What causes manhole and inspection chamber smells?

A few observations you can make without touching the cover or leaning over it narrow down the source of the smell quite a lot. When you call our team, describing these observations helps the job start with the right preparation.

A manhole cover is more than a lid: it is a seal that keeps the chamber's gas inside. A cover that does not sit fully in its frame, has a broken edge or is the grated type lets gas leak out all the time. In car parks where vehicles drive over them, covers gradually start to move in their frames, rattle as they are driven over and the gap grows.

For manholes inside buildings, especially in basements and boiler rooms, the cover has to be the airtight type. If a grated cover made for outdoor use has been fitted in a basement, the smell is unavoidable. Replacing the cover or reseating its frame often solves the problem on its own.

Standing water in the chamber means there is a blockage on the outlet side of the manhole. In a healthy manhole the water runs through the channel at the base and the chamber stays dry or nearly dry. If there is an obstruction in the outlet pipe, in the line beyond it or in the street line it connects to, the water starts to build up, turns foul quickly and a heavy smell rises from every gap in the cover.

In this case the smell only goes once the obstruction holding back the water is cleared. Cleaning the chamber alone is not enough. If there is wetness at the edge of the cover, slow drains on the ground floor or mud around the manhole, the smell is a warning of an overflow and waiting will only make the job bigger.

Under every drain inside a building there is a small water seal that keeps sewer gas out. This water, held in the sink trap, under the shower tray and in floor drains, stops air coming from the manhole and the shared stack from rising into the room. In a drain that has not been used for a long time, this water evaporates and a path opens for the gas.

This smell is most often met on returning from a summer house, in flats left empty after a change of tenant and in rarely used basement floor drains. The fix is simple: pouring a bucket of water into unused drains at regular intervals renews the seal. If the smell continues after the water has been poured, the cause has to be looked for elsewhere.

Even when water is flowing, a layer made of sludge, sand, grease and fibrous waste builds up at the bottom of the manhole over time. This layer stays permanently wet and becomes a source of smell in its own right. On lines serving restaurants and cafés, grease sticks to the chamber walls and speeds up this process.

Washing such a chamber with pressurised water and clearing the outlet line both reduces the smell and delays the next blockage. If the channel at the base is damaged, water pools in the corners. This is easy to see from above and with a camera.

Most manhole smells come from small oversights that could have been noticed before the problem grew. For building and site management, a few habits keep both the smell and overflows away.

The gas that collects inside manholes and inspection chambers does more than smell bad. It contains toxic, suffocating or flammable components such as hydrogen sulphide and methane, and the ones heavier than air gather at the bottom of the chamber. Because hydrogen sulphide dulls the sense of smell at high concentrations, you should not conclude that the area is safe just because the smell has faded.