Furnace Venting and Carbon Monoxide: What Homeowners Should Check

HVAC · 4 min read · By Rodney Jessen · Published November 13, 2026

A gas furnace burns fuel to make heat, and the exhaust from that flame has to leave your home through a vent pipe. When the venting works, you never think about it. When it does not, exhaust that includes carbon monoxide can end up indoors. A few simple checks, a working CO alarm, and a yearly service go a long way.

Why venting matters

Carbon monoxide, or CO, is a gas you cannot see or smell. The U.S. Consumer Product Safety Commission calls it the invisible killer for that reason. Any fuel-burning appliance can produce it, including furnaces, water heaters, gas ranges, fireplaces, and dryers.

A furnace vent carries exhaust outside. Older furnaces often use metal vent pipe that connects to a chimney. Many high-efficiency furnaces use plastic pipe that runs out a wall or the roof. Either way, a pipe that is loose, cracked, corroded, or blocked can let exhaust back into the house.

What homeowners can check

You do not need tools or to take anything apart. With the furnace off and cool, take a look at:

• The vent pipe near the furnace. Look for joints that have pulled apart, sagging sections, holes, or rust.
• Stains, soot, or water streaks on or around the vent pipe, the chimney, or the furnace itself. CPSC lists these as visible clues of a CO problem.
• The furnace panels. A loose or missing front panel is another warning sign on CPSC's list.
• The outside vent openings on high-efficiency furnaces. Keep them clear of leaves, plants, and snow. Wet Pacific Northwest falls can pile debris against low wall vents fast.
• Storage near the furnace. Keep boxes, laundry, and anything flammable away from the furnace and its vent, and do not block the openings that let air in.

CPSC also lists problems that can point to an appliance not working right, such as a furnace that cannot heat the house or runs all the time, an unfamiliar or burning smell, sooting, or more moisture than usual on the inside of windows.

If you have plastic vent pipe that is gray or black, look for the names Plexvent, Plexvent II, or Ultravent. CPSC has a past recall program for those vent pipes, and it is worth having a technician take a look if you find them.

CO alarms

CPSC recommends CO alarms on every level of the home and outside sleeping areas, ideally interconnected so they all sound together. Choose alarms that meet the UL 2034 standard, and keep them away from heating vents and places where curtains or furniture can cover them.

Test them often and replace the batteries as the instructions say. CPSC notes the test button checks the circuit, not the sensor, so replace alarms when the manufacturer says they have reached the end of their life. Washington has its own rules for CO alarms in homes, so check with your city or Washington L&I if you are building, remodeling, or selling.

Know the symptoms

Early CO poisoning feels a lot like the flu, without the fever. Headache, tiredness, shortness of breath, nausea, and dizziness are common. If more than one person in the house feels this way, or symptoms ease when you leave home, get outside and call 911.

The yearly service

CPSC recommends having your heating system, including its vents and chimney, inspected and serviced every year by a trained technician. Some problems, like a hidden blockage or a part failing inside the furnace, are not something a homeowner can see. A yearly visit is the best chance to catch them before the coldest part of winter.

When to call a professional

Call a heating technician if you see a loose, damaged, or rusted vent pipe, soot or stains near the furnace or chimney, or a furnace that runs but does not heat. Also call if your CO alarm has gone off and the fire department has cleared the house, so the source can be found and fixed.

OutToday serves Bellevue and the Eastside. If it has been a while since your furnace and venting were checked, call us at (425) 615-5000 and we will help you head into winter with peace of mind.