# heat loss

```4.3
Heat losses 失热量
4.3.1 Factors affecting heat loss影响建筑失热量的因素
Figure 3.1 Heat losses from a building
Fabric heat loss

Ventilation loss

Factors affecting heat losses

1) insulation of shell 建筑外围护结构的保温
Good insulation decreases the heat losses

Poorly-insulated increase the heat losses

2 ) area of the shell 建筑外围护结构的面积
A terraced house排房 and a detached house独立式住宅
Which one loses less heat ??
Table 4.7
3）temperature difference between inside and outside




Large temperature difference increases the heat losses
by conduction and ventilation.
Mainly depend on the design temperature for the inside
air.
Recommended comfort temperature for different types of
buildings are given in table 4.5
4) Air change rate 换气次数


Warm air leaving a building carries heat and is replaced
by cold air
Table 4.5 gives typical rates of air infiltration
5) Exposure to climate 建筑的暴露情况
External surface resistance 外表面热阻
Three types of exposure
 Sheltered: 受遮挡
Buildings up to 3 storeys in city centres.
 Normal: 正常
Most suburban and country buildings
 Severe:严重
Buildings on exposed hills or coastal sites.

Floors above the fifth in suburban or country sites.

Floors above the ninth in city centres.

6）efficiency of services 设备的效率


Flue is positioned inside the building Flue is positioned
inside the building
Flue is positioned on an external wall
7) Patterns of use 建筑使用模式


The number of hours per day and the days per year that
a building is used have a large effect on the energy
consumption of a buildings
Each parts needs to be considered as a separated
building for the heating calculations.
4.3.2 Calculation of heat loss失热量计算

Fabric heat loss 结构失热量
walls, windows, roofs and floors.
Pf  UAt

Applied in the following cases:
Pf  UAt




Ventilation loss 通风失热量
C V NV t
PV 
3600
External temperature 室外温度

In winter: = outside air temperature for design purpose

it is necessary to take account of solar radiation as well as air
temperature.

In summer: = sol-air temperature teo

1
Sol-air temperature（室外空气综合温度）

1）对流换热量
2）太阳辐射






3）长波辐射换热量
与大气之间的长波辐射
与环境表面之间的长波辐射
与地面的长波辐射

2 建筑物外表面单位

q   out (t air  t w )  aI  Qlw



Qlw
aI
  out (t air 

)  t w    out (t z  t w )


out
out



q
 out
t air
tw
a
I
Qlw
—建筑物外表面单位面积上得到的热量，W/m2
—围护结构外表面的对流换热系数，W/ m2℃
—室外空气温度，℃
—围护结构外表面温度，℃
—围护结构外表面对太阳辐射的吸收率
—太阳辐射照度，W/ m2
—围护结构外表面与环境表面的长波辐射换热量，
q   out (t air  t w )  aI  Qlw


Qlw
aI
  out (t air 

)  t w    out (t z  t w )
 out  out



Qlw 也被称为夜间辐射或有效辐射
Worked example 4.1
A window measuring 2 m by 1.25 m has an average Uvalue, including the frame, of 6.2 W/m2K. Calculate
the rate of fabric heat loss through this window when
the inside comfort temperature is 20℃ and the out
side air temperature is 4 ℃.
know U= 6.2 W/m2K A=2X1.25=2.5m2⊿t=20-4=16 ℃
using
Pf  UAt  6.2  2.5 16  248
So fabric loss=248W
Worked example 4.2
A simple building is 4 m long by 3 m wide by 2.5 m high. In the
walls there are two windows, each 1 m by 0.6 m, and there is
one large door 1.75 m by o.8 m.
The construction has the following U-values in W/m2K: windows
5.6, door 2.0, roof 3.0, floor 1.5.
The inside environmental or comfort temperature is maintained
at 18 ℃while the outside air temperature is 6 ℃. The volumetric
specific heat capacity of the air is taken to be 1300J/m3 ℃.
There are 1.5 air change per hour.
Calculate the total rate of heat loss for the building under the
above conditions.
Step1: sketch the building with its dimensions, as in
figure 3.2. calculate the areas and the temperature
difference.
Step 2: tabulate the information and calculate the rate of fabric
heat losses using
Pf  UAt
Step3: calculate the ventilation heat loss.
CV= 1300J/m3 ℃, N=1.5/h V=4X3X2.5=30m3, ⊿t=18-6=12 ℃
C V NVt
using
PV 
3600
1300 1.5  30  12

 195
3600
So rate of ventilation heat loss = 195W
Step4: total rate of heat loss = fabric heat loss + ventilation
heat loss= 1734.24+195=1929.24W



For situations where the steady state assumption is
invalid
it is necessary to consider the effects of
daily variations in the outside temperature
Changes in the internal heat input
Unit ： W/m2K
Thermal transmittance 传热系数
For very thin units, such as glass, the admittance becomes the
same as the U-value.

Figure 4.3 Thermal response
Heavyweight structures
have smaller
temperature swings(温度

structures.
damping, 衰减
McMullan
It is too difficult to solve
《空气调节》
4.4 Heat gains 建筑得热量
Figure 4.4 Typical heat gains in a building
McMullan
typical heat gains in a building
1）Solar heat gains from the sun 太阳辐射得热量
2）Casual heat gains from occupants and equipment in the
building 室内人员和设备形成的一般得热量
1）Solar heat gains from the sun

Depends on many factors
Table 4.9 seasonal solar gain through windows
Sun controls to Prevent excessive heat gain and
glare（眩光） caused by direct sunshine.
