---
title: "Human thermal comfort and thermal balance"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:human-thermal-comfort-balance"
source: https://www.bctengineer.com/q/semester_6/building_technology/human-thermal-comfort-balance
site: BCT Engineering
---

# Human thermal comfort and thermal balance

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2070 Chaitra (old course), 2+2+4 marks; 2065 Shrawan (old course), 16 marks; 2064 Jestha (old course), 16 marks; 2062 Baisakh (old course), 16 marks (asked 4 times)
- Video: [Thermal Comfort in Buildings Explained - HVACR Design](https://www.youtube.com/watch?v=yEWT_XmqCtQ) — The Engineering Mindset
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/human-thermal-comfort-balance
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

Explain human thermal comfort. What are the thermal factors affecting thermal comfort? How does the human body maintain its thermal balance? Explain with neat sketch (including thermal preferences).

## Short answer

Thermal comfort is a mental state of satisfaction with the thermal environment. Factors are air temperature, mean radiant temperature, humidity, air movement, clothing, activity, and personal factors. The body balances metabolic heat with radiation, convection, conduction, and evaporation (M ± R ± C - E = 0), using vasodilation/sweating or vasoconstriction/shivering.

## Answer

**Thermal comfort** is the condition of mind that expresses satisfaction with the thermal environment (ASHRAE). A person feels neither too hot nor too cold, and wants no change in the surroundings.

#### Thermal factors affecting thermal comfort
- **Air temperature** (dry-bulb): the main factor. Comfort range about 21 to 27 °C.
- **Mean radiant temperature**: radiation from the surrounding walls, roof and sun.
- **Relative humidity**: comfortable between 30 and 60 %. High humidity stops sweat from evaporating.
- **Air movement**: moving air increases evaporation and convection; 0.1 to 0.5 m/s is comfortable.
- **Clothing** (clo value) and **activity level** (met value).
- **Personal factors**: age, sex, health, body build and acclimatisation.

#### Thermal balance of the human body
The body keeps its core temperature near 37 °C. Metabolism produces heat (about 100 W at rest). The body is in balance when

$$
M \pm R \pm C - E = 0
$$

where $M$ is metabolic heat, $R$ is radiation, $C$ is convection and conduction, and $E$ is evaporation. In hot conditions, the skin blood vessels widen (vasodilation) and sweating cools the body by evaporation. In cold conditions, the vessels narrow (vasoconstriction), shivering raises metabolism, and heat loss is reduced.

```text
 Metabolism (M) ----> BODY 37 C
                       |  |  |  |
          Radiation R--+  |  |  +--Evaporation E
          Convection C----+  +-- Respiration
```

#### Thermal preferences (ASHRAE 7-point scale)
| Scale | Sensation |
|---|---|
| +3 | Hot |
| +2 | Warm |
| +1 | Slightly warm |
| 0 | Neutral (comfort) |
| -1 | Slightly cool |
| -2 | Cool |
| -3 | Cold |

The building should be designed to give a vote between -1 and +1 for most users. The comfort zone is shown on a bioclimatic chart (Olgyay or Givoni) using temperature and humidity. A combined index, **effective temperature (ET)**, expresses temperature, humidity and air movement as one number: the temperature of still, saturated air that gives the same sensation.

Design should cut heat gain in summer (shade, insulation, ventilation) and keep heat in during winter (insulation, solar gain), so that the body stays in balance without strain.

## Check yourself

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

1. Thermal comfort is best defined as
   A. The air temperature being exactly 37 C
   B. A state of mind satisfied with the thermal environment, wanting no change
   C. Absence of any air movement
   D. A sensation of slight warmth only
   Answer: B. ASHRAE defines it as satisfaction with the thermal environment; the person is neither too hot nor too cold. (Key point: Comfort definition)

2. In the balance M +/- R +/- C - E = 0, what does E stand for?
   A. Emission of CO2
   B. Energy of clothing
   C. Evaporation
   D. Efficiency of metabolism
   Answer: C. E is evaporation (sweating); M is metabolic heat, R radiation, C convection and conduction. (Key point: Heat balance)

3. In cold conditions the body responds by
   A. Narrowing skin vessels and shivering to raise metabolism
   B. Widening skin vessels and sweating
   C. Stopping metabolism
   D. Increasing evaporation
   Answer: A. Vasoconstriction reduces heat loss and shivering raises metabolism; vasodilation and sweating are hot-condition responses. (Key point: Cold response)

4. On the ASHRAE 7-point scale, what is the design target for most users?
   A. Exactly +3
   B. Between +2 and +3
   C. Exactly -3
   D. A vote between -1 and +1
   Answer: D. Buildings should give votes between -1 and +1, with 0 as neutral. (Key point: ASHRAE scale)

## Related in Building Technology

- [Thermal comfort and its factors](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Athermal-comfort-factors) — Building Technology, chapter 1, asked 2×
- [Factors affecting thermal performance](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Athermal-perf-factors) — Building Technology, chapter 1
- [Thermal comfort and its classification](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Athermal-comfort-classification) — Building Technology, chapter 1
- [Achieving thermal comfort, active and passive methods](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aachieve-thermal-comfort-active-passive) — Building Technology, chapter 1
- [Daylight factor, thermal factors, natural ventilation](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adaylight-thermal-natural-vent) — Building Technology, chapter 1
- [Factors affecting foundation design, improving bearing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afactors-design-improve-bc) — Building Technology, chapter 2

## Related in other subjects

- [Factors affecting human comfort](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Ahuman-comfort-factors) — Applied Thermodynamics, chapter 4
- [Factors affecting morale of engineer](https://www.bctengineer.com/question.md?key=semester_8%2Fengineering_professional_practice%3Afactors-affecting-morale) — Engineering Professional Practice, chapter 2
- [Factors affecting water demand; variation](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Afactors-affecting-demand) — Water Supply Engineering, chapter 3, asked 5×
- [Factors affecting bearing capacity; water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Abearing-capacity-factors-water-table) — Foundation Engineering, chapter 6, asked 5×