Thermal comfort 1

Question 1

Explain predicted mean vote and % dissatisfied for thermal comfort

Answer 1

PMV is a subjective seven point scale of thermal comfort from -3 (uncomfortably cold) to +3 (uncomfortably warm) with 0 being a thermally neutral sensation. This is a combination of factors including air temperature, air velocity, radiant heat

Predicted percentage dissatified is the percentage of people that will report dissatisfaction under a particular set of thermal conditions. When the predicted mean vote is 0 only about 5% of people will report being dissatisfied with the temperature.

Question 2

Explain the adaptive theory of thermal comfort.

Answer 2

People adapt as the temperature changes. One of the factors influencing a subjectively comfortable temperature is the temperatures of the recent past. A running mean of outdoor temperatures (Ø_rm) weighted according to their distance from the past will affect peoples idea of a comfortable indoor temperature.

Question 3

Explain the concept of free running buildings

Answer 3

In temperate climates thenheating and cooling should be aimed to minimise heating and cooling.

The upper margin of comfort it 20.8 + 0.33 (running mean)

Lower margin of comfort 16.8 + 0.33 (running mean)

A thermally successful building is one who indoor temperature changes only gradually in response to changes in the outdoor temperature and rarely stray beyond those bands.

Question 4

What is the definition of an overheated building?

Answer 4

If a building exceeds 28° for at least 1% of the occupied time.

Question 5

What are the reasons for localised thermal discomfort in a space

Answer 5

The person being close to a window and getting drafts

The person being near a source of heat or cooling such as a fan coil unit or a radiator

A person being near a ceiling fan.

Fluctuations in the air speed

A person being at a higher or lower level of a tall airspace

Asymmetrical radiation such as receiving sunlight through a window or an infra-red heating panel

Question 6

+

Question 7

Explain the concept of wet bulb temperature

Answer 7

Because we reduce body heat by evaporation of sweat then the relative humidity of the air will affect out subjective perception of temperature. wet bulb temperature will have an effect on our subjective perception of

Question 8

Explain the concept of operative temperature

Answer 8

Operative temperature is a function that allows us to combine the different elements of thermal comfort into a single temperature. These functions are: air temperature, mean radial temperature and air speed. Formula is (Ø_r + Ø_ax(10V_a)^1/2)/1+10V_a

Ø_r = mean radiant temperature

Ø_a= air temperature

V_a = air speed

In a still environment this simplifies to Ø_r + Ø_a

Question 9

What issues are to be considerent when selecting external design conditions?

Answer 9

Historical temperature and wind data from the area. Thermal mass of the building.

Insulation of the building. Ventilation heat loss. Altitude

Question 10

What is the formula for infiltration heat losses per hour.

Answer 10

0. 33XNXVXDT
1. 33 x number of air changes per hour, times volume of space x temperature difference.

This can be simplified into watts:

(0.33 x airflow(cubic metres) x temperature difference)/3600