External controls （外遮阳）
Internal controls （内遮阳）
Special glasses （特殊玻璃）

External controls
External controls
Special glasses
（特殊玻璃）
2）Casual heat gains from occupants and
equipment in the building

Heat from people
Heat from lighting
Heat from cooking and water heating
Heat from machinery, refrigerators , electrical appliances
Table 4.11
domestic seasonal heat gains
4.5 Heat balance
Fabric
Heat
ventilation
+ + Heat
Losses
Losses
solar
casual
= Heat
+ Heat
gains
gains
Energy
for
+ heating
or
cooling
This is a general expression of balance which is
true for summer and winter conditions.
E  Pt
Seasonal energy requirements





Temperature : average temperature
are valid for calculating total energy consumption and can be
used to predict the quantity of fuel required in a season and
how much it will cost.
can not be used to predict the size of the heating or cooling
plant required;
such a prediction needs consideration of the coldest and hottest
days.
Worked example 4.3
Over a heating season of 33 weeks the average rate of heat
loss from a certain semi-detached house（半独立式住宅） is
2500W for the fabric loss and 1300W for the ventilation loss.
The windows have areas:
6m2 south-facing, 5m2 east-facing, 6m2 north-facing.
The house is occupied by three people and cooking is by gas.
Use the values for seasonal heat gains given in table 3.7 and 3.9
and calculate :
(a) The seasonal heat losses
(b) The seasonal heat gains; and
(c) The seasonal heat requirements.
(a) total rate of heat loss= fabric loss+ ventilation loss
= 2500W+1300W=3800W
heat energy lost= rate of heat loss × time taken
=3800W × (33×7 ×24 ×60 ×60)s
= 75.842GJ(giga joules)千兆焦
so seasonal heat loss = 75.842GJ
=75842MJ(mega joules) 兆焦
(b) Heat gains
solar window gain ( table 3.7)
south (680MJ/m2×6)
4080
east (410MJ/m2×5)
2050
north (250MJ/m2×6)
1500
casual gains ( table 3.9)
body heat ( 1000MJ×3) 3000
cooking (gas)
6500
water heating
2000
electrical
3000
total
22130MJ
So seasonal heat gain=22130MJ
(c) Seasonal heat requirement = heat loss- heat gain
=75842-22130
=53712MJ(mega joules兆焦)
=53.712GJ(giga joules千兆焦)
Efficiency 效率
Efficiency is a measure of the effectiveness of a system
which converts energy from one form to another
efficiency%
useful energy

100
deliveredenergy



Domestic heating efficiency →table 4.12
Delivered energy（供给能量）
Useful energy （有用能）
Worked example 4.4
The seasonal heat requirement of a house is 54GJ, which is to
be supplied by a heating system with an overall house
efficiency of 67%. The solid fuel used has a calorific value of
31MJ/kg. calculate the mass of fuel required for one heating
season.
Efficiency = 67/100, output= 54MJ, input energy=?
Using
efficiency%
useful energy

100
deliveredenergy
67
54

100 input energy
Input energy = 80597MJ
Mass of fuel needed= energy required  80597MJ
calorificvalue
 2600kg
31MJ/kg
4.6 Energy regulations 能源规范
Why do we need energy regulations?
Can help to minimise energy use in buildings
control heat loss from buildings
Minimise the heat load for heating in winter
Minmise the cold load for air conditioning in summer
there are many regulations
4.6.1 Building regulations 建筑规范
How to realize energy efficiency in buildings by regulations?
(1) Heat loss by transmission through the fabric
(2) Heat loss by air leakage around openings and through the
fabric
(3) Control system for space heating and hot water
(4) Heat loss from vessels and pipes used for water
(5) Heat loss from hot water pipes and hot air ducts used for
space heating
(6) energy-efficient lighting sources and switching for the lighting
4.6 Energy regulations 能源规范
4.6.1 Building regulations 建筑规范
4.6.2 Energy rating, SAP 建筑能耗评级
4.6.3 Carbon Index，CI 碳指数
4.6.4 Insulation of the building fabric

4.6.5 Other measurements for energy
conservation







Controlling the insulation of the building fabric

Thermal bridging around openings

Infiltration

Space heating control

Hot water controls and insulation of storage

lighting

Important words in chapter 4
metabolic rate
Stack effect
Non-renewable energy
Renewable energy
Primary energy
Secondary energy
Calorific values
Dry resultant temperature
sol-air temperature


Air change rate
1 SAP Energy Ratings are expressed on a
scale of (
)
A 0 to 1.0
B 0 to 10
C 0 to 100
D 0 to 100%
2 Carbon Index energy ratings are calculated
using the information for a SAP rating and
expressed on a scale of (
)
A 0 to 1.0
B 0 to 10
C 0 to 100
D 0 to 100%
3 ( ) may contribute to energy
efficiency
A Controlling the insulation of the building fabric
B avoid Thermal bridging around openings
C mimise Infiltration
D Space heating control
E Hot water controls and insulation of storage
F energy-efficient lighting source and control
4 Casual heat gains in a building include (
A heat from people
B heat from lighting
C heat from sun
D heat from cooking and water heating
E heat from machinery, refrigerators
F heat from electrical appliances
)

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