By the team at Homegrown Heating and Air Conditioning.
Snow blocking a furnace vent is the most common storm-related no-heat call we run in Northern Colorado, and one of the few HVAC problems a homeowner can check and often prevent without tools.
It is a safety issue rather than a comfort issue. Below is what the two pipes on the side of your house do, what the code requires, and what to check after a storm.
Your Furnace Probably Breathes Through the Wall, Not the Roof
Almost every 90 percent efficiency or better furnace installed in Northern Colorado in the last two decades is a condensing furnace, and a condensing furnace does not vent up a chimney.
It vents through PVC, out a sidewall, usually low to the ground. Beside that exhaust pipe there is normally a second PVC pipe of the same size bringing combustion air in from outside. One pushes flue gas out. The other pulls burner air in.
Both pipe ends sit at a height where snow lives. That is the whole problem.
Three things block them here:
- Drifting snow piling against the foundation on the windward side
- Melt from a roof line above the termination, running down and refreezing into a cap over the pipe
- A plow berm or a shoveled pile thrown against the side of the house or across a window well
The chinook pattern makes the second one worse here than in most cold climates. Wet snow drifts against the wall, a warm westerly partly melts it, and a hard refreeze turns the drift into an ice shell around the pipe.
What Happens Inside When a Vent Is Blocked
A condensing furnace runs a pressure switch that proves the inducer fan is actually moving air through the vent system before the gas valve opens.
Block either pipe completely and that switch does not close. The furnace locks out. You wake up to a cold house and a blinking error code, and that is the system working correctly.
The case that matters more is a partial blockage. A vent narrowed rather than sealed, or a snow cave around both terminations that lets exhaust get pulled back into the intake, can leave the furnace running on combustion air that is short on oxygen or already full of flue gas. That is incomplete combustion, and incomplete combustion is how carbon monoxide gets made.
The U.S. Consumer Product Safety Commission is direct about this in its carbon monoxide questions and answers: carbon monoxide is a deadly, colorless, odorless, poisonous gas produced by the incomplete burning of fuels including natural gas, and malfunctioning furnaces are among the consumer products that produce it. CPSC also tells homeowners to keep appliance vents and chimneys unblocked and to have a professional check flues for blockages and disconnections.
CPSC lists the early symptoms as headache, fatigue, shortness of breath, nausea and dizziness, and notes they resemble the flu without a fever. That is why a blocked vent in February gets written off as a bug going around the house.
The Clearance Northern Colorado Code Actually Requires
Vent height is not a rule of thumb here. Two of our jurisdictions put a number on it.
Fort Collins requires all gas vents to terminate a minimum of 22 inches above the surface or grade directly below, under the City’s amended fuel gas code. That is a local amendment, and it is stricter than what most homeowners assume.
Estes Park goes further, because it has to. Its amended residential code requires vents to terminate at least 12 inches above the anticipated snow level and 22 inches above the surface or grade below. At roughly 7,530 feet and about 7,558 heating degree days, anticipated snow level is a design input rather than a formality.
If your vent terminates below those heights, it predates the current amendments or it was installed wrong. Either way, raising or relocating a termination is a smaller job than most people expect. Our furnace repair service covers vent and combustion air corrections.
The Five Minute Check After Every Significant Snow
Do this in daylight, from outside, before you need it.
- Find both pipes. They are white PVC, usually the same diameter and within a foot of each other on an exterior wall. Learn the spot once.
- Clear a wide space, not a narrow hole. Aim for two to three feet of open air in every direction around both terminations, including above.
- Clear snow away with your hands or a soft broom or brush. Nothing rigid.
- Look up. Check whether a roof valley, gutter seam or ice dam above the pipes is dripping onto them.
- Check where snow from your driveway and walk ends up. A plow berm against the house is the easiest of these to prevent.
What not to do. Do not chip, pry, hammer or dig at ice on or near the pipes with a shovel, scraper, screwdriver or ice pick. PVC goes brittle in cold weather and cracks, and a cracked vent leaks flue gas into the house. Do not run the furnace to melt ice off the termination. If ice is frozen onto the pipe and will not come off with a gloved hand, shut the furnace off at the switch and call us.
