Boiler or Radiator Smells? Gas, Burning, Oil or Sewage

Rotten eggs, sulfur or gas

Never wait it out. PG&E says not to rely on your nose alone, and Peoples Gas says the smell can fade, be masked by other smells, or stop being noticed after a while. Washington Gas says not every suspected gas smell is a leak, but only qualified personnel can decide that.

Propane. The Mississippi State Fire Marshal says propane smells like rotten eggs, a skunk’s spray or a dead animal, and to act on even a faint smell. Weil-McLain says propane can collect at floor level and its odorant can fade, and says to check the odorant level now and then.

Before lighting a boiler. Navien says to smell all around it and next to the floor, because some gas is heavier than air. Once you smell gas, Peoples Gas says not to try to find the leak: leave.

Shutting the gas off. PG&E says leaving comes first, and once a meter has been shut off, not to turn it back on yourself. For propane, the Propane Education & Research Council says to turn off the gas at the tank, unless the leak may be at the tank, and never to turn it back on yourself.

Detectors. Con Edison says most carbon monoxide alarms do not detect natural gas, so check the label. National Grid says a methane detector does that job for natural gas, and the Propane Education & Research Council says to use a propane gas detector for propane.

Rotten eggs from the hot tap only. The Minnesota Department of Health says a rotten egg smell in hot water can come from a water heater (hot water smells), and Bosch’s Greenstar installer manual lists hot water with an unpleasant odor among its faults.

Flue gas, exhaust or fumes

What to do. Viessmann and Bosch give the same three steps for a flue gas smell: switch the boiler off, open the windows and doors, and call a heating contractor. DOE says a smell of combustion gases means a venting problem, to be repaired immediately.

Why it matters. Viessmann says a blocked or partly blocked vent or chimney can leak flue gases into the house, and that flue gases carry carbon monoxide. CPSC lists an unfamiliar or burning smell among the signs that an appliance may not be working properly.

Things you can see. CPSC lists rust or water streaks on the vent or chimney, soot, and loose or disconnected vent pipes. Burnham says soot in or near the burner area is a sign of a possible carbon monoxide hazard and the boiler should be shut down. Weil-McLain says a loose flue pipe on its oil boiler means switch it off and call.

Outside at the vent. If one of its condensing boilers stops, Burnham says to check a wall vent you can reach from the ground for a blockage such as a bee’s nest or ice, and to call a technician if there is none or you cannot reach it. Peerless says to keep the vent and air inlet clear of snow and debris, and to look for gaps where the pipes pass through the wall, which could let flue gases back in.

Fans in the house. Washington Gas and Peerless say an exhaust fan near a boiler that vents up a chimney can make flue gases spill back into the room.

A safety switch that tripped. Burnham says a blocked vent switch should only be reset after a technician has found and fixed the venting fault.

A condensing boiler: flue gas at the drain

The trap. A condensing boiler drains water through a trap that must stay full, or flue gas comes out of the drain, Weil-McLain and Lochinvar say.

The check. Weil-McLain’s Ultra manual has the owner hold their fingers in front of the drain hose while the boiler runs. Flue gas there means the trap is dry; that manual gives the steps to fill it, and says to call a technician if it keeps drying out. Weil-McLain’s Eco-Tec manual, Triangle Tube’s 2019 Prestige manual and Peerless say to call a technician instead, and Peerless says this exhaust can cause carbon monoxide poisoning. Follow your own boiler’s manual.

A recall. Triangle Tube recalled Prestige Solo and Prestige Excellence boilers made from December 2011 to April 2019, with a serial starting AE, PT, PA, PTE or PEA, because they can release carbon monoxide. Its page says registration for the recall closed on 31 January 2026 and the company stopped trading in November 2025, so have a technician check one of these boilers.

The water. Navien says its condensate has an acidic pH of 3 to 5, so never drink it or let pets drink it.

