Potterton Osprey CF 125 Installation & Service Instructions Manual page 10

Installation Requirements - Page 10
To size the expansion vessel it is first necessary to calculate
the volume of water in the system in litres. The following
volumes may be used as a conservative guide to calculating
the system volume.
Boiler Heat Exchanger:
Small Bore Pipework:
Micro Bore Pipework:
Steel Panel Radiators:
Low Water Capacity Radiators:
Hot Water Cylinder
(Secondary Store):
If the system is extended, the expansion vessel volume may
have to be increased unless provision has been made for
extension. Where a vessel of the calculated size is not
available, the next available larger size should be used.
The boiler flow temperature is controlled at approximately
85°C.
The vessel size can now be determined from the information
in Table 4 where V = System volume in litres.
Vessel Charge Pressure (bar)
Initial System Pressure (bar)
Expansion Vessel Volume (litres)
Circulation Pump Selection
The resistance through the heat exchanger
when operating with a water flow rate
producing an 11°K temperature rise at
maximum boiler output are shown in Table
5. If other controls such as 2 position valves
are used in the system, the resistance
through them, quoted in their manufacturers
literature must be taken into account. The
pump may be fitted on either flow or return
and MUST be wired directly to the boiler
terminal block. It must be fitted with two
isolating valves which are positioned as
close to the pump as possible. Closing of
any valve must always leave the open vent
and pressure relief valve unobstructed.
See Technical Data
1 litre per kW of system
output
7 litres
8 litres per kW of system
output
2 litres per kW of system
output
2 litres
0.5 10.
1.0 1.0
V x0.11 V x 0.087
Table 4.
Table 5. Hydraulic Resistance for 11k Temp. difference
Boiler
Output kW
35.0
43.0
52.8
64.5
Cylinder
The hot water cylinder must be an indirect coil type or a
direct cylinder fitted with an immersion calorifier suitable for
operating at a gauge pressure of 0.3 bar (5 lbf/in2) in excess
of safety valve setting. Single feed indirect cylinders are not
suitable for sealed systems.
Method of Make-up
Provision shall be made for replacing water loss from the
system either:-
i) from a make-up vessel or tank mounted in a position
higher than the top point of the system, and connected
through a non-return valve to the system on the return
side of the hot water cylinder or the return side of all
heat emitters.
or
ii) where access to a make-up vessel would be difficult by
using the mains top up method or a remote automatic
pressurisation and make-up unit as shown in Fig. 8.
Mains Connection
There shall be no connection to the mains water supply or to
the water storage tank which supplies domestic hot water
even through a non-return valve, without the approval of the
Local Water Authority.
Filling Point
The system shall be fitted with a filling point always on the
return at low level which incorporates a stop valve to BS
1010 and a double check valve (approved by the National
Water Council) to be fitted in this order from the system
mains, see Fig. 8.
Water Flow Hydraulic Resistance
Rate L/min mbar
45.6 9.7
56.0 18.7
68.8 27.5
84.0 55.9
m
0.1
0.19
0.27
0.56