Hey there! It’s Jake, from the gas furnace supply team—you know, the guys who make sure your home’s heating works when it’s 20 degrees out and you don’t want to freeze mid-movie. Lately, I’ve been getting so many DMs and calls from customers panicking about their furnace acting weird: odd smells, weird noises, even that constant “is this gonna blow up?” feeling when you turn it on. And 9 times out of 10, the culprit isn’t the furnace itself—it’s the venting system. That’s why today I’m walking you through how to check your gas furnace venting like a pro (no fancy tools or HVAC degree required), and yeah, I’m throwing in some inside tips from our techs who service every system we ship. Gas Furnaces
First, let’s cut to the chase: why does venting matter so much? Your gas furnace burns fuel to make heat, right? That burn produces exhaust gases—carbon monoxide, water vapor, nitrogen oxides… stuff you definitely don’t want breathing into your home. The venting system is the pipe (or pipes) that shoves all that gunk outside. If that vent is clogged, cracked, or just not doing its job, CO can leak back into your house, which is no joke—deadly, even. And before you go “my furnace is new,” I’ve seen 6-month-old systems with vent issues because a squirrel chewed a hole in the pipe during installation. Vent checks are non-negotiable—ideally before every heating season, but honestly, if you’ve been putting it off, stop what you’re doing and do this today.
Let’s start with the basics: what even is my furnace vent? Most residential gas furnaces use either a Category I vent (the old metal pipe sticking out your roof or side of the house) or a PVC vent (newer, two-pipe system for high-efficiency models). Either way, the first step is finding it. Inside your house, it’s usually a white PVC pipe in the basement (if you have a basement) or a metal pipe running up through the attic. Outside, it’s that pipe poking out the side of your home or your roof—don’t mix it up with your dryer vent or bathroom fan vent, those smell way different if they’re messed up.
Now, let’s do the indoor check first—super easy, no ladder needed. Head to wherever your furnace is: basement, utility closet, garage. Look at the vent pipe connected directly to the furnace (this is the start of the whole vent system). Grab a flashlight if it’s dark. First, check for leaks or cracks. Run your hand along the pipe joints—do you feel any wet spots, soot, or dust buildup? That’s a bad sign. For metal pipes, rust is a huge red flag—rust eats through metal fast, and a rusted pipe means exhaust is leaking. For PVC pipes, look for cracks, especially near elbows or where the pipe goes through walls/ceilings. If you see a crack, even a tiny one, that’s a problem. Next, check the pipe connections. Are the metal joints (called flanges or slip joints) tight? Sometimes they wiggle loose if your house shifts a little. For PVC pipes, make sure the glue joints are smooth and no gaps there—bad glue when installing leads to leaks later. Pro tip from our techs: if you have a metal pipe that connects to a vent hood or a draft inducer fan, make sure the connection isn’t corroded. Corrosion there means the fan can’t pull exhaust out properly, which backs up fumes.
Now the outdoor check—this is where people usually mess up, because the vent is outside and easy to miss debris on. First, go outside and locate the vent cap—this is the cap on the end of the pipe that keeps rain, snow, leaves, animals from getting in. Start with the obvious: is the cap fully intact? No cracks, no broken parts? A missing or busted cap lets all kinds of stuff clog the pipe. Next, look down the pipe—can you see debris? Leaves, pine needles, bird nests, even small toys (we found a toy soldier in a customer’s vent last month, don’t ask). Don’t stick your hand in there! Use a long stick (like a broom handle, but clean) to gently prod the inside—if it’s clogged, you’ll feel the resistance. For sidewall vents, check the area around the base of the pipe for debris—sometimes leaves pile up right below and block the air flow. For roof vents, be careful if you go on the roof (I don’t recommend it if you’re not comfortable on ladders or in high places—slip and fall is a real risk). If you do get up there, look for debris on the roof around the vent pipe, and make sure the pipe isn’t tilted. A tilted vent can cause water to pool inside, which leads to rust or mold later.
