ABS pipe for gas furnace flue venting Ontario: Is It Safe for High-Efficiency Furnaces?

Is ABS pipe safe for venting a high-efficiency furnace in Ontario? Learn what’s allowed, what changed after 2007, and what homeowners should do today.

Introduction

If you’ve ever looked at your furnace and noticed a black plastic pipe (ABS) venting exhaust outside, you’re not alone. Many Ontario homes—especially those built or updated in the 1990s and early 2000s—have this exact setup; ABS pipe for gas furnace flue venting

But here’s the question homeowners may be asking:

Is ABS piping still safe and allowed for venting a high-efficiency furnace?

The answer isn’t a simple yes or no. Let’s break it down in plain language. 

What Type of Furnace Are We Talking About?

Most newer furnaces are high-efficiency (condensing) furnaces. These systems:

• Extract more heat from combustion gases

• Produce cooler, moist exhaust

• Create acidic condensation inside the vent pipe

Because of this, the venting system plays a critical safety role in your home to contain and exhaust products of combustion to the outside.

Why Was ABS Pipe Used in the First Place?

Back in the 1990s and early 2000s, ABS piping was widely available, easy to install and commonly used for furnace venting. So if your home was built or updated during that time, seeing ABS is very normal. However, there’s an important detail, ABS pipe was never specifically designed or certified in Canada for venting furnace exhaust gases.

At the time, it was more of an industry practice than a fully approved solution.

What Changed? (The 2007 Turning Point)

As of August 1, 2007, the CSA B149.1 – Natural Gas and Propane Installation Code requires that plastic venting systems used with gas appliances be certified to ULC S636, venting be installed as a complete, matched system (pipe, fittings, cement, etc.) and installations follow both the manufacturer’s instructions and the certified vent system requirements

What this means:

• New installations must use certified venting systems

• Regular ABS plumbing pipe is no longer allowed for exhaust venting

So… Is ABS Venting Allowed Today?

If you already have ABS installed, yes — you can remain in place as long as it’s in good condition, there are no signs of damage or leakage and the system is functioning properly. For new furnaces or replacements, no — ABS is not allowed. Today’s systems must use certified materials like PVC (ULC S636 rated), CPVC, Polypropylene vent systems.

What Are the Risks with ABS Venting?

Over time, ABS can develop issues—especially under the conditions created by a condensing furnace. Potential problems can include cracking or becoming brittle, joints loosening or separating, damage from acidic moisture inside the pipe

If problems develop, this can lead to exhaust gases leaking into the home, moisture damage inside walls or ceilings, reduced furnace efficiency, in worst cases, carbon monoxide risks

What Should Homeowners Do?

If your home has ABS venting have it checked during your next home inspection or HVAC maintenance service. Look for visible signs like cracks or discoloration, loose fittings, sagging pipes. You don’t necessarily need to replace it immediately—but you should keep an eye on it.

If you’re replacing your furnace:

Know and plan that the venting will need to be updated. To avoid surprises, look up in your ceiling and follow the existing path of the ABS piping to the outside. If your ceiling is un-finished you are in luck. If your ceiling is finished, it will need to be removed in order to install the new piping. You may want to do this ahead of time to lower your furnace installation cost.

Make sure your contractor installs a ULC S636-certified system with matching components (no mixing pipe types) from the new furnace all the way to the outside.

If you’re buying a home:

ABS venting should be noted as a “legacy system” on your home inspection report. It’s not unusual—but it’s something to monitor and budget for replacing in the future.

Bottom Line for Homeowners

Older homes: ABS venting is common and often still in use

New installations: ABS is no longer permitted

Long-term: Plan to upgrade when replacing your furnace

Need Help or Have Questions?

Understanding furnace venting might not be the most exciting part of homeownership—but it’s an important one for safety and long-term planning. If you’re unsure about your system or want peace of mind, a professional inspection can help you identify potential issues early, avoid costly repairs and plan ahead with confidence.

Feel free to reach out if you’d like a professional opinion on your home’s furnace and venting system.