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FLUE RESISTANCES
Flue Systems
For concentric flue systems with elbows fitted, use the table to correct the maximum flue extension capability.
Alternatively use the table to design the flue system, deducting the individual resistance of components from
the maximum pressure drop allowed in the flue for that boiler. The maximum pressure drop allowed in the flue is
given below.
PERMISSIBLE FLUE LENGTH
The maximum permissible flue lengths for each model are shown in Table 1 below, these lengths are inclusive of
the terminal resistance. The value shown is the maximum available length for extension. The equivalent length of
elbows is shown in Table 2.
TABLE 1
TABLE 2
MAX PERMISSIBLE EQUIVALENT FLUE LENGTH
(INC TERMINAL RESISTANCE)
METRES
CONCENTRIC
OPEN FLUE
Flue Size 80/125
100/150
80
100
Model
30/30P
33
-
65
-
40/40P
30
-
70
-
60/60P
17.5
28
25
-
80/80P
10
18
15.3
-
100
-
10
-
20
120
-
9
-
49
150
-
6
-
32
EQUIVALENT LENGTH OF ELBOWS
(METRES)
CONCENTRIC
OPEN FLUE
Size
80/125
100/150
80
100
45˚
0.85
1.25
0.45
0.60
90˚
1.6
1.9
1.0
1.0
EXAMPLES OF FLUE LENGTH CALCULATION
MODEL
FLUE TYPE
MAX
PERMISSIBLE
EQUIVALENT
LENGTH
(TABLE 1)
ELBOWS
MAX
PERMISSIBLE
STRAIGHT
LENGTH
TYPE
EQUIVALENT
LENGTH
(TABLE 2)
NO
TOTAL
EQUIVALENT
LENGTH
60
80/125
17.5
90
1.6
2
3.2
14.3
60
100/150
28
90
1.9
2
3.8
24.2
80
80/125
10
90
1.6
3
4.8
5.2
120
100/150
9
90
1.9
4
7.6
1.4
FOR OWN BUILT OPEN FLUE USE THIS:
EXAMPLES OF CALCULATING FLUE RESISTANCES/LENGTHS
EXAMPLE 1 (CONCENTRIC)
MAXIMUM ALLOWABLE PRESSURE DIFF & FLUE LENGTH
FLUES INCLUDING TERMINAL
MODEL
FLUE SIZE
PRESSURE DIFF (Pa)
30/30P
80/125
180
40/40P
80/125
227
60/60P
80/125
150
80/80P
80/125
312
100
100/150
220
120
100/150
473
150
100/150
332
Therefore this installation is acceptable as only a 2m run.
HORIZONTAL FLUE FOR EVOMAX 2 40
Resistance (m)
Flue size
80/125
Max permissible flue run
30
2 x 90˚ elbow
2 x 1.6 = 3.2
Total flue length available
30 - 3.2 = 26.8
b
7
5
3
5-
0
1
2
6
0
2
