12
0020008877-04
4 Water System
Table 3. Flow Rate
OUTPUT
MINIMUM FLOW RATE
18
773.8 litres/hr (12.9litres/min)
31
1289 litres/hr (21.5litres/min)
This is equal to 20
o
C differential acros the appliance.
Diagram 4.1
12430
4.1 Draining Tap
A draining tap must be provided at the lowest point of the
system, which will allow the entire system and hot water system
to be drained.
Draining taps shall be to the current issue of BS 2879.
4.2 Safety Valve
A safety valve need not be fitted to an open-vented system.
4.3 Pump
The pump should be fitted on the flow pipe from the boiler and
have isolating valves each side.
A variable duty pump should be set to give a temperature
difference of no greater than 20
0
C between the flow and return,
with the thermostat set at “MAX”, which is about 80
0
C, to give
a flow rate as shown in table 3.
See chart for pressure loss of the boiler, diagram 4.1.
High resistance microbore systems may require a higher duty
pump.
4.4 Flow Rate
If it is necessary to alter the flow rate, the system can be fitted
with a lockable balancing valve in the main flow or return pipes
shown as valve “A” in diagram 4.3. The flow rate through the
boiler must not be allowed to fall below that given in Table 3.
IMPORTANT Flow and Return Pipe Sizes.
Heating circuit flow and return pipes should be sized as follows.
Upto 18 kW 22mm. over 18 kW 28mm.
4.5 Bypass
A system bypass will be required fitted at least 1.5 metres away
from the boiler, refer to the current issue of the central heating
system specifications (CHeSS).
4.6 Water Treatment
In the case of an existing installation, it is ESSENTIAL that prior
to installing the new boiler the system is thoroughly flushed. For
optimum performance after installation of a new system, the
boiler and its associated central heating system should also be
flushed. Flushing should be carried out in accordance with
BS7593: 1992 using a cleanser such as Fernox or Salamander
products.
For long-term corrosion protection, after flushing, an inhibitor
suitable for stainless steel exchangers should be used, refer to
the current issue of BS 5449 and BS 7593 on the use of
inhibitors in central heating systems. Examples are Fernox or
Salamander products.
4.7 Open (Vented) Water System
The boiler must be supplied from an unrestricted water supply
taken from a feed and expansion cistern situated at a maximum
height of 27 metres (90ft) above the boiler.
The cold feed must be 15mm minimum size.
The vent must rise continuously and be unrestricted.
It is important that the relative positions of the pump, cold feed
and open vent are as shown in diagram 4.2.
Diagram 4.2
9235
Open (vented) system.
Recommended
relationship between
pump, cold feed and vent.
22mm (MIN.) VENT
FEED AND
EXPANSION CISTERN
15mm (MIN) COLD FEED
15mm (MIN.)
AUTOMATIC BY-PASS
RETURN
FLOW
HEATING
CYLINDER
PUMP
BOILER
150mm MAX.
1000mm
MIN.
450mm
MIN.
HEIGHT
