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TECHNICAL DATA - Page 9
Specification for Domestic Wet Central Heating Systems’
should also be consulted.
Safety Valve
A non-adjustable spring-loaded valve, pre-set to operate at
3bar (451bf/in
2
) shall be used. It must comply with BS6759
Part 1 and include a manual testing device. It shall be
positioned in the flow pipe either horizontally or vertically
upwards and close to the boiler. No shut-off valves are to
be placed between the boiler and safety valve. The valve
shall be installed into a discharge pipe which permits the
safe discharge of steam and hot water such that no hazard
to persons or damage to electrical components is caused.
Pressure Gauge
A pressure gauge incorporating a fill pressure indicator,
covering the range 1
-
4 bar (60lbf/in
2
) shall be fitted to the
system. It should be connected to the system, preferably at
the same point as the expansion vessel. Its location should
be visible from the filling point.
Expansion Vessel
A diaphragm-type expansion vessel to BS4814 Part 1 shall
be fitted close to the inlet side of the pump. The connecting
pipework should not be less than 15mm (½ in nominal).
Pipework connecting the expansion vessel should not
incorporate valves of any sort. Methods of supporting the
vessel are supplied by the manufacturer. The nitrogen or
air charge pressure of the expansion vessel shall not be
less than the hydrostatic head, (height of the top of the
system above the expansion vessel). 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
6.5 litres
Small Bore Pipework
1 litre per kW of
system output
Micro Bore Pipework
7 litres
Steel Panel Radiators
8 litres per kW
of system output
Low Water Capacity
Radiators
2 litres per kW
of system output
Hot Water Cylinder
2 litres
If the system is extended, the expansion vessel may have
to be increased unless previous provision has been made
for the 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
82 ºC.
The vessel size can now be determined from the following
table where V
=
System volume in litres
Vessel Charge
Pressure (bar)
0.5
1.0
Initial System
Pressure (bar)
0.5
1.0
Expansion Vessel
Volume (litres)
Vx0.833
Vx0.11
This is a general guide; if it is required to be more accurate
refer to BS7074.
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 pressure of 0.3bar (5 lbf/in
2
) 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 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
illustrated in Fig 7 METHODS 1 & 2.
Mains Connection
There shall be no connection to the mains
water supply or to the water storage tank which supplies
domestic water even through a non-return valve, without
the approval of the local Water Authority.