Building Services Engineering 5th Edition Handbook

68 Heat loss calculations
The ventilation heat Q v required to warm the natural infiltration of outdoor air is:
Q v = 0.33 NV (t ai − t ao ) W
The total heat requirement for each room is:
Q p = Q u + Q v
The values of environmental and air temperature used in the calculations depend upon the
type of heating system employed, and the following temperature ratios are used:
F 1 = t ei − t ao
t c − t ao
F 2 = t ai − t ao
t c − t ao
These two ratios are substituted into the equations for heat requirements Q u and Q v . The total
heat requirement Q p then becomes:
[ ∑ ]
Q p = F 1 (AU) + 0.33F2 NV (t c − t ao ) W
For buildings with average external U values in the range 0.60–3.0 W/m 2 K, including openings,
which covers the majority of habitable structures, the temperature ratios have the following values
(with an accuracy to 5.0%):
F 1 = 1.00 F 2 = 1.10
for panel radiator heating systems:
F 1 = 0.92 F 2 = 1.23
for forced warm-air heating systems. Further values are tabulated in the CIBSE Guide.
To check the comfort conditions produced by the heating system in a room we use:
t ai = F 2 (t c − t ao ) + t ao
where t c is the dry resultant temperature specified for the centre of the room from consideration
of the application and t ao has been specified for the location. The environmental temperature
produced in the room is given by:
t ei = F 1 (t c − t ao ) + t ao