Publication No. 5105703
9
Installation Requirements
1.9
The System
If plastic pipe is used for the central heating circuit there
must be a run of at least 1 m of un-insulated copper pipe
from the boiler flow and return connections.
The boiler must be used on Indirect hot water systems only.
It is suitable for use on open vented gravity domestic 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 30.5 m (100 ft) and a minimum of 150
mm (6 in).
Gravity domestic hot water circuits should have a minimum
circulating head of 1.2 m (4 ft).
To ensure optimum boiler performance on both gravity hot
water and fully pumped systems a change-over switch is
incorporated in the boiler control box. The boiler is supplied
with the switch set for sealed or open vented fully pumped
systems.
If the boiler is to be used on a system with gravity hot water
the switch must be set to the GRAVITY position. In
addition, the bulb of the overheat thermostat should be
repositioned as shown in Fig. 21.
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.
20.
On all systems the pump live connection should be wired
to the boiler terminal block, it will then be controlled by the
pump overrun. This will ensure that the pump will continue
to run after boiler shut down if the water temperature is high,
thus preventing nuisance operation of the overheat
thermostat.
It is important that where electrically operated zone valves
are used the boiler is wired so 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 from within the boiler.
Drain off taps should be fitted in the pipework close to the
boiler and in the low points of the system.
Note: Although the system can be emptied using the drain
off taps installed in the pipework around the system, to
empty the boiler it is necessary to remove the drain off cap
positioned within the boiler case.
Combined Gravity Hot Water, Pumped Central Heating
Systems
Where a cylinder thermostat and zone valve is used to
control the temperature of the 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. 8 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 or partially closed
MUST NOT BE FITTED.
Fully Pumped Systems
With the change-over switch in the control box set at the
FULLY PUMPED position, the pump will be controlled
by an overrun thermostat. This will ensure that the
pump will continue to run after boiler shut down if the
water temperature is high, thus preventing nuisance
operation of the overheat thermostat.
If a three port valve is used as shown in Fig. 5 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 22 mm pipe. It should be fitted with a
lockshield valve and be adjusted to maintain a minimum
flow through the boiler of 4.5 litres/min (1 gal/min).
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 of:-
1. Dissipating a minimum of 1 kW (3400 Btu/h).
2. Maintaining a minimum water flow rate 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 lockshield
valves is shown in Fig. 5. Additional system information
can be found in the Control Systems, pipework and
Wiring Guide.