Is Your Furnace Vent Safe? Why ABS Pipe Can Be Dangerous & 636 Venting Matters

When most homeowners think about furnace safety, they think about filters, tune-ups, and maybe carbon monoxide alarms.
Almost nobody thinks about the plastic pipe running from the furnace to the outside wall.

But that vent pipe is one of the most important parts of your heating system. If it’s wrong, damaged, or installed with the wrong material — like ABS drain pipe instead of certified 636 venting — it can lead to serious problems, including carbon monoxide entering your home.

At Over the Top Heating in Stony Plain, we’re starting to see more homes in Stony Plain, Spruce Grove, Parkland County, and West Edmonton where furnaces were vented with the wrong type of plastic. This blog will walk you through:

• What furnace venting is actually supposed to do

• The difference between ABS and 636 venting (in plain English)

• The real-world risks of using the wrong pipe

• How this is similar to the story of poly-B plumbing

• Simple visual checks you can do as a homeowner

• How we correct unsafe venting and make your system safe

What Your Furnace Vent Is Actually Supposed to Do

Modern high-efficiency gas furnaces produce combustion gases as they burn natural gas or propane. Those gases include:

• Water vapour

• Carbon dioxide

• Small amounts of carbon monoxide (CO)

• Other byproducts of combustion

The job of the venting system is to:

1. Safely carry those gases outside the home

2. Keep them contained so they don’t leak into living spaces

3. Handle heat and condensate without breaking down, melting, or cracking

4. Meet gas code and manufacturer requirements, so your system is safe and your warranty remains valid

If the vent pipe is the wrong material or poorly installed, it can:

• Fail prematurely

• Sag and hold water

• Crack or separate at joints

• Allow combustion products — including CO — to escape where they shouldn’t

That’s where the ABS vs 636 issue comes in.

ABS vs 636 Venting: What’s the Difference?

You might look at the plastic pipe on your furnace and think,
“Plastic is plastic… what’s the big deal?”

There’s actually a huge difference between ABS and 636 when it comes to furnace venting.

What is ABS Pipe?

ABS (Acrylonitrile Butadiene Styrene):

• Is commonly used for drain, waste, and vent (DWV) plumbing

• Is designed to carry room-temperature or warm water and sewer gases

• Is not listed or certified for high-temperature flue gases from gas appliances

• Is not designed for long-term exposure to acidic condensate produced by high-efficiency furnaces

In other words: ABS is meant for drains, not furnace exhaust.

What is 636 Venting?

System 636 is a venting system (usually PVC or CPVC) that:

• Is specifically tested, listed, and certified for flue gas venting

• Is designed to handle the temperature and chemistry of furnace exhaust

• Has its own fittings, primers, and cements that must be used as a system

• Is recognized by gas code and furnace manufacturers as an acceptable vent material

You’ll typically see “System 636” or similar markings printed on the pipe and fittings.

If your furnace is vented with 636, and it’s installed properly, you’re on the right track.

Why Using ABS for Furnace Venting Is a Problem

Using ABS for furnace venting isn’t just a “little shortcut.” It can create serious safety and reliability issues, including:

1. Heat and Condensate Damage

Furnace flue gases and condensate:

• Can be hot (especially at the appliance connection)

• Are acidic, which can slowly eat away at some plastics

ABS was never designed to deal with this long-term. Over time, you can see:

• Softening or warping

• Discolouration

• Cracking or brittleness

• Joint failures

2. Risk of Carbon Monoxide Leaks

If vent pipe fails, sags, cracks, or pulls apart at a joint, combustion gases can escape. Those gases can include carbon monoxide, which is:

• Colourless

• Odourless

• Potentially deadly at high enough concentrations

While CO detectors are a must, the first line of defence should always be a properly installed, code-compliant venting system.

3. Code & Manufacturer Violations

Using ABS where the manufacturer calls for 636, PVC, or CPVC venting:

• Can violate local gas code

• Can void the furnace warranty

• Can cause issues during a home inspection, insurance claim, or real estate sale

If there was ever a problem, an inspector or adjuster will absolutely look at the venting.

4. Long-Term Reliability Issues

Bad venting often doesn’t fail on day one. It quietly degrades over time. By the time there’s a visible problem, it’s already been an issue for a while.

Remember Poly-B Plumbing? This Is the Same Story, But More Dangerous

If you’ve heard of poly-B plumbing, you already know how this kind of thing can play out.

Poly-B was once a mass-produced, widely accepted material used in tons of homes because it was:

• Cheap

• Easy to install

• Approved at the time

For a while, it looked like the perfect solution. Then the long-term problems started showing up:

• Pipes and fittings degrading over time

• Leaks starting inside walls and ceilings

• Hidden damage turning into floods and expensive repairs

Now, poly-B is something most homeowners and inspectors watch out for — and a lot of people are spending good money to replace it.

