---
title: "Air change per hour and effective temperature"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:air-change-effective-temp"
source: https://www.bctengineer.com/q/semester_6/building_technology/air-change-effective-temp
site: BCT Engineering
---

# Air change per hour and effective temperature

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2080 Chaitra, 3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/air-change-effective-temp
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

Define the term air change per hour and effective temperature used during study of functional requirement of ventilation.

## Short answer

Air change per hour (ACH) is the number of times the room air volume is replaced per hour, calculated as ACH = Q/V where Q is airflow rate (m³/h) and V is room volume (m³). Effective temperature (ET) is an index combining dry-bulb temperature, humidity, and air movement into a single value representing the temperature of still, saturated air that would produce the same thermal sensation; comfort zone is 20–27°C ET.

## Answer

#### Air change per hour (ACH)
**Air change per hour** is the number of times the whole volume of air in a room is replaced by fresh air in one hour.

$$
ACH = \frac{Q}{V}
$$

where $Q$ is the volume of air supplied or removed per hour (m³/h) and $V$ is the room volume (m³). For example, a 4 m × 5 m × 3 m room with a supply of 360 m³/h has $V = 60$ m³, so $ACH = 360/60 = 6$. Typical values: living rooms 3 to 6, kitchens 15 to 30, toilets 6 to 10, assembly halls 8 to 12.

#### Effective temperature (ET)
**Effective temperature** is an index that combines dry-bulb temperature, humidity and air movement into one number. It is the temperature of still, saturated air (100 % RH, no air movement) that gives the same feeling of warmth or cold as the actual conditions. It is read from the ET chart (Yaglou and Houghton) using dry-bulb and wet-bulb temperatures and air velocity. The comfort zone is about 20 to 27 °C ET for most people, with the preferred ET at 22 to 25 °C.

## Check yourself

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

1. Air change per hour is
   A. Q x V
   B. Room air volume replaced by fresh air times in one hour, ACH = Q/V
   C. V/Q
   D. Q/3600
   Answer: B. Q is the hourly volume of air supplied (m3/h) and V is room volume. (Key point: ACH)

2. A 60 m3 room with supply of 360 m3/h has ACH
   A. 6
   B. 0.17
   C. 21
   D. 60
   Answer: A. 360/60 = 6. (Key point: ACH example)

3. Effective temperature is the temperature of
   A. Dry air at 50 % RH
   B. Moving air at wet-bulb
   C. Still saturated air giving the same sensation
   D. Outdoor air only
   Answer: C. It uses 100 % RH and no air movement as reference. (Key point: Effective temperature)

## Related in Building Technology

- [Functional requirements of ventilation](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Avent-requirements) — Building Technology, chapter 1, asked 3×
- [Ventilation versus air-conditioning](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aventilation-vs-ac) — Building Technology, chapter 1
- [Building types, mechanical ventilation, air flow](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aairflow-078c) — Building Technology, chapter 1
- [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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