Technical data

14
GEMINI HSC
10.3 secondary
circulation
It is recommended that long dead legs are avoided and that a secondary circulation system is installed
as shown in figures 3 - 6.
If the installation is unvented then the following information regarding system water capacity is of the
utmost importance. If the specific installation has a water capacity greater than can be accommodated by
the (standard) expansion vessel supplied with the water heater, then additional expansion vessels must be
installed. Contact MHS Boilers Ltd for advice and guidance on selection of additional expansion vessel/s.
Model
60i
100i
150i
200i
300i
500i
Standard Expansion
Vessel Size
12 litre
12 litre
12 litre
18 litre
25 litre
40 litre
Max allowable system
content with standard
expansion vessel
200 litre
200 litre
200 litre
250 litre
415 litre
665 litre
Max allowable pipework
length with standard
expansion vessel
227m of 28mm Tube
162m of 28mm Tube
81m of 28mm Tube
81m of 28mm Tube
186m of 28mm Tube
267m of 28mm Tube
Worked Example
The example below is G 3/4 Temperature and Pressure Relief valve with a discharge pipe (D2) having
4 no. elbows and length of 7m from the Tundish to the point of discharge.
From Table 1
• Maximum resistance allowed for a straight length of 28mm copper discharge pipe (D2) from G3/4 T&P
valve is 9m.
• Subtract the resistance for 4 no. 28mm elbows at 1.0m each = 4.0m.
• Therefore the maximum permitted length equates to 5.0m.
• As 5.0m is less than the actual length of 7m therefore calculate the next largest size.
• Maximum resistance allowed for a straight length of 35mm pipe (D2) from G 3/4 T&P valve equates
to: 18m.
• Subtract the resistance for 4 no. 35mm elbow at 1.4m each = 5.6m
• Therefore the maximum permitted length equates to: 12.4m
• As the actual length is 7m, a 35mm (D2) copper pipe will be satisfactory.
Table 1
Sizing of copper discharge pipe "D2"
for common valve outlet sizes
Valve
Outlet
Size
G 3/4
Minimum Size
of Discharge
Pipe D1*
22mm
Minimum Size of
Discharge Pipe D2*
From Tundish
28mm
35mm
42mm
Maximum Resistance
Allowed, Expressed
as a Length of Pipe
(ie. No Elbow or Bends)
Up to 9m
Up to 18m
Up to 27m
Resistance Created
by Each Elbow
or Bend
1.0m
1.4m
1.7m