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TECHNICAL DATA - Page 9
THE SYSTEM
The boiler must be used on INDIRECT hot water systems
only. It is suitable for use on open vented gravity hot
water/pumped central heating systems or fully pumped
systems which may be sealed or open vented
The system should be designed so that the maximum static
head does not exceed 30.5m (l00ft) and a minimum on fully
pumped systems of 150mm (6in). See Fig 4.
Gravity domestic hot water circuits should have a minimum
circulating head of 1.2m (4ft). See Fig 5. Excessive
horizontal runs should be avoided, but if this is not possible
the circulating head should be increased.
If the boiler is to be used on a system with gravity hot water
the bulb of the overheat thermostat should be repositioned
as shown on Fig 2.
To prevent reverse circulation in the gravity circuit when the
pump is running an injector disc is provided to form an
injector tee at the return connection on the boiler. See
Fig 11.
To prevent nuisance operation of the overheat thermostat, it
is important that where electrically operated zone valves are
used the boiler is wired so that it does not cycle when the
zone valves are closed. Also systems fitted with controls that
close both hot water and central heating circuits while the
boiler is still hot must be fitted with a by-pass circuit to
dissipate the residual heat within the boiler.
Further information on by-pass arrangements is provided in
later notes and illustrations.
Drain-off taps should be fitted in the pipework close to the
boiler and in low points in the system.
NOTE
Although the system can be emptied using the drain-off taps
in the pipework around the system, to empty the boiler it is
necessary to remove the drain-off plug situated in the return
header at the lower right-hand side of the casting. See Fig 2.
COMBINED GRAVITY HOT WATER AND PUMPED
CENTRAL HEATING SYSTEMS
Where a cylinder thermostat and zone valve are fitted to
control the temperature of the domestic hot water it is
recommended that a by-pass be installed
in the gravity circuit. A suggested method of doing this is
shown in Fig 5 where the bathroom radiator is connected
into the gravity circuit and is fitted with two lockshield valves.
Mechanically operated thermostatic domestic hot water
temperature control valves which allow the boiler to operate
when the valve is closed MUST NOT BE FITTED.
FULLY PUMPED SYSTEMS
The pump must always be wired directly to the boiler
terminal block as shown in Fig 13.
If a three port diverter valve is used as shown in Fig 4 a by-
pass is not necessary since one circuit is always open.
Where a pair of two port valves are used, a by-pass is
necessary. The total length of the by-pass circuit taken from
the boiler connections should be greater than 4 metres of
22mm pipe. It should be fitted with a lockshield valve and
adjusted to maintain a minimum flow through the boiler of
4.5 litres/min (1 gal/mm), see Figs 4 & 6.
Systems fitted with controls which allow the boiler to operate
when both the hot water and central heating circuits are
closed (i.e. mechanically operated thermostatic control
valves) must be fitted with a by-pass circuit capable of:-
1. Dissipating a minimum of 1 kw (3400 Btu/h).
2. Maintaining a minimum water flow through the boiler of 9
litres/min (2 gal/min).
A suggested method of meeting these requirements by using
a bathroom radiator fitted with two Iockshield valves is
shown in FIGS. 4,6.
Additional system information can be found in the Control
Systems, Pipework and wiring guide.
SEALED SYSTEMS (FULLY PUMPED)
The installation must comply with the requirements of
BS6798 and BS5449 Part 1. The 'British Gas publication
British Gas 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 (45lbf/in2) shall be used. It must comply with BS6759
Part 1 and include a manual
9
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