Potterton Combi 80 Installation & Service Instructions Manual page 11

Installation Requirements - Page 11
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Frost Protection
The appliance is fitted with an internal frost thermostat,
however this device is purely for the protection of the
appliance. If any other part of the central heating system
requires frost protection, an external frost thermostat
should be fitted.
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Mains Water Feed: Central Heating
A connection must be incorporated into the central
heating system to facilitate filling. There must be no
direct connection to the mains water supply, even
through a non-return valve, without the approval of the
Local Water Authority.
Permissible methods of filling
(1) Direct Method - Fig. 6
A detachable flexible hose is connected to a stop valve
fitted to an outlet on the service main. The other end of
the hose is connected to a second stop valve and a
double check valve. The double check valve is fitted to
an inlet connection on the central heating return pipe
under the appliance. The hose should be disconnected
after filling. Where the mains pressure is excessive a
pressure reducing valve shall be used to make filling
easier. The following fittings shall form a permanent part
of the system and shall be fitted in the order stated.
a. A stop valve complying with the requirements of BS
1010 Part 2, (the temporary hose from the draw off
tap shall be connected to this fitting).
b. Double check valve of an accepted type.
(2) Booster Pump Method
The system may be filled through a self contained unit
comprising a cistern, pressure booster pump and if
necessary, an automatic pressure-reducing valve or flow
restrictor.
The pressure booster pump must be capable of
pressurising the system to a minimum of 1.0 bar (14
p.s.i.) measured at the appliance.
The cistern should be supplied through a temporary
connection from a service pipe or cold water distributing
pipe. The unit may remain permanently connected to the
heating system to provide limited water make-up.
Provisions for make up water
Provision should be made for replacing water loss from
the system by re-pressurisation of the system.
Reference should be made to British Gas Publications
"Material and Installation Specifications for Domestic
Central Heating and Hot Water".
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Pump Performance Curve
The circulating pump fitted within the appliance should
be capable of satisfying most system requirements. For
central heating systems greater than 15 kW it may be
necessary to use a higher "design temperature drop
across system" to meet the system pump head
requirement. The design temperature drop should not
exceed 20 °C (36 °F). Fig. 7 indicates the amount of
pump head available for the system. The boiler
resistance is already taken into account in this curve.
How to Use Fig. 7.
Starting with the required SYSTEM HEATING LOAD,
draw a vertical line downwards so that it intersects the
chosen SYSTEM DESIGN TEMPERATURE DROP line,
normally this would be 11 °C (20 °F) although up to 17
°C (30 °F) can be used without increasing radiator sizes.
From this intersection point draw a horizontal line so that
it intersects the PUMP CURVE. From this intersection
draw a vertical line upwards.
The AVAILABLE PUMP HEAD can now be read from
the horizontal scale, and the corresponding FLOW
RATE from the vertical scale in the centre of the chart.
The system must be designed so that its hydraulic
resistance does not exceed the available head unless
an additional circulating pump is fitted.
If the designed flow rate through the boiler is less than
11.5 l/min, the boiler could be noisy and ultimately
overheat. See note on system by-pass in Section 1.8.
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Central Heating Range Rate
For most installations the boiler will automatically adjust
the central heating output to match your heating system
requirement.
Systems which have very low water content and a low
heating requirement may benefit by reducing the
maximum boiler output. This is achieved by rotation of
the adjustment knob, located on the electronic
modulation control board. (Fig. 19, Page 30).
The boiler central heating output can be adjusted
between the maximum and minimum values stated on
the data badge. Operation of this adjuster will not affect
the maximum boiler output when domestic hot water is
being drawn off.