26
Sigma
7.07 Gas Valve (fi g. 48)
a.
Remove 4 screws
"a"
that secure the gas inlet isolation
cock to the bottom of the gas valve.
b.
Remove the full sequence PCB
"b"
and housing from
the gas valve (1 screw) and place to one side.
c.
Remove balance tube and earth lead from the gas
valve.
d.
From inside the combustion chamber remove the 3
screws
"c"
which hold the gas valve in place.
e.
Remove gas valve and replace in reverse order.
7.08 Control PCB (fi g. 49)
a.
Turn boiler thermostat down to the off position and
pull the thermostat knob off
"a"
.
b.
Remove fixing screw securing the control PCB
"b"
housing in place and hinge down the housing.
c.
Remove the electrical connections from the PCB,
noting their position.
d.
Remove the 2 plastic nuts from outside the housing
that hold the fixing pins in place and gently push the
pin inside the housing.
e.
Very carefully, push the reset button
"c"
inside the
housing and remove the PCB.
f.
Remove the fixing pins from the PCB and swap over
to the replacement.
g.
Replace in reverse order.
7.09 Heat Exchanger (fi g. 50)
a.
Lift fan and flue hood assembly
"a"
out of boiler, remove
all connection from the fan, noting positions.
b.
Remove air pressure switch
"b"
.
c.
Drop control PCB compartment out of the way
"c"
.
d.
Disconnect flow and return unions
"d"
from heat
exchanger.
e.
Remove heat exchanger connection nipples and lift
out heat exchanger.
f.
Replace in reverse order.
C
B
A
Fig. 49
B
A
Fig. 50
B
A
C
Fig. 51
B
A
D
C
Fig. 48
Summary of Contents for SIGMA
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