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.

Rectangular Building dimensions : 6.0 metres long x 3.0 metres wide x 2.5 metres high.
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 200C.

For a Workshop environment a temperature as low as 16oC 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 m3.


Q = U . A . dt


1.Calculate the heat loss through the windows:
Total window area = 4 (1.8 x 0.6 ) = 4.32 m2.
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 m2
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 m2
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 m3 air volume.

In the previous example this is:

18,793.3 / 656.25 = 28.6 W/m3 .

Some typical approximate values of heat loss per m3 for buildings are shown below:

Room

Heat Loss

W/m3

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


Science - Thermal Transmission - Page 1 2 3 4 5 6 7