© Baxi Heating UK Ltd 2008
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3.10 Water Circulating Systems
The boiler is suitable for use with fully pumped open
vented and sealed systems with an indirect coil type
high efficiency cylinder.
The following conditions should be observed on all
systems:
• The static head must not exceed 30.5 m (100ft)
(3 bar) of water or be less than 150 mm (6 in).
• The flow pipe from the boiler must always be higher
than the return pipe.
• To ensure correct operation, the pump must be wired
to the boiler terminal block.
3.10.1 Treatment of Water Circulating Systems
All recirculatory water systems will be subject to
corrosion unless they are flushed and an appropriate
water treatment is applied.
To prevent this, follow the guidelines given in BS 7593
“Treatment of water in domestic hot water central
heating systems” and the treatment manufacturers
instructions.
Failure to flush and add inhibitor to the system may
invalidate the boiler warranty.
Treatment must involve the use of a proprietary
cleanser, such as BetzDearborn Sentinel X300 or
X400, or Fernox Superfloc and an inhibitor such as
BetzDearborn Sentinel X100, or Fernox MB-1 or
Copal.
Full instructions are supplied with the products, for
further information contact BetzDearborn (+44 151 420
9563) or Fernox (+44 1799 550 811) directly.
It is important to check the inhibitor concentration after
installation, system modification and at every service
in accordance with the inhibitor manufacturer’s
instructions. (Test kits are available from inhibitor
stockists.)
3.10.2
System Design
Pipe Sizing
The flow and return connections on the boiler are 22
mm. The sizes of the system flow and return pipes
from the boiler should be determined by normal
methods, according to the system design
requirements. See Fig. 30. and Table 2.
Air Vents
It is recommended that automatic air vents are fitted
within the system pipework to prevent air locks during
filling, commissioning and normal operation.
To ensure correct circulation, it is essential to vent all air
from the system prior to commissioning the boiler.
System Drain Off Points
Drain off points should be fitted in the pipework close to
the boiler and in the low points of the system.
Plastic Pipework
If plastic pipe is used for the central heating circuit there
must be a run of at least 2 metres of uninsulated copper
pipe from the boiler flow and return connections
including any branches.
3.10.3
Bypass Requirements
The system must be designed to always provide a route
for water to circulate between the boiler flow and return.
Best Practice is to use an automatic bypass although it
may be possible to use an existing fixed bypass such as
a landing or bathroom radiator fitted with two lockshield
valves or a bypass fitted with a balancing valve. See
Fig. 30.
Systems with Two Port Valves
Where a pair of two port zone valves are used, the total
length of the by-pass circuit taken from the boiler
connections should be greater than 4 metres of 22 mm
pipe. The bypass should be capable of maintaining a
minimum flow through the boiler of 4.5 litres/min (1
gal/min).
Systems with Thermostatic Radiator Valves (TRV’s)
Where mechanically operated thermostatic control
valves are used, the total length of the by-pass circuit
taken from the boiler connections should be greater
than 2 metres of 22 mm pipe. The bypass should be
capable of maintaining a minimum flow through the
boiler of 9.0 litres/min (2 gal/min).
3.10.4
System Controls
For optimum operating efficiency, the heating system
into which the boiler is installed should include a control
system. Best Practice suggests such a system should
comprise: a programmer, room and cylinder
thermostats, control valves giving boiler interlock.
Frost Protection - System
The system pipework may require insulation and the
protection of an additional frost thermostat.
If the system is drained isolate the external gas and
electrical supplies.
