Science - Thermal Transmission - Page 1 2 3 4 5 6 7
Total Heat Loss Calculations
Example 8
Calculate the total heat
loss from the building shown below i.e. the fabric and ventilation losses.
The window size is 2.0 m long x 1.0 m high.
The air change rate due to natural ventilation is 2 air changes per hour.
It is normal to ignore
the door without glazing and add it into the wall area in most calculations
although for very accurate methods the door could be calculated separately.
Q = ‘U’
. A . dt
First calculate the heat loss through the window ( as in
example 6)
Q window = 2.8 x 2.0 x 1.0 x ( 20 - - 2 )
Q window = 2.8 x 2.0 x 22
Q window = 123.20 Watts
Second calculate the heat loss through the blockwork.
Q front wall = 0.317 x ( 15.0 - 2.0) x ( 20 - - 2 )
Q front wall = 0.317 x 13 x 22
Q front wall = 90.66 Watts
Q rear wall = 0.317 x 15 x 22
Q rear wall = 104.61 Watts
Q side walls = 0.317 x 2 ( 3.0 x 2.5 ) x 22
Q side walls = 104.61 Watts
Q walls total = 90.66 Watts + 104.61 Watts + 104.61
Watts = 299.88 Watts
Third calculate the heat loss through the floor.
Q floor = 0.7 x 6.0 x 3.0 x 22
Q floor = 277.20 Watts
Fourth calculate the heat loss through the roof
Q roof = 0.8 x 6.0 x 3.0 x 22
Q roof = 316.80 Watts
Fifth calculate the heat loss by ventilation ( as in
example 7)
Q = N . V . Sp.ht. . dt
Q = 2.0
x 6.0 x 3.0 x 2.5 x 0.34 ( 20 - - 2)
Q = 2.0
x 45 x 22
Q = 673.2
Watts
Finally calculate the total heat loss
Q total = heat loss
window + heat loss blockwork + heat loss floor + heat loss roof + ventilation
heat loss
Q total = 123.20
+ 299.88 + 277.20 + 316.80 + 673.2
Q total = 1690.28 Watts
It can be seen from the
above calculations that the ventilation heat loss accounts for:
( 673.2 / 1690.28) x 100
= 40% of the total heat loss for the building.
It is therefore important
to establish an accurate figure for the air
change rate and to minimise unnecessary
infiltration and exfiltration in buildings.
Example
9
Calculate the total heat
loss from the building shown below.
Workshop
DATA:
Building dimensions : 15.0 metres long x 7.0 metres wide x 5.0 metres high to
eaves.
Roof ridge height is 7.5 metres.
The four identical window sizes are 1.8 m long x 0.6 m high.
The air change rate due to mechanical and natural ventilation is 3 air changes per hour.
Note: The
design internal temperature for the building in this example is 20^{0}C.
For a
Workshop environment a temperature as low as 16^{o}C
is often satisfactory.
The building
volume may be calculated from:
V = ( L x W x H _{to eaves} ) + ( 0.5 x W x perpendicular height from
eaves to ridge x L )
V = ( 15 x 7 x 5 ) + ( 0.5 x 7 x 2.5 x 15 )
V = 525 + 131.25 = 656.25 m^{3}.
Q = ‘U’
. A . dt
1.Calculate the heat loss through the windows:
Total window area = 4 (1.8 x 0.6 ) = 4.32 m^{2}.
Q windows = 2.8 x 4.32 x ( 20 - - 2 )
Q window = 266.1 Watts
2. Calculate the heat
loss through the blockwork.
Total blockwork area = ((15.0 x 5.0 ) - 4.32 ) + (15.0 x 5.0 ) + ( 2 ( 7.0 x 5.0 ))
+ ( 2 ( 3.5 x 2.5 ))
Total blockwork area = ( 75 - 4.32
) + ( 75 ) + ( 70 ) + ( 17.5 )
Total blockwork area = ( 75 - 4.32
) + ( 75 ) + ( 70 ) + ( 17.5 )
Total blockwork area = 233.18 m^{2
}
Q blockwork = 0.317 x 233.18 x ( 20 - - 2 )
Q blockwork = 1626.2 Watts
3. Calculate the heat loss through the floor.
Q floor = 0.45 x 15.0 x 7.0 x 22
Q floor = 1039.5 Watts
4. Calculate the heat loss through the pitched roof.
Roof area = 2 x building length x rafter length
Rafter length h = ( 2.5 ^{2 }+
3.5 ^{2 })^{ 1/ 2 }= 4.30 metres
Therefore, Roof area = 2 x 15.0 x 4.30 = 129.0 m^{2
}
Q roof = ‘U’ . A . dt
Q roof = 0.4 x 129.0 x 22
Q roof = 1135.2 Watts
5. Calculate the heat
loss by ventilation.
Q = N . V . Sp.ht. . dt
Q = 3.0 x 656.25 x 0.34 ( 20 - - 2)
Q = 14,726.3 Watts
Finally calculate the
total heat loss
Q total = heat loss
window + heat loss blockwork + heat loss floor + heat loss roof + ventilation
heat loss
Q total = 266.1
+ 1626.2 + 1039.5 + 1135.2 + 14,726.3
Q total = 18,793.3 Watts
Heat Loss Calculation
sheets help to tabularise the data.
It can be seen from the
above calculations that the ventilation heat loss accounts for:
( 14,726.3 / 18,793.3) x 100 = 78% of the total
heat loss for the building.
If the mechanical
ventilation is not continuous then this air
change rate of 3
AC/h may be reduced, thus saving on heating equipment sizes.
Heat
Loss Comparison
A useful comparison for
heat losses is to calculate the heat loss per m^{3} air volume.
In the previous example
this is:
18,793.3 / 656.25 = 28.6 W/m^{3} .
Some typical approximate values
of heat loss per m^{3} for buildings are shown below:
Room |
Heat Loss W/m^{3} |
Modern
passive house |
1.0 to 2.5 |
Modern
low energy building |
3 to 10 |
Modern house |
8 to 20 |
10 year old House |
20 to 30 |
20 year old House |
30 to 40 |
House with no cavity wall insulation and double
glazing |
40 to 50 |
House with no cavity wall insulation and single
glazing |
50 to 60 |