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10.2.5 Pressure Relief Valve
Any pressure relief valve fitted to the boiler or system must be able to discharge externally to a
drain where the discharge can be seen but cannot cause any injury or damage. No other valves
should be positioned between the relief valve and discharge termination. The pressure relief
valve should be manually activated periodically in order to assess valve opening, normally as
part of servicing.
10.2.6 Low Pressure Switch
Where there is a catastrophic loss of water from the system the boiler thermostats may fail to
operate which would result in serious damage to the appliance. To prevent this it is
recommended that a low pressure cut out switch set at 0.3 bar is fitted to
the system and wired
in series with the boiler limit thermostat. Combination Boiler models are factory-fitted with a low
system pressure shutdown as standard.
10.3 Combination Boiler Domestic Hot Water
10.3.1 Mains Water Pressure
To protect the appliance and to prevent excessive flow rates, a pressure reducing valve is
factory-fitted to limit the maximum supply pressure to 3 bar.
A mini expansion vessel is factory-fitted after the pressure reducing valve in order to protect the
appliance from the expansion due to heating of the water in the domestic hot water pipework.
Depending on DHW system volume, additional expansion may be required.
We strongly advise that all installation pipework is run in copper. However, if plastic pipe or
plastic coated pipe is used, it must be recommended by the pipe manufacturer for use with oil
fired appliances and, in any case, the last 1000mm of pipework connected directly to the
appliance must be of copper.
Note that the flow rate from individual taps is dependent on the number of outlets being
operated together, as well as the length and size of pipework and the mains supply pressure.
10.3.2 Water Hardness
Although many of the DHW components are designed to resist lime scale formation, in areas of
hard water it may still be necessary to fit an inline chemical water softener. For further
information contact Warmflow and your local water company.
10.3.3 Boreholes
In order to minimise the pressure variations it is recommended that the differential setting on the
borehole pump pressure control is kept as low as possible without adversely affecting the pump
motor. The accumulator must be as large as possible in order to reduce the rate of pressure
change and a pressure reducing valve (PRV) must be situated between the pump and
accumulator and the boiler. The PRV must be set slightly below the minimum setting of the
pressure switch on the pump.