The cause
Every plumbing fixture has a P-trap — the curved section of pipe below it — holding a small volume of water. That water is the only thing separating your building's interior from the sewer system. It is a water seal, and it works well.
It also evaporates. In an unused fixture the trap can dry within a few weeks, faster in dry winter air with heating running. Once it is gone, sewer gas enters the building continuously.
The fixtures that dry first are the ones nobody ever uses: floor drains in basements and mechanical rooms, laundry sinks, guest bathroom showers, and any bathroom in an unused part of the house. Floor drains are the most common culprit and the most commonly forgotten.
Why it matters commercially
For a listing this is not a cosmetic issue. A buyer walking into a vacant house and smelling sewer gas does not think "dry trap" — they think "plumbing problem", and that suspicion attaches to everything else they see afterwards.
For an estate property being shown to family, or a rental about to be handed to a tenant, the same applies. It is a very cheap problem producing a disproportionate impression.
The fix
- Run every fixture briefly at every visit — thirty seconds each is enough.
- Pour water directly into every floor drain, including the mechanical room.
- Flush every toilet, and close the lids to slow evaporation from the bowls.
- For fixtures that will not be used at all, add a small amount of mineral oil to the trap. It floats on the water and slows evaporation substantially.
- If the water is shut off and drained, add plumbing antifreeze to traps instead.
When it is not the traps
If running every fixture and treating the traps does not resolve it within a few days, the smell is something else and worth investigating properly. Genuine possibilities include a dead rodent in a wall or duct, mould from a moisture source, a failed sewer line, or a gas leak — which is an emergency and means leaving the building and calling the utility immediately.
A persistent unexplained odour in a vacant property is worth a professional opinion rather than more air freshener.