Quick Answer:
If a sewer-like or musty odor becomes stronger when your heating runs on cold days, start by checking whether the smell lines up with low outdoor temperatures, then inspect venting and traps. Common causes are roof vent blockages from ice or snow, pressure changes when the furnace cycles, dried traps, or cold drafts pulling sewer gas into the basement.
Why This Happens
Cold weather changes how air moves through a house. A running furnace creates airflow and brief pressure shifts. If the roof vent is blocked by ice or snow, sewer gases can’t escape properly. Very cold, dry air can also speed evaporation of water in traps on rarely used drains, letting gases through. In basements, cracks and gaps in walls or around pipe penetrations can let cold drafts pull sewer gas inside.
If the odor is concentrated in one area, such as the cellar, consider whether you have a Smell only in basement pattern; that points toward traps, cracks, or local venting problems rather than the whole-house sewer line.
Step-by-Step What to Do
1. Note the weather and when the smell appears
- Keep a simple log for a few days: note outdoor temperature, whether the furnace was running, and when the smell shows up. If the odor gets stronger on very cold days or during furnace cycles, that’s a useful clue.
2. Check the roof vent for ice or snow from the ground
- From a safe spot on the ground, use binoculars or a camera zoom to look for ice, packed snow, or sagging at the vent stack. Do not climb onto the roof yourself if it’s icy or you’re unsure—call a pro.
- If the vent looks blocked, the stack can trap sewer gases and delay their escape until pressure shifts push them back indoors.
3. Watch for pressure changes when the furnace cycles
- When the furnace turns on, stand where you notice the smell (basement or main level) and pay attention for any increase in odor during the run cycle.
- Briefly turn on a strong exhaust appliance—like the kitchen hood—while watching the smell. If the odor changes noticeably when the hood runs, that indicates pressure-driven air movement. Do this only for a few seconds to test; you’re just observing, not trying to fix it by leaving exhaust fans on.
4. Make sure traps and primers are working
- Floor drains, seldom-used sinks, basement toilets and other traps can dry out in cold, dry weather. Pour a couple of cups of water into floor drains and unused sinks to refill the trap seal.
- Some homes have trap primers—small devices that add water automatically to traps. If you have them, confirm they’re not stuck or empty. If you don’t have primers and traps dry out repeatedly, ask a plumber about adding them.
5. Inspect for cracks and cold-draft entry points
- Look along basement walls and around where pipes enter the foundation for gaps, visible cracks, or loose seals. Cold drafts near drains or pipe penetrations can draw sewer gas into living spaces.
- Seal small gaps with appropriate caulk or foam designed for plumbing penetrations; larger cracks or structural issues should be repaired by a professional.
What Not to Do
- Don’t rely on keeping a window open constantly as a winter fix—this wastes heat and can mask pressure problems rather than solve them.
- Don’t climb onto an icy roof to clear vents yourself; you can fall. Use ground inspection or call a pro.
- Don’t try makeshift heating or thawing methods on frozen vents or pipes (open flames, torches, or unsafe electrical devices). This can cause fire or damage.
- Call a professional when the smell consistently worsens in cold weather, or when you discover vent blockages or frozen venting and can’t safely clear them yourself.
When to Call a Professional
- If you see or suspect a blocked or frozen vent stack on the roof.
- If the smell persists after refilling traps and sealing gaps, or if you can’t locate the source.
- If you find large foundation cracks, damaged sewer piping, or plumbing you’re not comfortable repairing.
- If you detect an unfamiliar strong gas-like odor alongside the sewer smell—get help promptly.
Safety Notes
- Never go onto a wet, icy, or steep roof. Use binoculars, photos, or a professional roof/vent technician for inspections.
- Sewer gas is unpleasant and can be hazardous in high concentrations. If the odor is strong and makes people ill, leave the building and call for help.
- Keep carbon monoxide detectors and smoke detectors in working order when troubleshooting furnace-related issues; do not disable detectors.
- Wear gloves when working with drains and avoid breathing directly over open drains or sewer covers.
Common Homeowner Questions
- Why does the smell get worse only when the furnace runs?
Furnace operation changes indoor air pressure and airflow; if vents are blocked or traps are dry, those pressure shifts can push sewer gas into the house. - Can I clear a frozen roof vent myself?
Not safely in most cases—don’t climb an icy roof. Have a pro assess and clear vent stacks if needed. - Is sewer gas dangerous?
Low levels are mainly unpleasant, but strong or persistent odors can be hazardous; get fresh air and call a pro if people feel sick or the smell is strong.
More in this topic
For more related fixes and similar symptoms, see Sewer Gas Smell from Floor Drains.