Cambridge Engineering, Inc.
32
M-Series Technical Manual
switch in the closed position, and wait
30 seconds to read the manometer. If the
reading is greater than 0
inch
WC, refer
to the Maintenance Instruction Section for
information on Gas Valve Cleaning for
cleaning the second gas valve and retest.
If the reading is 0
inch
WC, remove the
manometer and reinstall the pipe plug.
c. To check the gas tightness of the first
valve in the gas train on heaters with
separate redundant valves, connect the
manometer to the leak test port between
the valves and verify that the manom-
eter is properly zeroed. Wait 30 seconds
to read the manometer. If the reading
is greater than 0
inch
WC, refer to the
Maintenance Instruction Section for infor-
mation on Gas Valve Cleaning for the
first gas valve and retest. If the reading is
0
inch
WC, remove the manometer and
reinstall the pipe plug.
o
9. GAS TRAIN LEAK CHECK
a. Turn the blower and burner service
switches to the
“
LOCAL
”
position.
b. With the burner operating, spray the com-
plete gas train with leak detector solution,
checking all pipe connections and plugs.
c. Turn the blower and burner service
switches to the
“
OFF
”
position.
m
WARNING:
Do not use flame to leak check piping.
m
WARNING:
Any gas leak detected must be repaired before the
make-up air heater is placed into service.
10. MINIMUM FIRE SETTING
IMPORTANT
All access doors must be closed during this procedure.
a. Reinstall the #3 wire previously removed
from the amplifier.
b. Connect a meter set to read DC micro-
amps (
m
A) in order to verify the minimum
fire flame signal during the adjustment
process. Connect the meter by removing
the red wire from terminal FP of the flame
safe
g
uard relay (FSR) and attaching the
positive lead of the meter to terminal FP
and the negative lead to the red wire.
c. Turn the blower and burner switches to
the “LOCAL” position.
d. After the burner has ignited remove #8
wire from the amplifier to drive the modu-
lating valve to minimum fire.
e. Look through the burner porthole in the
front of the heater to verify that the burner
flame is a very small ribbon (approxi-
mately 1-1/2" to 2
"
long
)
that is evenly
spread across the burner (without gaps).
f. Verify that the minimum fire flame signal
is steady and above 2.0 microamps (
m
A).
IMPORTANT
If any of the conditions in steps 10.e and 10. f are not
met, an adjustment is required.
g. Minimum Fire Adjustment
i. To adjust the minimum fire, remove
cap (A) exposing the mini-
mum fire adjusting screw.
On the MR212 modulating/
regulating valve, remove the housing
cover, then cap (A), and then loosen
lock screw (C). Turn the adjusting
screw (B) to obtain the desired mini-
mum fire setting. Repeat steps 10.e
and 10.f.
