Gas Line Basics for Furnace Installation: Don’t Blow It (Literally)

Let’s Talk Gas—The Kind That Heats Your House 🛠️🔥

Hey, Mike here. And look, I’ll be straight with you—working with gas lines isn’t something you do on a whim, right after a YouTube binge and two cups of strong coffee. There’s zero room for mistakes, because the stakes are sky high (as in boom 💥).

But if you're serious about understanding the right way to handle gas lines for furnace installation—and you're willing to follow codes and know your limits—you’re in the right place. I’ll walk you through the basics, the do's and don’ts, and when to tag in a pro.

This isn’t just about getting heat—this is about keeping your house and family safe.

 

What Kind of Gas Lines Are We Talking About?

When we say “gas line,” we’re usually referring to natural gas or propane (LP gas). Most residential furnace installs connect to natural gas provided by your local utility.

The furnace connects to a branch line from the main gas supply in your home, typically using one of the following materials:

• Black Iron Pipe (the classic and code-friendly choice)

• CSST (Corrugated Stainless Steel Tubing) – flexible and easier to route

• Copper Tubing – sometimes allowed for propane, depending on your local code

• PE Pipe – only used outdoors and underground

The International Fuel Gas Code (IFGC) governs how these materials should be installed. Trust me, it’s not a “pick your favorite” kind of deal.

 

What Size Gas Line Do You Need?

Sizing the gas line isn’t a guessing game. Undersize the line, and your furnace won’t get the fuel flow it needs. Oversize it, and you’ve wasted time and money on material.

Here’s what affects gas line size:

• BTU rating of your furnace (you’ll find this on the label)

• Length of the run from the gas meter to the furnace

• Number of appliances sharing the same gas line

• Pipe material (CSST has different flow properties than black pipe)

Most furnace installers use a gas sizing chart based on these factors. You can find a solid one in this Engineering Toolbox calculator.

Example: A 100,000 BTU furnace might need a 3/4” line if it’s a short run. But if you’re going 60 feet with other appliances on the same line, you might need to bump up to 1”.

 

The Right Tools and Fittings for the Job

If you’re using black iron, you’ll need:

• Pipe wrench set

• Pipe cutter or threader

• Pipe dope or yellow Teflon tape (rated for gas)

• Unions, nipples, elbows, tees (depends on the layout)

Using CSST? Then you’ll need:

• Manufacturer-specific fittings

• Bonding clamp and wire (don’t skip this!)

• Termination fittings to transition to appliance connection

⚠️ Pro Tip: Always install a manual shutoff valve within 6 feet of the furnace. It’s required by code, and it’s a life-saver when things go sideways.

Need a visual? Supreme Steel Pipe Corp has a solid walkthrough on running black iron gas pipe safely.

 

Common Furnace Gas Line Mistakes (and How to Avoid 'Em)

Let’s talk about the mistakes I’ve seen on job sites that made me shake my head harder than when someone uses duct tape to fix a flue.

1. Using the Wrong Pipe Sealant

That white Teflon tape for plumbing? Not safe for gas. Use yellow gas-rated tape or pipe dope specifically marked for gas.

2. Failing to Pressure Test the System

You can’t just tighten everything up and hope for the best. You need to do a pressure test using either:

• Soapy water test (good for DIYers)

• Manometer or gas leak detector (preferred method)

NFPA 54 requires all new gas line installs to be pressure tested. Leaks are no joke.

3. No Bonding Wire on CSST

Flexible CSST must be bonded to the home’s grounding system to prevent arc-induced holes during lightning strikes. That’s right—a zap from afar can literally poke holes in your gas line.

Read Gastite’s bonding instructions if you're using flexible CSST.

4. Improper Support or Sagging Lines

Gas lines must be secured at regular intervals (typically every 4 to 6 feet). Sagging = stress = leaks over time.

 

Local Code Is King 👑

No matter what your buddy or some blog tells you, your local building code wins every time. Codes vary by city and county, especially when it comes to:

• What kind of pipe is allowed

• Whether you can use unions or elbows inside walls

• If a permit is required for furnace replacement

• Whether homeowners can pull their own gas permits

Not sure? Call your local building department or check your city’s permit website. Also check out the National Fuel Gas Code (NFPA 54) for national standards.

 

Step-by-Step: Connecting the Gas Line to the Furnace

This is the moment where things get real. Follow this rough outline if you’re DIY-ing the connection (and again, only if local code allows it).

1. Turn off the gas at the meter

You don’t want any pressure in the line.

2. Thread and connect the pipe or CSST

Follow manufacturer instructions. Keep joints clean and use approved sealant.

3. Install the manual shutoff valve

Position it close to the furnace and easily accessible.

4. Add a sediment trap (drip leg)

This collects debris or moisture before it hits your burners. It’s a short vertical stub just before the furnace inlet. Required by most codes.

5. Pressure test the full line

Use soapy water on every joint—or a combustible gas leak detector. If bubbles form or alarms sound, turn everything off and fix it.

6. Restore gas and light the furnace

Follow the manufacturer’s startup procedure, usually involving purging air from the line and watching the burners ignite.

 

What About Propane (LP) Systems?

If you’re on propane instead of natural gas, most of the same principles apply. But:

• Your furnace may need a conversion kit

• Pressures are different (11” WC for propane vs. 3.5” WC for natural gas)

• Copper pipe is sometimes allowed (again—local code!)

• Propane tanks must be installed outdoors and regulated properly

For a clear breakdown of propane furnace requirements, check out this guide from Propane.com—they know their stuff.

 

When Should You Call in a Pro?

Honestly? As soon as you feel unsure. Here’s when it’s a non-negotiable:

• You’ve never worked with gas before

• Your local code prohibits DIY installs

• You need to size a complex system

• You smell gas or can’t find a leak

• You fail the pressure test (don’t ignore it!)

Sites like Angi or HomeAdvisor can help you find licensed gas installers near you. Look for folks with solid reviews and clear pricing.

 

Final Thoughts from Mike

Look, I love a good DIY project as much as the next guy, but gas line work? It’s not like hanging drywall or installing a light fixture. This stuff matters.

If you’re gonna do it, do it right—follow code, use the right materials, and don’t cut corners. And if you’re ever in doubt, call a pro. There’s no shame in playing it safe when lives are on the line.

And when you’re ready to connect that shiny new furnace, check out The Furnace Outlet. They’ve got high-efficiency units, great support, and shipping that doesn’t take forever.

Does your furnace keep shutting off? Visit my guide: Here’s Why—and What to Do About It.

Stay safe out there—and may your heat always kick on when you need it.
– Mike, your cool HVAC guide 🔧