Two smells where you stop reading and act
Most household smells are a puzzle you can take your time over. Two are not, and they go first because getting them wrong is the only outcome on this page that kills people.
Rotten eggs, sulfur, or a struck-match smell
Natural gas is odorless. Utilities add mercaptan specifically so that a leak smells like rotten eggs, and the American Gas Association’s instruction is not hedged: leave the building immediately and take others with you.
On the way out, touch nothing electrical. AGA spells it out — do not light a match or smoke, do not turn appliances or lights on or off, do not use a flashlight, do not start a car, do not use a telephone. A light switch, a garage door button, a thermostat, a phone screen: every one of those is an ignition source in an atmosphere that is waiting for one.
Once you are outside and well clear of the building, call 911 or your gas utility’s emergency line. Both respond free and around the clock, and utilities would rather attend a false alarm. Do not go back in for anything, including to shut the gas off at the meter — that is their job. The full sequence covers the repair afterward, and what changes on propane.
A faint whiff as a burner lights, gone in seconds, is normal; a smell that persists, or that you notice with every appliance off, is not. The odorant is also mistaken for drain smell constantly — if it is strongest at bathrooms and floor drains and tracks water use, see the sewer branch below. When unsure, call the utility: that call is free and the mistake is not.
Anything burning, hot plastic, or a sharp acrid smell with the power on
Hot plastic, burning insulation, an acrid scorched smell, or a sharp fishy chemical odor from an outlet, switch plate, light fixture, or breaker panel: this is not a smell you diagnose. Overheating wiring, an arcing connection, and a failing breaker all produce it, and by the time plastic is hot enough to smell, it is already at a temperature that starts fires inside a wall cavity you cannot see into.
In order:
- Kill power to that circuit at the breaker panel. If you cannot tell which circuit it is, kill the main. Losing the whole house for an hour is a cheap trade.
- Unplug what is on that circuit, only if you can reach the plug without touching a device that is hot or discolored.
- Call the fire department — 911. Not an electrician. Fire crews carry thermal imaging, they will open a wall or a panel to find a hot spot, and they do not charge to look. A burning smell with no visible fire is exactly the call they would rather take early.
- If there is smoke, sparking, scorching, melting, or a panel that is warm or buzzing, get everyone out of the house first and call from outside.
Never put water on an electrical fire. A dry-chemical extinguisher rated for electrical fires is the right tool, only for something small and visible, and only from a doorway you can back out of.
The electrician comes after the fire department, not instead of it — an electrician gives you Thursday, and the failure mode here is measured in hours. ESFI lists frequent breaker tripping, lights dimming when other devices switch on, buzzing from switches or outlets, and discolored outlets as signs of an overloaded system; any of those alongside the smell tells the electrician where to start.
How to identify a smell you can’t name
Everything below this line is non-emergency. Stop describing the smell and start describing its behavior — the pattern identifies the source far better than the adjective does.
- Does it change when the HVAC runs? Switch to fan-only for ten minutes. A smell that appears or strengthens at the supply vents is in the ductwork, air handler, condensate pan, or filter. One that is identical with the system off is elsewhere; the HVAC is only moving it.
- Is it worse in one room, or does it move? Walk the house slowly at three heights — floor, chest, ceiling. A smell with a hot spot is a located problem; one that is even everywhere is being distributed by the air system or drawn in from outside.
- Is it worse when it’s hot? Heat accelerates decomposition and off-gassing from damp material.
- Is it worse after rain? That is water: failed grading, a wet crawl space, bad flashing, a saturated foundation wall.
- Is it worse in the morning? A closed-up house concentrates anything emitted continuously. It is also the pattern for combustion problems — a morning smell plus a morning headache is a different page.
- Does it track water use? Showers, laundry, the dishwasher. Drains and vents.
- Are there flies? Blowflies clustering at one point on a wall or ceiling is the most reliable locator anyone gets.
Do the free eliminations first: trash out, disposal cleaned, refrigerator and freezer checked including the drip pan, every rarely used drain filled, and a look behind the stove, fridge, dishwasher, and washer.
The branches
Sewer, drains, or a flat wet-rot smell
Pour a quart of water into every drain you rarely use — floor drains especially, plus guest showers, basement sinks, and laundry tubs. The U-shaped trap under each holds a water plug, and that plug is the only thing between your rooms and the sewer line. It evaporates in drains nobody runs, and refilling it costs nothing.
If the smell survives that, the pattern picks the fault: strongest in one bathroom after showers is a failed toilet wax ring; tied to wind or cold snaps rather than water use is the roof vent stack; slow drains or gurgling is a partial clog or a cracked line. All three are plumber work, and the full triage has the costs.
Sweet, putrid, unmistakably dead
Decomposition. It is strongest within a foot or two of the source, it worsens in a warm closed-up house, and it follows a curve — building through the first week or two, then fading. Follow the flies, then look for a yellow-brown seepage stain; where smell, flies, and stain agree, that is the spot.