Wait, what about when your furnace is running? You should always check the vent while the furnace is on, because that’s when the exhaust is moving, and problems show up easier. So after the visual checks, turn on your furnace and let it run for 5-10 minutes. Now, head back outside. Do you see steady, light vapor coming out of the vent? That’s normal—water vapor is a byproduct of burning gas, so it should look like a little cloud, not thick smoke or a huge billow. If it’s thick smoke, that means the vent isn’t pulling air properly, so exhaust is backing up. Next, take a piece of toilet paper (or a tissue, but toilet paper is thicker) and hold it a few inches away from the vent cap, where the exhaust is coming out. What does it do? It should pull gently toward the vent— that means the draft is working, so exhaust is moving outside. If it blows away from the vent, or just flutters randomly, there’s no draft, which is a big issue. That means the vent is blocked, or the fan (draft inducer) in the furnace is broken. Another thing: if you smell gas outside, near the vent, that’s a red flag. Wait, no—natural gas is odorless, utilities add mercaptan to make it smell like rotten eggs. If you smell that near the vent, turn off the furnace, open windows, and call a pro right away. But usually, if you smell exhaust inside the house, that’s from a bad vent, not gas.
Let’s talk about common mistakes people make here, because I see them all the time. First, not checking in the winter—snow can pile up around roof vents and sidewall vents, completely blocking them. Last year we had a customer who turned on their furnace after a snowstorm, and their vent was covered in 6 inches of snow. Don’t skip the post-storm vent check, especially if you get heavy snow. Second, ignoring small issues: that tiny crack in the PVC pipe? It’ll get worse in a year, and by then you might have CO in your house. Third, using the wrong materials to fix the vent. A lot of people try to seal a leak with duct tape or silicone caulk. Duct tape melts from the heat of the exhaust, silicone caulk can crack. If you need to fix a leak or connect pipes, use the correct metal vent tape for metal pipes, or PVC cement for PVC pipes—we sell both on our site, by the way. Fourth, mixing up vent pipes: if you have a high-efficiency furnace (90% AFUE or higher), it uses two PVC pipes—one to pull fresh air in, one to push exhaust out. Don’t block the fresh air pipe! We had a customer who stacked firewood right next to the fresh air pipe, and the furnace stopped working because it couldn’t get air to burn. Oops.
Wait, what if you check all this and nothing’s obviously wrong, but your furnace still smells weird, or your CO detector goes off? Then it’s time to call a professional, right? But wait, some issues you can fix yourself. Like, if the vent is clogged with leaves, use a long stick to clear it out. If the cap is loose, tighten the screws. If the pipe connections are wiggly, tighten the clamps. But if you don’t feel comfortable on a ladder, or if you see rusted metal pipe, or if the draft is totally dead, don’t mess around—call a tech. We work with a network of reliable HVAC folks, and if you need a referral, just let us know.
Here’s the thing: we sell gas furnaces, and we want you to be happy with them for years. That means helping you keep every part working, including the vent. A good vent check takes 15 minutes, costs nothing (except a toilet paper roll), and can save you from a ton of hassle—like a service call in the middle of a blizzard, or worse, a CO scare. I can’t tell you how many times a customer has told me they did this check themselves and fixed the issue before it became a big problem. Last month, a guy from Michigan sent us a pic of his vent—full of acorns, right above his sidewall. He cleared them out, and his furnace stopped making that weird gurgling noise. Game over.
Now, before you go, let me hit you with a quick reminder: if you do find that your vent is worn out, or you need a new furnace (hey, if yours is 15+ years old, it’s probably time for an upgrade anyway), we’ve got you covered. Our furnaces are built with vent systems that are designed to last, but even the best needs a check. If you have questions about your current vent, or want to chat about upgrading to a more efficient model, reach out to our sales team—we’re here to help, no pushy sales stuff, just real answers. We know home heating is a big deal, and we want you to feel confident in whatever you choose.
Resistance Furnaces References:
- U.S. Consumer Product Safety Commission. (2022). Carbon Monoxide and Gas Furnace Safety.
- American Society of Heating, Refrigerating and Air-Conditioning Engineers. (2021). Ventilation for Residential Gas Appliances.
- National Fire Protection Association. (2020). NFPA 54: National Fuel Gas Code.
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