Where the smell in an empty house usually comes from
| Source | Why vacancy makes it worse | What usually fixes it |
|---|---|---|
| Floor drains and unused sinks | Water in the trap evaporates, letting sewer gas through | Run water in every fixture regularly; add water to floor drains |
| Basement or crawl space | No air movement; moisture builds | Keep the HVAC fan cycling; check for damp spots and running sump |
| Refrigerator and freezer | Unplugged units with doors shut grow mildew | Clean, dry, and prop doors open, or leave running and empty |
| Dishwasher and washing machine | Standing water and residue sit for weeks | Run an empty cycle, then leave doors ajar |
| Trash, recycling and garage bins | Forgotten containers ripen | Empty and rinse before vacating |
| HVAC system | Odor settles in ducts, then blasts out on first use | Fresh filter at move-out; run the fan periodically |
Denver's climate helps and hurts
The region's dry air means many vacant homes do not develop the heavy mildew smell seen in humid climates. But dry air speeds evaporation from drain traps, which is why sewer smell in an empty basement is such a common complaint. Winter adds its own twist: sellers often set the thermostat low to save on heating, and a cold, still basement concentrates whatever odor it has. When the heat finally runs hard for a showing, the furnace pushes that air through the whole house.
Summer has the reverse problem. A closed house with the AC off can heat up enough that wood, carpet and paint release odors faster, and the first buyer through the door gets the full effect. Moderate thermostat settings and some air movement prevent both extremes.
Vacancy also removes the people who would notice a problem early. A small roof leak, a slow drip under a sink, or a mouse in the attic can go unseen for weeks if nobody is walking the house.
A weekly routine for an empty listing
The seller, a family member, or the listing agent can share this. It takes less than an hour.
Run every fixture
Sinks, tubs, showers and toilets for a few seconds each, plus a pitcher of water into each floor drain.
Air the house before a showing day
Open windows for a short time when the weather allows, then close up.
Check the lowest level
Look and sniff for damp spots, sump activity and any new smell near the furnace or water heater.
Look for signs of animals
Droppings, chewed insulation or a new, sweet-rotten odor in one room deserve attention right away.
Confirm the HVAC settings
Moderate temperature, fan cycling, filter not overdue.
Walk out the front door and back in
The re-entry is the closest you will get to a buyer's first impression.
General information for the Denver area, not a quote or professional advice. In an emergency, call 911.