Common Home Smells + Diagnosis Reference
Why this matters
Customers often report smells without knowing the source. Different smells suggest different problems - some cosmetic, some dangerous. The tech who can identify the likely source narrows the diagnosis quickly. This is the field card.
Why smell matters
Smell is the body's early warning:
- CO has NO smell (you need a detector)
- Natural gas has added scent (mercaptan) for safety
- Mold has musty smell (warning of moisture problem)
- Electrical fire smell (acrid) warns of imminent fire
Customers may dismiss a smell that's actually serious. The tech who investigates respectfully helps.
Common smells + likely sources
Rotten egg / sulfur:
- Natural gas leak (mercaptan added; smells like rotten egg) - DANGEROUS
- Hydrogen sulfide (sewer gas) - failed trap, dry trap, vent issue
- Anaerobic bacteria in water heater (smell from hot tap only)
- Well water (some areas naturally have sulfur)
Action: gas suspected = 911 / utility immediately; sewer gas = investigate traps + vents; hot-water sulfur = check anode rod.
Burning electrical / acrid:
- Overheated wire / outlet
- Failing motor (HVAC, appliance)
- Overheated transformer
- Heating element burnout
Action: turn off breaker; investigate; replace component before fire.
Burning plastic / chemical:
- Melting wire insulation
- Heated PVC (high temp; abnormal)
- Air filter clogged + struggling motor
- Heating element on dust accumulation
Action: investigate immediately; could be fire risk.
Musty / mildew:
- Mold growth
- Damp basement / crawlspace
- HVAC drain pan
- Carpet padding
- Wet drywall
Action: find moisture source + address; mold remediation if significant.
Sewage:
- Failed P-trap (dry; allows sewer gas)
- Sewer line damage / break
- Cracked toilet flange / wax ring
- Failed vent
- Septic system backup
- Cleanout cap missing
Action: trace source; correct trap / vent / repair as needed.
Wet pet / dog:
- Pet present + wet
- Pet bedding/carpet odor
- Cosmetic; cleaner addresses
Action: cleaning recommendation; not service emergency.
Cat urine:
- Cat sprayed area
- Soaked into carpet / padding / subfloor
- Hard to fully remove
- Sometimes specialized enzyme cleaners
- Sometimes subfloor replacement
Action: deep cleaning; subfloor inspection.
Cigarette smoke:
- Past smoker in home
- Ventilation system carries scent
- Permeated walls / drywall
- Difficult to fully remove
- Sealer paint may help
Action: remediation services; HVAC duct cleaning.
Sweet / chemical (refrigerant):
- Refrigerant leak (HVAC)
- Some refrigerants slightly sweet OR no smell
- R-22 + R-410A largely odorless
Action: refrigerant leak detector; HVAC tech evaluation.
Mothball / chemical:
- Stored mothballs
- Customer used heavily
- Older homes
- Removing may help
Vinegar / sour:
- Old food
- Refrigerator failure
- Wet basement (anaerobic conditions)
Burning oil / petroleum:
- Oil furnace problem
- Tank leak
- Service issue
Action: oil furnace check.
Sweet / acetone (banana):
- New paint / solvent
- Sometimes diabetic ketoacidosis (medical issue; not building issue)
Garlic / onion-like:
- Some industrial chemicals
- Methane / decomposition (sometimes)
- Unusual sources
Investigation process
When customer reports smell:
- Ask questions:
- When did it start?
- Where strongest?
- Time of day pattern?
- Worse when HVAC running?
- Worse when water running?
- Worse in specific room?
- Find the strongest source:
- Walk through home methodically
- Note where smell intensifies
- Outdoor vs indoor source
- Investigate likely sources:
- Match smell to common causes (above)
- Check most-likely first
- Verify with second method:
- Detector (gas, CO, moisture)
- Visual inspection
- Smell comparison
- Repair OR refer:
- Within your trade: fix
- Outside trade: refer
Sources by location
Bathroom:
- Sewage from drain (failed trap, vent)
- Mold (shower)
- Plumbing leak under sink
Kitchen:
- Disposal (organic buildup)
- Refrigerator (failed compressor, expired food)
- Range / oven (food residue)
- Sink drain (anaerobic buildup)
Laundry:
- Washer (mold in seal / drum; front-loaders especially)
- Dryer vent (lint buildup; fire risk)
Basement:
- Damp / mold
- Sewer (floor drain, sump pit)
- Stored items
Crawlspace:
- Damp / mold
- Animal infestation
- Sewer / septic
Attic:
- Pest activity
- Wet insulation
- Heater venting issues
Garage:
- Vehicle exhaust
- Stored chemicals
- Mold (uninsulated)
Smells suggesting urgent action
Gas (rotten egg + chemical):
- Evacuate
- Call 911 / gas utility
- Don't operate switches / phones inside
- Wait outside
Burning electrical:
- Turn off breaker
- Investigate
- Don't dismiss
- Possible imminent fire
Sewage strong + sudden:
- Possible major leak / backup
- Identify source
- May need plumber emergency
Mold smell + symptoms (respiratory):
- Significant exposure
- Investigate + remediate
Smells with low urgency
Most other smells can wait for scheduled investigation:
- Mild mustiness
- Pet odors
- Stale air
- Persistent kitchen smells
Refrigerator + freezer
Cold but smelly:
- Expired food
- Mold growth in drain pan
- Failed defrost system
Stops working + smells:
- Refrigerant leak (less common with modern hermetic)
- Failed compressor (lubricant burned)
HVAC system smells
Burning / hot metal:
- New furnace first season (dust burning off heat exchanger)
- Motor overheating
- Belt slipping
Musty (cooling mode):
- Drain pan stagnant water
- Mold on coil
- Filter saturated
Dirty sock / pungent (cooling):
- "Dirty sock syndrome"
- Bacteria / mold on evaporator
- Coil cleaning needed
Burning electrical:
- Capacitor / contactor issue
- Wiring problem
- Investigate immediately
Plastic / chemical:
- Overheated component
- Burned wire insulation
- Possible fire risk
When customer's smell isn't real
Sometimes:
- Customer is hypersensitive
- Family member smells things differently
- Cultural / generational differences in perception
- Sensitivity vs reality
You may detect nothing. Document your investigation. Customer may need different help (medical evaluation if persistent).
When you smell something customer doesn't
You arrive + immediately notice smell customer ignores:
- Customer's nose adapted (you can adapt back)
- Customer doesn't have the chemical sensitivity
- Some smells fade with exposure
Mention factually: "I'm noticing a smell of [X]. Let me look into where it's coming from."
Common cross-trade observations
HVAC tech: musty air handler, sewer smell from condensate drain (failed trap), burning smell during operation.
Plumber: sewer gas at fixtures (trap issues), well water sulfur (anode rod / chemistry).
Electrician: burning insulation (panel, outlet, switch), overheated motor (HVAC, appliance).
References
- EPA Indoor Air Quality (IAQ) resources
- ASHRAE 62.2 (Indoor Air Quality)
- Manufacturer information on common appliance odors
- Manuall internal: Mold Remediation Basics, Indoor Air Quality Testing