volume
Run a combustion test using a digital analyzer
Check for CO, O₂, and stack temperature

“But Tony, I don’t have a combustion analyzer.”
Then you shouldn’t be installing a gas furnace solo. Period.

Want to learn how it’s done? HVAC School’s guide to combustion testing is gold. Read it. Twice.

Red Flag #5: You Didn’t Properly Configure the Blower for 3.5 Tons 💨🧾

Even if you nail the install mechanically, your system can still underperform if you don’t configure the blower correctly.

Most Goodman 3.5-ton air handlers and furnaces ship with blower speeds set for default 2 or 3-ton loads. You must adjust:

Blower tap settings
DIP switches (for multi-speed or ECM motors)
Cool/heat delay timers

Fail to do this, and:

Cooling is weak
Heating cycles are uneven
SEER2 rating drops
Utility bills spike

Need help adjusting speeds? Goodman’s own Installation & Operating Instructions have detailed blower setup tables based on tonnage and static pressure.

Red Flag #6: You Didn’t Pull a Permit or Plan for Inspection 🧯📋

If your furnace install doesn’t go through inspection, you’re risking:

Code violations
Voided homeowner’s insurance
Resale headaches
And worst of all—liability

Even if you DIY the labor, get it permitted and inspected. Many counties allow homeowner installs—but require documentation and review.

You can check your state’s policy with this Residential Code Lookup from the International Code Council.

Red Flag #7: No Test Run or Commissioning Log 📉😩

You’d be shocked how many installs I get called to fix because the homeowner never actually tested airflow, temperatures, or refrigerant charge.

After installing your 3.5-ton R-32 AC and gas furnace system, do this:

Measure delta T (18–22°F is ideal)
Check refrigerant charge by subcooling/superheat
Log static pressure, return/supply temps, and blower settings
Verify thermostat operation

Still guessing? ESCO Institute’s AC troubleshooting chart is a great final QA checklist.

Final Words from Tony 👋

Look, if you’re thinking of installing a 3.5-ton R-32 AC and gas furnace system from scratch—like the ones from The Furnace Outlet—I give you credit.

But don’t confuse confidence with competence. This ain’t IKEA furniture. This is flammable refrigerant, live gas, high voltage, and potential carbon monoxide.

So before you fire it up, run back through this list. Ask yourself: