---
title: "Types and design of ventilation"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:ventilation-types-design"
source: https://www.bctengineer.com/q/semester_6/building_technology/ventilation-types-design
site: BCT Engineering
---

# Types and design of ventilation

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2072 Magh, 6 marks; 2072 Asoj, 8 marks (asked 2 times)
- Video: [Ventilation Basics Series #2 - System Types](https://www.youtube.com/watch?v=d9btBXMocsc) — Aereco Channel
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/ventilation-types-design
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

Explain different types of ventilation and their design (design methodology of ventilation).

## Short answer

Types include natural (wind/stack), mechanical (extract/plenum/balanced), and mixed mode. Design: find occupant number or air change rate, compute required flow, size openings for natural or fan capacity for mechanical.

## Answer

**Ventilation** is the supply of fresh air to, and removal of stale air from, an enclosed space.

#### Types of ventilation
1. **Natural ventilation**: Uses wind pressure and stack (thermal) effect through openings.
   - *Wind-driven (cross) ventilation*: windows on windward and leeward sides.
   - *Stack effect*: warm air rises and leaves through high openings (ventilators, ridge, chimney), and cool air enters low.
2. **Mechanical (forced) ventilation**: Fans and ducts are used.
   - *Extract (exhaust) system*: fan removes air; used in kitchens, toilets.
   - *Plenum / supply system*: fan forces fresh air in.
   - *Balanced system*: both supply and exhaust fans.
3. **Combined (mixed mode)**.

Mechanical ventilation is preferred where natural ventilation cannot give the needed rate: basements, large halls, interior rooms, kitchens, workshops.

#### Design methodology
1. Find the **number of occupants** and the use, then the required fresh-air rate (e.g. 25 to 30 m³/h per person) or the **air change rate** (ACH) from the code.
2. Required flow: $Q = \frac{V \times N}{3600}$ (m³/s), where $V$ is room volume and $N$ is ACH.
3. **Natural**: Wind flow $Q = E A V$ (m³/s), with $E$ effectiveness (0.5 to 0.6 for wind perpendicular to opening, 0.25 to 0.35 diagonal), $A$ open area, $V$ wind speed (m/s). Stack flow: $Q = C_d A \sqrt{2 g h \frac{T_i - T_o}{T_i}}$.
4. Fix **opening areas** (generally 15 to 20 % of floor area) and locate inlets low and outlets high, on opposite walls.
5. **Mechanical**: Choose fan capacity for $Q$, size ducts for 3 to 6 m/s, and select grilles.
6. Check the result against the Nepal National Building Code (NBC 206).

## Check yourself

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

1. In stack ventilation, air moves because
   A. Wind pushes through windows
   B. Warm air rises out of high openings while cool air enters low
   C. Fans force it
   D. Air is cooled below dew point
   Answer: B. Stack uses thermal buoyancy; wind-driven flow is the cross type. (Key point: Stack effect)

2. A plenum system is one where
   A. A fan forces fresh air into the space
   B. A fan only removes air
   C. Both supply and exhaust fans are used
   D. No fans are used
   Answer: A. Plenum is supply; extract removes air; balanced uses both. (Key point: Mechanical systems)

3. The required flow from the ACH is
   A. Q = V / N
   B. Q = N x 3600 / V
   C. Q = V x N / 3600 m3/s
   D. Q = E x A x V x N
   Answer: C. Volume times air changes per hour gives m3/h; dividing by 3600 gives m3/s. (Key point: Flow formula)

4. In Q = E A V, E is
   A. Efficiency of fan
   B. Energy of air
   C. Extraction ratio, always 1
   D. Effectiveness of the opening, 0.5 to 0.6 for perpendicular wind
   Answer: D. E is 0.5 to 0.6 for perpendicular wind and 0.25 to 0.35 for diagonal wind. (Key point: Wind formula)

## Related in other subjects

- [Short note: check list methodology](https://www.bctengineer.com/question.md?key=semester_8%2Finformation_system%3Ashort-note-checklist-design) — Information System, chapter 5