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High-Efficiency Furnace Venting Explained (Cold-Climate Edition)

Discover why your high-efficiency furnace has two PVC vent pipes, what happens when they freeze in Calgary’s winter, and how to prevent costly shutdowns, CO risks, and airflow issues before they start.
Intro: Welcome to the Two-Pipe Club
If you've ever looked behind your new high-efficiency furnace and wondered, "Why are there two white plastic pipes sticking out of my wall?" — this guide's for you.
Unlike older furnaces that used a metal chimney, high-efficiency models (95%+ AFUE) use PVC venting to move air in and out more safely and efficiently. It's part of why you're saving on gas — but it's also why your furnace can suddenly quit in the middle of a cold Calgary night… if one of those pipes gets blocked.
Let's break down what each pipe does, why venting matters, and how to keep yours from freezing solid when it's -30°C.
What the Two Pipes Actually Do
Think of your furnace as a breathing machine.
1
Pipe #1: Intake
Pulls in fresh outdoor air for combustion.
2
Pipe #2: Exhaust
Pushes out the gases created during heating.
By using outdoor air instead of indoor air, your furnace runs cleaner, safer, and more efficiently.
That's also why you don't see the old "flame view" like in older furnaces — the entire combustion process is sealed and balanced.
Why It's So Important (and Why It Can Go Wrong)
Calgary's frigid winters make venting one of the most common furnace failure points. When temperatures drop, frost, snow, and ice can build up around the pipe ends, blocking airflow.

When that happens, your furnace's safety controls trip and shut the system down — not because it's broken, but because it's doing its job: preventing carbon monoxide (CO) from backing up into your home.
If your furnace won't start and you see frost or ice near the pipes — don't panic. Turn off the system, gently clear the obstruction, and restart. If it still fails, call a pro.
⚠️ Don’t Pour Hot Water Down Your Vent Pipes
It’s tempting — we get it. You see ice clogging your furnace vent and think a little hot water will melt it fast.
But here’s the problem: that water doesn’t drain outside — it runs straight back into your furnace.
It can fill the inducer motor housing, short electrical components, and leave you with a flooded furnace and a service bill you’ll really feel.
If your vent’s frozen, gently clear the ice by hand or use warm air (like a hair dryer) on the outside section only. If it’s badly frozen or reoccurs often, it’s time to call a pro — not your kettle.
Common Calgary Venting Issues (and Fixes)
The Basics of Proper Venting
For high-efficiency furnaces, correct venting = safety and performance.
Materials:
Use PVC, CPVC, or ABS depending on manufacturer specs — no metal pipes allowed.
Termination spacing:
  • Intake and exhaust should be 12–18 inches apart horizontally depending on manufacturers specs.
  • At least 12 inches off the ground (24 inches is better in Calgary)
  • 6 feet from windows or doors
Slope:
All venting must slope ¼" per foot toward the furnace to allow condensation to drain properly.
Elbows:
The fewer, the better. Too many turns can cause pressure issues.
Warning Signs of Bad Venting
If you notice any of these, call a pro immediately:
1
Strong fuel or exhaust smell indoors
2
Water dripping or pooling near the furnace
3
Loud gurgling or "sucking" noises
4
Condensation or frost forming on the vent pipes indoors
These signs can indicate blocked airflow, incorrect vent slope, or a condensate drain issue — all of which can lead to CO buildup or furnace failure.
Calgary-Specific Tips
01
Elevate your vent ends
If your pipes are low to the ground, they can get buried in snow. A 2-foot riser or vent elbow upgrade can prevent midwinter shutdowns.
02
Keep a 3-foot "clear zone" around the vent
Avoid piling snow or leaving garbage bins near your vent sidewall. Airflow is everything.
03
Don't chip at ice!
If the vent or condensate line freezes solid, never strike it — you can crack the pipe or fittings. Use warm air (not boiling water) or call a tech.
How Often Should You Check Your Furnace Vents?
  • After any major snowfall
  • During chinooks (melting and refreezing can block intakes)
  • Daily during deep winter for peace of mind
Add it to your "outside checklist" along with clearing eavestroughs and shoveling walks.
When to Call a Pro
DIY vent checks are fine — but modifying or replacing venting is always pro territory. If you suspect improper installation, weird noises, or recurring frost issues, a certified tech can:
  • Measure exhaust pressure
  • Verify slope and termination distances
  • Inspect for internal ice or damage
  • Confirm proper condensate drainage
Bottom Line
High-efficiency venting is amazing when done right — but unforgiving when done wrong.
Keep your pipes clear, check them after storms, and don't ignore weird noises or leaks. Your furnace is smarter than you think — if it shuts off, it's probably protecting you.
Future-Proofing Your Furnace
If you're upgrading or replacing your furnace soon, ask your installer:
  • "How will the venting handle Calgary's snow levels?"
  • "Where will the intake and exhaust terminate?"
  • "Is the slope back to the furnace correct?"
A quick discussion upfront can prevent years of winter headaches.
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