ABS Furnace Venting Is the Same Idea, With a Different Outcome

Using ABS for furnace venting is a similar story:

• It was convenient and cheap

• It was used in places it shouldn’t have been

• It may “look fine” for years

But instead of ending in a flooded basement, the failure mode is far more serious:

With furnace venting, failure can mean carbon monoxide entering the home, not just water.

So while poly-B was a plumbing shortcut that eventually showed its weaknesses through leaks and water damage, improper furnace venting with ABS is a shortcut that can turn into a CO safety hazard.

The lesson from poly-B is simple:
Just because something was commonly installed in the past doesn’t mean it’s safe or the right material today.

How Homeowners Can Do a Quick Visual Check

To be clear: only a licensed gasfitter can properly inspect and diagnose your venting.
But there are some simple things you can look for as a homeowner.

?? Safety note: If you ever smell gas, feel unwell around your furnace, or your CO alarm is going off, leave the home and call for emergency service immediately.

Step 1: Find the Vent Pipe

Go to your furnace and look at the plastic pipes connected to it. High-efficiency furnaces typically have:

• One pipe for exhaust

• Sometimes a second pipe for combustion air intake

These pipes usually go up and out through a wall or ceiling.

Step 2: Look for Markings

On the plastic pipe, look for:

• “System 636” or 636 markings

• Manufacturer markings and vent ratings

If the pipe is clearly labelled as 636, that’s a good sign. If it looks like standard ABS drain pipe (often black ABS or unlabelled), it may not be appropriate for venting.

Step 3: Check for Obvious Red Flags

Things to watch for:

• Sagging or “belly” sections in horizontal runs

• Joints heavily taped over instead of properly glued

• Discolouration, soft spots, or cracking

• White scaling or crust around joints

• Vent terminations outside that are too close to windows, doors, or grade level

If you see anything that looks questionable, it’s worth having a professional take a look.

What We Do When We Find Unsafe Venting

At Over the Top Heating, we take venting and CO safety very seriously. When we come out to inspect a furnace vent, here’s what we typically do:

1. Full Visual & Code-Based Inspection

We:

• Inspect the venting material from the furnace to the termination (as much as is visible)

• Check pipe size, slope, length, and support spacing

• Look at terminations outside for clearances and proper configuration

2. Identify Wrong or Damaged Materials

If we find:

• ABS or other non-approved materials being used as vent

• Poorly glued, unsupported, or sagging 636/PVC

• Cracked or heat-damaged sections

We’ll document it and explain the issue clearly.

3. Replace With Proper 636 Venting

If replacement is needed, we:

• Remove the improper pipe and fittings

• Install properly sized and supported 636 venting system

• Use the correct 636 primers and cements, as required by the manufacturer

• Confirm vent slope and termination clearances meet requirements

4. Safety Testing Before We Leave

Once everything is installed, we:

• Run and test the furnace

• Check for proper operation and good draft

• May perform combustion/CO checks as needed

• Confirm that there are no obvious issues with the venting or furnace operation

Our goal is simple: your system should be safe, code-compliant, and reliable.

Local Homes We Help With Venting & CO Safety

We regularly work on homes in:

• Stony Plain

• Spruce Grove

• Parkland County

• West Edmonton

A lot of these homes have had:

• Furnaces replaced cheaply, with corners cut on venting

• Older installs that no one has really looked at in years

• DIY modifications that seemed harmless at the time

If you’re not sure what kind of venting your furnace is using, it’s worth having a professional check it — especially heading into peak heating season.

When to Call Over the Top Heating

You should give us a call if:

• You suspect your furnace vent is ABS or another non-636 material

• You see visible sagging, cracks, heavy taping, or discolouration on the vent

• Your CO alarm has gone off or you’ve had unexplained headaches or symptoms near the furnace

• You’re replacing your furnace and want to ensure the new venting is done right

? Call/Text: 780-870-4328
? Website: overthetopheating.com
? Serving: Stony Plain, Spruce Grove, Parkland County & Edmonton

We’re licensed, insured, and experienced with furnace venting, gasfitting, and CO safety inspections. If you’d like us to take a look at your venting and give you straight answers, we’re happy to help.

Final Thoughts

Furnace venting isn’t glamorous, but it’s one of the most important safety systems in your home. Using ABS drain pipe instead of certified 636 venting might look fine on the surface, but it can lead to:

• Premature pipe failure

• Carbon monoxide risks

• Code and warranty issues

Just like poly-B showed us that “common and convenient” doesn’t always mean “safe long-term,” improper furnace venting is another place where cutting corners can cost you.

If you’re not sure what’s on your furnace now, you don’t have to guess. A quick inspection from a licensed gasfitter can tell you whether your system is safe — and what it would take to make it right.