Energy Loss in buildings

Question 1

What is heat flux

Answer 1

Heat transfer per unit area, driven by temperature difference

Question 2

What is a heat pump?

Answer 2

Moves heat from low temperature regions to high temperature regions, against the natural direction

Question 3

What is conduction?

Answer 3

Heat transfer through a stationary medium, with a temperature gradient

Question 4

What is ficks law?

Answer 4

q’’ = -kDT/dx

Question 4

What is convection?

Answer 4

Heat transfer between a moving fluid and stationary object at different T

Question 5

Free Convection
Benefits/drawbacks

Answer 5

Entirely due to density gradients. Low heat flux, plenty of space and low noise

Question 6

What is radiation

Answer 6

Heat transfer between two or more surfaces at different temperatures with the absence of an intervening medium

Question 7

How to find total U for a non homogeneuous wall?

Answer 7

1/Ut = Rtot = R1+R2+…

Question 8

What is U and R in terms of ficks law

Answer 8

U = k/d
R = 1/U

Question 9

What is the equation for heat flux in radiation

Answer 9

q’’ = sigma T^4

Question 10

Equation for convection heat flux

Answer 10

q’’ = UdTm

Question 11

What is the logarithmic mean temperature

Answer 11

(dTa-dTb)/ln(dtA/dTB)

Question 12

Sources of heat loss and gain in buildings

Answer 12

Heat loss - conduction and convection through walls and openings.

Heat gain - people, equipments, sun radiation.

Question 13

Impact of sunlight on heat gain. Factors influencing

Answer 13

Raises surafce temperature and therefore DT

Depends on angle of rays, colour, roughness, reflectivity

Question 14

Ventilation heat flow equation

Answer 14

Q = MCN (Ti-To)/3600

Question 15

What is the ventilation factor

Answer 15

Heat loss due to room air changes - 0.33

Question 16

What is the building design heat loss?

Answer 16

H = Qloss/dT

Question 17

How to reduce error in building heat loss calculations. Why is this needed?

Answer 17

The surface temperature is rarely equal to internal temperature due to radiative and convective heat transfer.

Use the environmental temperature.

Question 18

Environmental temperature calculation

Answer 18

Te = 0.33Ti + 0.66Tr where Tr is the mean temp of surfaces in the room

Question 19

What is comfortable temperature?

Answer 19

Subjective and affected by air temp, velocity, humidity etc
Tc = 0.75Te + 0.25Ti

Question 20

Procedure for energy loss in buildings

Answer 20

1.) select recommended internal and external temp
2.) Determine factors to account for Te and Ti
3.) Determine Q for each zone and hence total heat loss
4.) Note design heat loss

Question 21

What are degree days used for?

Answer 21

Estimating heating costs over a long time period, allowing for seasonal variations

dT x time

Question 22

How are degree days calculated?

Answer 22

average of max and min temperatures, ignoring above base.

Question 23

What are Y values?

Answer 23

Measure of response of a material to temperature change and depends on thermal inertia of material

If