Call a wildlife or dead animal removal company, not an exterminator, because the job is locating and cutting rather than treating. The exception: worst at the supply vents and intensifying when the system runs means something died in a duct or the air handler, which is HVAC work. Details, including who declines this job.
Musty, earthy, like a damp basement
Microbial growth on something wet. EPA says you can suspect hidden mold when a building smells moldy but you cannot see the source, and that the key to mold control is moisture control. The smell is the symptom; the water is the problem, and remediating without finding it buys months.
So look for water first: a drip under a sink, a window that sweats, condensation on cold ductwork, a crawl space with no vapor barrier, a basement wall that darkens after rain. EPA’s rule of thumb is that a moldy patch under about ten square feet on a hard surface is yours to clean with detergent and water. Bigger, or inside walls or the HVAC, and it is a remediation company — with the testing bought from someone who is not also selling the removal. Splitting those two purchases matters more than the price does.
Cat urine or ammonia, and you have no cat
Two very different answers share this smell. Some molds produce a sharp ammonia-like odor, so if the area is damp, treat it as the musty branch above. But concentrated rodent urine smells almost exactly like this and accumulates in nesting areas — wall cavities, under insulation, behind the range.
The tell is noise. Scratching at night in the same area makes it rodents, and the fix is exclusion — sealing the entries — not a spray schedule. What that job looks like, and why poison indoors turns a noise problem into a smell problem.
Sweet and chemical, or a chemical smell near the furnace or AC
Refrigerant is often described as faintly sweet or ether-like, though plenty of people smell nothing and notice only that the AC has stopped cooling, that there is ice on the lineset, or that the system runs constantly. HVAC diagnostic.
The other case here needs saying plainly. A furnace burning incompletely can produce a sharp, slightly sweet chemical smell from combustion byproducts, and what travels alongside it is carbon monoxide, which has no smell at all. You cannot detect CO by nose. If anyone has headaches, dizziness, nausea, or confusion that lifts when they leave the house and returns when they come back, or if a CO alarm sounds, that is a different emergency — everyone outside, then 911. A gas detector does not detect CO and a CO alarm does not detect a gas leak; homes with fuel-burning appliances need both.
Fishy, with nothing electrical suspected
Rule out electrical anyway. A fishy or urine-like smell with no fish in the house is the classic signature of overheating plastic in an outlet, switch, or fixture — the flame retardants in the housing produce it as they cook. Check every outlet and switch on that wall for warmth, discoloration, or buzzing; find any, and you are in the emergency branch at the top of this page. New plastics and adhesives do off-gas this way for a few weeks, but benign is the second guess here, never the first.
Wet dog or dirty socks, but only when the HVAC runs
The trade calls this dirty sock syndrome: biological growth on a cold evaporator coil, released for the first minutes of a cycle. Its cousin is a blocked condensate drain and a standing pan of water under the air handler.
Change the filter and check the condensate pan and drain line. If the smell persists it is a coil cleaning, not a mystery — book it as “odor on startup, suspect the evaporator coil” and you get a technician who already knows the job. If that system is also failing to heat, deal with that fault first.
Oily, kerosene, or diesel
In an oil-heated home this is normally the oil system: a clogged nozzle or filter, a cracked fuel line, a spill around the tank, or a burner that puffs on startup. That is an oil burner or HVAC technician, and worth doing promptly, because the faults that make the smell also make soot. With no oil equipment, look for stored fuel — a generator, a mower, a can in an attached garage, where a fuel smell is a ventilation question as well as a spill.
What each handoff costs
The free branches are genuinely free: refilling dry traps, changing a filter, cleaning a condensate pan, pulling out the refrigerator. So is every emergency call at the top of this page — fire department and gas utility both respond at no charge, and neither repairs anything afterward.
The paid ones start with a diagnostic. Fixr puts HVAC technician labor at $87 to $162 per hour and furnace repair at a national average range of $349 to $650. Mold inspection and testing runs $450 to $800, national average $500 — worth buying only when you cannot see the growth and need to know what and how much before authorizing remediation.
What not to buy is a diagnostic from every trade in turn. Pick the one the pattern points at and give them the pattern, not the adjective: “musty in the hall closet, worse after rain, ceiling stain above it” reaches the right person first time. “Weird smell” does not.
If you rent
Report it in writing, with the date, before you book anyone. Several branches above — sewer gas, mold with an active leak, a dead animal in a wall, overheating wiring — are habitability issues in most states with defined landlord response windows, and most leases require written notice and a chance to repair before you can charge anything back. Call your own contractor first and the bill is likely yours.
That does not apply to the top of this page. If you smell gas or anything burning, call 911 or the utility immediately and tell the landlord afterward. No lease outranks that. If the notice then goes ignored, the escalation ladder, with the deadlines and the retaliation protections, is what to have open when you write the second message.