---
title: "Short note: Ventilating shaft"
subject: "Sanitary Engineering"
chapter: 4
key: "semester_6/sanitary_engineering:ventilating-shaft-note"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/ventilating-shaft-note
site: BCT Engineering
---

# Short note: Ventilating shaft

Sanitary Engineering · Chapter 4: Sewer Appurtenances

- Asked in: 2068 Magh (old course), 4 marks
- Video: [sewerage, systems of sewerage, Ventilation of sewers](https://www.youtube.com/watch?v=m7vu3A8MLNc) — Explore with Athi
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/ventilating-shaft-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-4

## Question

Write a short note on ventilating shaft.

## Short answer

Ventilating shafts are 100-150 mm vertical pipes (CI/PVC) with cowls, extending 2-3 m above roofs to release toxic, explosive, and corrosive sewer gases (H₂S, methane) and admit fresh air. Provided at branch ends, high points, and along trunk sewers, they complement perforated manhole covers and house vent pipes. Workers must still test gases before entry.

## Answer

A **ventilating shaft** (vent stack) is a vertical pipe or shaft connected to the sewer or to a manhole, which lets the sewer gases escape to the atmosphere at a safe height and lets fresh air in.

#### Need
Sewage decomposes and produces gases such as **hydrogen sulphide, methane, carbon dioxide, ammonia and carbon monoxide**. These are
- poisonous (H₂S, CO) and dangerous to sewer workers,
- explosive or inflammable (methane), and
- corrosive; H₂S changes into sulphuric acid and attacks the concrete crown of the sewer.
Ventilation also reduces sewer odour and lets air in for the aerobic condition.

#### Arrangement
```text
        ___ Cowl
       |   |
       | V |  vertical pipe, 100-150 mm CI,
       | e |  fixed on a building wall or a pole
       | n |  at least 3 m above ground (above roofs)
       | t |
   ____|   |____
  Manhole / sewer
```

- A 100-150 mm cast iron or PVC pipe is fixed along a building wall and connected to the sewer; the top has a cowl to keep rain out. Its height is above the roofs (at least 2-3 m above the highest window).
- Ventilation is also obtained through the **perforated manhole covers** (which let air in), and the **house vent pipes** (soil pipes carried up above the roof), and **ventilating columns** at the end of the sewers.
- Ventilating shafts are provided at the ends of branch sewers, at the highest point of the network, and spaced at regular distances along trunk sewers; if the sewer is long they may use forced draught or fans.

#### Precautions
Workers must still test for gases and ventilate by blowers before entering a sewer.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the main purpose of a ventilating shaft?
   A. Collect silt
   B. Let sewer gases escape at a safe height and fresh air in
   C. Flush the sewer
   D. Cross obstacles
   Answer: B. It vents gases to the atmosphere. (Key point: Purpose)

2. Which sewer gas is explosive?
   A. Nitrogen
   B. Oxygen
   C. Methane
   D. Water vapour
   Answer: C. Methane is inflammable and explosive; H2S and CO are poisonous. (Key point: Gases)

3. What does H2S do to the sewer?
   A. Strengthens concrete
   B. Cleans pipes
   C. Dissolves grease
   D. Changes to sulphuric acid and attacks the concrete crown
   Answer: D. It forms sulphuric acid which corrodes the crown. (Key point: Corrosion)

4. How is the shaft arranged?
   A. 100-150 mm pipe with a cowl, above roofs
   B. Pipe ending at ground level
   C. Open drain
   D. Underground only
   Answer: A. The top has a cowl and rises well above roofs. (Key point: Height)