---
title: "Building types, mechanical ventilation, air flow"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:airflow-078c"
source: https://www.bctengineer.com/q/semester_6/building_technology/airflow-078c
site: BCT Engineering
---

# Building types, mechanical ventilation, air flow

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2078 Chaitra, 10 marks
- Video: [The RIGHT Way to Ventilate Your Building](https://www.youtube.com/watch?v=32mkWLPOFCU) — Passive House School with Anton
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/airflow-078c
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

Define residential and institutional building. Explain types of mechanical ventilation system. Calculate the rate of air flow through two square openings of size 0.32m and 0.42m in a 4m×4m×4m room, with a wind of 5 km/hr blowing inclined at 45° to the opening. Take Coefficient of effectiveness =0.33.

## Short answer

Residential building: for living/sleeping (houses, flats, hotels). Institutional building: for medical care/custody (hospitals, clinics, jails). Mechanical ventilation types: extract (exhaust) system, plenum (supply) system, balanced (supply+exhaust) system, ducted system with air-handling unit. For given openings, effective area A = 0.08856 m²; airflow Q = E×A×V = 0.33 × 0.08856 × 1.389 = 0.0406 m³/s = 146.1 m³/h.

## Answer

#### Residential building
A building used for living and sleeping: houses, flats, apartments, hostels, hotels.

#### Institutional building
A building used for medical treatment, care or custody of people, where they stay or attend: hospitals, clinics, sanatoriums, homes for the aged, orphanages, jails. (Some textbooks also include schools and colleges, but the building code treats educational buildings as a separate class.)

#### Types of mechanical ventilation
1. **Extract (exhaust) system**: Fans remove air from the room, and fresh air comes in through openings. Used in kitchens, toilets and factories. It creates negative pressure.
2. **Plenum (supply or positive-pressure) system**: A fan forces filtered air in, and stale air leaves through openings. Used in hospital operating theatres and clean rooms.
3. **Balanced (combined supply and extract) system**: Both supply and exhaust fans are used, so the pressure is balanced. Used in theatres, halls and basements.
4. **Ducted system with air-handling unit**: A central fan, filters and ducts distribute the air; it can be combined with air conditioning.

```text
 Extract:  out <-[fan]  room  <- air leaks in
 Plenum:   in ->[fan]   room  -> air leaks out
 Balanced: in ->[fan] room [fan]-> out
```

#### Given
- Inlet and outlet openings: 0.32 m × 0.32 m and 0.42 m × 0.42 m, so $A_1 = 0.1024\ \text{m}^2$ and $A_2 = 0.1764\ \text{m}^2$
- Wind speed $v = 5$ km/h $= 5/3.6 = 1.3889$ m/s, at 45° to the opening
- Coefficient of effectiveness $E = 0.33$ (already accounts for the wind direction; the room size does not enter the formula)

#### Formula
For wind-driven ventilation through two openings in series, the effective area is

$$
A = \frac{A_1 A_2}{\sqrt{A_1^2 + A_2^2}}, \qquad Q = E\, A\, V
$$

#### Calculation
$$
\begin{aligned} A &= \frac{0.1024 \times 0.1764}{\sqrt{0.1024^2 + 0.1764^2}} = 0.08856\ \text{m}^2 \\ Q &= 0.33 \times 0.08856 \times 1.3889 = 0.04059\ \text{m}^3/\text{s} \end{aligned}
$$

**Answer: $Q = 0.0406$ m³/s $= 146.1$ m³/h.**

## Check yourself

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

1. In a mechanical extract (exhaust) system, what pressure does the room have and where is it typically used?
   A. Positive pressure; operating theatres
   B. Negative pressure; kitchens and toilets
   C. Balanced pressure; basements
   D. Positive pressure; kitchens
   Answer: B. Fans remove air so the room is at negative pressure and fresh air leaks in. This suits kitchens, toilets and factories. (Key point: Extract system)

2. Which system forces filtered air into the room so that stale air leaves through openings?
   A. Extract system
   B. Balanced system
   C. Plenum (supply) system
   D. Natural stack system
   Answer: C. The plenum system uses a supply fan for positive pressure, as in operating theatres and clean rooms. (Key point: Plenum system)

3. For two openings in series, the effective area is:
   A. A1 + A2
   B. A1*A2 / sqrt(A1^2 + A2^2)
   C. sqrt(A1*A2)
   D. (A1 + A2)/2
   Answer: B. The effective area for openings in series is A = A1A2/sqrt(A1^2 + A2^2), then Q = E A V. (Key point: Effective area formula)

4. With A1 = 0.1024 m^2, A2 = 0.1764 m^2, E = 0.33 and V = 5 km/h, what is Q?
   A. About 0.0406 m^3/s
   B. About 0.406 m^3/s
   C. About 0.0146 m^3/s
   D. About 0.0881 m^3/s
   Answer: A. A = 0.0886 m^2, V = 1.389 m/s, so Q = 0.33 x 0.0886 x 1.389 = 0.0406 m^3/s (146 m^3/h). (Key point: Result of calculation)

## Related in Building Technology

- [Air flow through openings numerical (0.36 m, 0.4 m)](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aairflow-078p) — Building Technology, chapter 1, asked 2×
- [Air flow through openings numerical (0.3 m, 0.4 m)](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aairflow-079a) — Building Technology, chapter 1, asked 2×
- [Mechanical ventilation and building code](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Avent-mech-conditions-nbc) — Building Technology, chapter 1

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- [Design wind pressure, large openings, 100 yr](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Awind-large-openings-hill-160m) — Design of Steel and Timber Structure, chapter 8