Condensing Boiler
8
22/10/02
minimum of 19mm pipe. External runs MUST be
made in a minimum of 32mm pipe, starting inside
the building, and if possible insulated to defer
problems from freezing.
Whichever method is used it should be checked
during commissioning to prove there is a leakfree
working connection from the boiler to the drain. The
simplest way to do this is to carefully pour some
water into the boiler flue and check it emerges at the
drain.
The British Gas booklet “Guidance for the installa-
tion of Domestic Condensing Boilers” should also be
referred to.
System Design
General principals
The system should be designed to conform with
the latest reuirements of the Building Regula-
tions.
Condensing Boilers give their best performance
when the system water temperature is as low as
possible and still achieve the desired room tempera-
tures. Oversized radiators make this easier to
achieve but are not essential as, even in a standard
designed installation, most of the boilers’ working
hours are spent with the system producing much
less than the designed output. This is because the
outside temperature is often higher than the figure
used for the design, typically -1 to -2º C.
A design using 3-port flow-share valve, room ther-
mostat and cylinder thermostat, is a perfectly
suitable minimum. The boilers include a pump
overrun device so it is essential that there is an
open circuit for water to flow between the flow and
return. In some system designs this may require the
addition of a bypass, an automatic bypass is pre-
ferred.
They also contain integral frost protection, the boiler
will fire if the temperature within it approaches
freezing. It will do this even when turned to Off, and
can only be inhibited by removing the Permanent
Live supply to the boiler.
Thermostatic radiator valves can be used but it is
essential to fit a bypass to maintain minimum flow
conditions. An automatic bypass is preferred.
The boiler waterside is completely copper so all
good corrosion inhibitors are suitable.
Designs
The following designs are all acceptable:
1. Sealed system to BS.6798 and BS.5449:1
2. Combined Feed and Vent systems.
3. Separate Cold Feed and Vent systems.
The pump may be on the flow or return, but careful
note of its position relative to the feed and vent
MUST be made to avoid pumping over, or sucking
in air. It should always be installed with isolating
valves as close as possible on both its flow and
return. The design temperature difference across
the boiler for optimum operating efficiency is 20ºC. If
thermostatic valves or zone valves are used ensure
the minimum flow shown in the Technical Data on
page 1 is possible.
Electrical Connections
WARNING: The appliance MUST be earthed.
All wiring for the boiler and system controls MUST
conform to I.E.E. Wiring Regulations, and work
should be tested using a suitable meter, for Earth
Continuity, Polarity, Short Circuit and Resistance to
Earth.
The supply must be through a common isolator, a
double pole 3A fused isolating switch with a contact
separation of 3mm minimum on both poles. The
cable used should be no less than 0.75mm
2
to
BS.6500 PVC, 3 core, and fixed ensuring the earth
connection is longer than the Live and Neutral.
Access to the boiler connections is made by with-
drawing the Electrical Tray then releasing the single
screw retaining the cover over the push fit screw
terminals. The boiler internal wiring is shown in
Fig.3 and Fig.4.
Connections are as follows :-
4 Way Terminal
Supply Connection
Earth
N
Mains Neutral
Ls
Switched Live
L
Permanent Live
3 Way Terminal
Pump Connection
Earth
N
Pump Neutral
L
Pump Live
The boiler includes a pump overrun and an external
pump MUST be powered from the Pump push-fit
connector.
For systems using an external programmer, the
electrical wiring should follow the relevant control
manufacturer’s recommendations, with the switched
live from the controls returning to Ls in the 4 way
push-fit connector.