Oil, smoke or soot (oil boiler)

Oil leaks. Weil-McLain says to check for oil leaks in the piping and around the burner, and to call a technician at once if you find one. Connecticut’s health department says to look at the oil tank for blisters, wet spots, rust, dents, bulges or drips, and at the lines from the tank. New York State says to call your oil delivery company right away if you see a problem with the tank.

The reset button. If the burner will not fire, Weil-McLain says to press reset only once, because repeated presses put oil in the combustion chamber, and to call a technician if it still will not fire. Beckett says the boiler must be free of oil and oil vapor before anyone resets the burner, or hot gases can puff back.

Oil on the floor. Connecticut says to report an oil smell or standing oil to the fire department, put out flames such as pilot lights, keep away from switches and outlets, and open the windows. Wisconsin’s health department says you can usually clean up less than a gallon yourself, but not with a shop vacuum; more needs professional help.

Feeling ill. Connecticut says heating oil vapors can cause headaches, nausea, dizziness and eye or throat irritation. New York State says not to stay in a building with strong oil odors.

Smoke or soot. NORA’s manual for oil burner technicians lists delayed ignition, combustion problems, a dirty chimney, too little air, air leaks, a bad heat exchanger and a house fan pulling air down the chimney. It says a fan doing that draws odors, soot, smoke and carbon monoxide into the house. Burnham says soot is a sign of a possible carbon monoxide hazard and its boilers should be shut down until a technician has found the cause.

Switching it off. Slant/Fin says its oil boiler is wired through an emergency switch with a red plate, placed where you see it on entering the basement. Beckett says there could be more than one disconnect switch.

Burning plastic or a hot electrical smell

Power off. Carrier’s advice for a furnace that smells of burning plastic is to shut off its power and have a technician inspect it. NFPA says to call a qualified electrician or your landlord for a burning or rubbery smell from an appliance.

Fire. Viessmann says that if you see fire coming from the boiler, call the fire department at once and do not try to put it out unless you are qualified.

At a radiator. USFA says to keep anything that can burn at least 3 feet from all heat sources, radiators included.

Dust, a new boiler or fresh paint

Dust after summer. Stelrad says to check radiators for dust after a long summer, and to clean them with the radiator off and cool. Carrier says a burning dust smell from a furnace at the start of the season is typically temporary, but one that lasts more than a few hours or gets stronger means turn it off and have it inspected.

A new boiler. Viessmann’s installation supplement says that when a new Vitola 200 is first fired, the insulation in its door needs half an hour to an hour to cure, and an odor may occur. Energy Kinetics says a slight odor is common when its oil boilers first light. A new condensing boiler is different: Burnham says an unfilled trap at start-up can let flue gas into the building, as in check 3.

Freshly painted radiators. Runtal says to let paint dry completely before heating a radiator. Rust-Oleum says its high heat paint, made for radiators among other things, may give off smoke and a harmless odor while it cures, so open the windows.

Sewage, chemicals or a dead animal

A dry floor drain. The South Davis Sewer District says a drain trap holds water that blocks sewer gas, and the water in a drain that is not used often can dry out. Pour a cup or two of water into it, and if the smell stays, call a plumber. The City of Green River, Wyoming says sewer gas usually smells of rotten eggs, so if it does, treat it as gas first, as in check 1.

Chemicals near the boiler. Burnham says bleaches, fabric softeners, paints, cleaners, refrigerants and cat litter boxes near the boiler can cause corrosion and objectionable odors. Weil-McLain says to remove such products at once, and if they have been there a long time, to have a technician check the boiler and vent for acid damage.

A dead animal. In a propane home, go to check 1 first: propane can smell like a dead animal. Washington’s health department says a rodent can die inside a wall and cause a bad smell. To clean up, it says to air the space for at least 30 minutes, wear gloves, wet everything with a bleach solution or another disinfectant, and never sweep or vacuum. Next to the boiler, use the other disinfectant: Weil-McLain lists chlorine bleach among the products to keep away from it.

A musty smell after a leak. EPA says you may suspect hidden mold if a building smells moldy but you cannot see the source, or if there has been water damage and people in the home report health problems: a radiator leaking.