Building Services Engineering 5th Edition Handbook

98 Heating
Table 4.2 Heat output from steel double-panel radiators of 500
and 700 mm height.
Radiator length (m)
Heat output (kW) for 55 K difference
500 mm 700 mm 900 mm
1.720 2.00 2.60 2.90
1.920 2.30 2.90 3.30
2.200 2.60 3.25 3.75
2.400 2.85 3.60 4.25
2.600 3.10 3.90 4.80
[ ] (82 + 71)/2 − 22 1.30
temperature correction factor =
55
= 0.988
radiator heat output at 55 K difference = 4250 W
0.988 = 4302 W
The 2.6 m × 900 mm radiator is needed. Verify that the wall space is available and that there
is not a clash with furniture and furnishings in the room. The water flow rate is:
q =
1.1 × 4.25
4.19 × (82 − 71)
= 0.1014 kg/s
A first estimate of pipe sizes can be made using an average pressure loss rate p/EL for the
whole system (EL is the equivalent length). Either an arbitrary figure of say 300 N/m 3 is chosen,
or a figure is evaluated from the expected pump head. The index circuit length is found from the
total flow and return pipe length from the boiler to the furthest radiator. Pipe fittings increase
frictional resistance and an initial 25% increase in measured pipe length is made in order to find
the equivalent length of the system:
pump head p
N p
= EL m ×
m2 EL
N
m 3
This is often converted into head H m water gauge:
pump head H = p
N m 2 × m3
ρ kg × kg
gN
p
=
9.807 × 1000 m
= p
9807 m
Water flow rate through the pump is the sum of all the radiator water flow rates. Figure 4.19
shows typical pump performance curves. The allowed pressure loss rate can be assessed from