If a Carbon Monoxide Alarm Sounds
This part is not negotiable.
- Get everyone out of the house immediately, including pets.
- Call 911 from outside, from a neighbor’s home, a car or a mobile phone in the open air.
- Do not go looking for the source. Do not open windows first. Do not check the furnace.
- Do not go back inside for any reason until responders tell you it is safe.
If anyone has a headache, nausea, dizziness or shortness of breath, tell the dispatcher.
Only after the fire department clears the house does diagnostic work begin. That is when a blocked termination, a cracked heat exchanger or a disconnected flue gets found, and where our emergency HVAC repair service picks the job up.
A Northern Colorado Homeowner on Being Told the Truth
Dianne Whitaker called us about a fireplace. Our technician examined it, decided it was past what he could safely take on, and brought his supervisor in rather than pressing ahead. Together they told her the work belonged to a fireplace specialist and that our focus is furnace and air conditioning.
“Homegrown was so responsive and gave me an appointment right away. The technician was very polite and was respectful of my property.”
That instinct matters more on a combustion call than on any other kind. A blocked vent, a suspect flue or a cracked heat exchanger is not a place to guess, and knowing where our work ends is part of doing it safely. Read more customer reviews from Northern Colorado homeowners.
Why Northern Colorado Homeowners Call Us After a Storm
- Licensed and insured throughout Northern Colorado
- We check vent and intake termination height against the town the house sits in
- We test the pressure switch and inducer rather than resetting the lockout and leaving
- We service seven days a week, 7 a.m. to 7 p.m., including weekends and holidays
- We carry combustion analysis on heating calls instead of guessing at it
Most HVAC companies in this market are closed on Sunday, and the morning after a Saturday night storm is the most likely time for this failure. Our heating maintenance service catches marginal vent terminations in the fall, before the first storm finds them.
Get the Vent Checked Before the Next Storm
If you have never located your vent and intake terminations, do it on the next clear day, and have them measured against the code your town adopted. Book a heating service visit online.
Snow and Furnace Vent Questions From Northern Colorado Homeowners
How high does a furnace vent have to terminate in Fort Collins?
Fort Collins requires all gas vents to terminate a minimum of 22 inches above the surface or grade directly below, under the City’s amended fuel gas code. Measure from the ground the pipe sits above, not from the foundation or the sidewalk nearby.
Why does Estes Park require more than Fort Collins?
Because it sits at roughly 7,530 feet and runs about 7,558 heating degree days, roughly 28 percent more than Fort Collins. Estes Park requires vents to terminate at least 12 inches above the anticipated snow level as well as 22 inches above grade. Our Estes Park service page covers the town’s other altitude-specific requirements.
My furnace quit the morning after a storm and started again later. Was something actually wrong?
Probably yes. That pattern usually means the pressure switch opened on a blocked or partly blocked vent, then closed again once enough snow melted or shifted. The safety worked, but the blockage is still a blockage and it will happen again on the next storm.
Can I clear the vent myself?
Snow, yes. Use your hands or a soft broom and open up two to three feet in every direction around both pipes. Ice, no. Never chip or pry at ice on or near PVC, and never run the furnace to melt it. Cold PVC cracks, and a cracked vent puts flue gas inside the house.
My furnace is older and vents up the chimney. Does this apply to me?
Differently, but yes. A mid-efficiency furnace vents through a metal flue up a chimney rather than out a sidewall, so drifting snow is not the risk. Bird nests, animal entry, a failed chimney liner and a disconnected flue pipe are, and CPSC calls out flue blockages and disconnections specifically.
What should I do if the carbon monoxide alarm goes off?
Leave the house immediately with everyone in it, and call 911 from outside. Do not look for the source, do not open windows first, and do not go back in until responders tell you it is safe. Have the appliance and its venting inspected before it is used again.
