MBD200
7
14/02/04
Heating Design
General principals
The system should be designed to conform with
the latest requirements 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.
The Manufacturer recommends the use of a voltage
free, programable room thermostat, alternatively a
separate timeswitch and room thermostat can be
used.
The boilers include pump overrun devices so it is
essential that there is an open circuit for water to
flow between the LPHW flow and return. In some
system designs this may require the addition of a
bypass, an automatic bypass is reccommended.
They also contain integral frost protection, a 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. An
automatic bypass is incorcporated in the appliance
for the heating circuit.
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 is on the return, careful note of its posi-
tion relative to the feed and vent MUST be made to
avoid pumping over, or sucking in air. The design
temperature difference across the boiler for opti-
mum operating efficiency is 20ºC. If thermostatic
valves or zone valves are used ensure the minimum
flow shown in the
Technical Data on page 21
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 screw
retaining the cover over the push fit screw terminals.
The boiler internal wiring is shown in
Fig.3,
Fig.4
and Fig.5.
Connections are as follows :-
Supply Connection
Earth
N
Mains Neutral
L
Permanent Live
Pump Connections
Earth
N
Pump Neutral
Lp1
Pump No1 Live
Lp2
Pump No2 Live
Controls
1
+12 V DC to External controls.
2
+12V DC from Dehumidifier
3
+12V DC from Heating control
E
Earth, 2 Port Valve
N
Neutral, 2 Port Valve
4
2 Port Valve, Br
5
2 Port Valve, Or
6
2 Port Valve, Gy
The appliances includes pump overrun and the
Pumps have been factory connected to their respec-
tive push-fit connectors.
External controls
MUST BE
voltage free and con-
nected to terminals 1, 2 and 3.
Commissioning
General
The elimination of air from systems is essential to
minimise the occurrence of heat exchanger and
general system noise. It is particularly important for
lightweight boilers.
Metal swarf and flux chemicals make a very aggres-
sive mixture, the destructive nature of which can be
drastically reduced by flushing. Consequently this is
a mandatory step in the commissioning procedure.
