Why does my house smell like rotten eggs? What to do first

Natural gas has no smell of its own. Before it reaches your house, Enbridge adds an odorant called mercaptan so that a leak smells like sulphur or rotten eggs. Sewer gas and smelly hot water get the same smell from a different chemical, hydrogen sulphide. Your nose cannot tell them apart, which is why the safe order is always gas first, plumbing second.

Is a rotten egg smell always a gas leak?

No, but where you smell it is the best clue, and it is not your job to confirm a leak. A smell that is strongest near the furnace, water heater, stove, dryer, gas fireplace or the gas meter outside is a gas call. A smell that is strongest at a drain, a toilet or only when the hot tap runs is almost always plumbing. If you are not sure, call. Do not walk around sniffing pipes or appliances to narrow it down.

Where is the smell strongest? Narrow it down

Why does my basement smell like rotten eggs near the floor drain?

Most often the drain's trap has dried out. Every drain has a U-shaped bend that holds water, and that water is what stops sewer gas coming up. A basement floor drain may go months without water, and in October the furnace starts drying out the basement air, so the seal evaporates faster. Pour a few litres of water into the drain, then a spoonful of cooking oil on top to slow evaporation. If the smell is gone within an hour, that was it. If it comes back in a day or two, the trap may be cracked, the cleanout plug inside the drain may be missing or loose, or the drain may be partly blocked, and that is a plumber's job.

Why does only my hot water smell like rotten eggs?

If the cold tap is fine and the hot tap smells, the source is almost always inside the water heater. Tank heaters have a metal anode rod that corrodes on purpose to protect the steel tank. In some water, bacteria react with a magnesium anode and give off hydrogen sulphide. It is common after a tank has sat unused, such as after a holiday, and in one RedFlagDeals thread the smell showed up only after hot showers.

The usual fixes are to flush and disinfect the tank and to swap the magnesium anode for an aluminium-zinc or powered anode. Bradford White, a water heater maker, gives these as the remedies. Do not simply pull the anode out: without one, the tank rusts from the inside. Many Ontario water heaters are rented, and if yours is, the rental company is responsible for servicing it, so call them before paying a plumber.

Can sewer gas in the house make you sick?

A faint sewer smell from a dry drain is mainly a sign that there is an open path from the sewer into the house, and it should be closed. A strong smell that does not go away, or anyone feeling unwell, is a reason to air the house out, leave if needed, and get help: call 911 in an emergency, or Health811 for health advice. Do not use bleach or drain chemicals to mask the smell, because they do not fix a dry or broken trap and can react with each other.

In Toronto and York Region homes, what is most common?

Enbridge Gas supplies natural gas in Toronto, Vaughan, Richmond Hill, Markham, Newmarket and Aurora (Ontario Energy Board records), so the emergency number is the same across the area. Beyond that:

• Basements: most houses here have a basement floor drain, a laundry tub or a basement bathroom that is used rarely. They are where dry traps happen.
• Older houses: 29% of the City of Toronto's homes were built in 1960 or earlier, and about 30% of Newmarket's before 1981 (Statistics Canada, 2021 Census). Older drains and vent pipes are more likely to have cracked or been altered during past renovations.
• Hard water: York Region's tap water is hard (141 mg/L in Newmarket, 130 in Aurora, 127 in Richmond Hill in 2026). Hard water leaves scale and sediment in tank heaters, which is one more reason a flush is part of the fix.
• Rented tanks: if the water heater has a rental company's sticker on it, start with them.

What to tell the pro

• Whether Enbridge or the fire department has already checked for gas, and what they said.
• Exactly where the smell is strongest: which drain, which bathroom, which tap, hot or cold.
• When it happens: all the time, after showers, after the washing machine drains, on windy days, or after you were away.
• The water heater: tank or tankless, gas or electric, its age from the label, and whether it is rented.
• Anything else: a slow drain, gurgling, a toilet that rocks, or water around a floor drain.
• A photo of the drain or fixture and of the water heater's label.

What does a plumber cost on Doyo for this?

On Doyo, a smelly toilet base is booked as Toilet Repair, $224.20, and a slow or smelly drain as Drain Cleaning, $259.60, both all-in with Doyo's fee included. A cracked trap, a vent pipe or a water heater anode swap is booked as Pipe Repair, priced by written quote once the plumber has seen it. Plumbing is a compulsory trade in Ontario: plumbers on Doyo hold a Certificate of Qualification (306A), and Doyo checks it before they are offered plumbing work.