M-Series Technical Manual
31
Cambridge Engineering, Inc.
setting. If the ambient temperature exceeds its
highest setting, remove the set point jumper
and rotate 90˚ for the bypass mode.
d. Remove the 1/8" NPT plug from the manual
shut-off valve located just prior to the burner
and connect a manometer for the purpose of mea-
suring the manifold pressure.
e. Refer to the heater nameplate for the Manifold
Differential Pressure (MDP) (
inch
WC) for
which the heater was specified.
Nameplate Manifold Differential Pressure
f. Turn the disconnect switch on.
g. Turn the blower service switch to the
“
LOCAL
”
position. The blower motor will
start. Monitor the pressure reading on the manom-
eter. Record the Manifold Static Pressure (Blower
Only).
h. Using the formula below, calculate the required
manifold pressure:
Nameplate Manifold Differential Pressure
___________
inch
WC
+ Manifold Static Pressure (Blower Only)
___________
inch
WC
= Calculated Manifold Pressure
___________
inch
WC
IMPORTANT
The high limit may trip on warm or mild temperature
days. A jumper may be necessary to complete the mani-
fold pressure adjustment.
i. Turn the burner service switch to the
“
LOCAL
”
position. After a delay for pre-
purge and igniter warm-up, the burner will
light.
j. Observe the manometer reading and compare
to the Calculated Manifold Pressure above. If
the manifold pressure reading does not equal this
value, adjust the manifold pressure regulator
until the proper manifold pressure is obtained.
IMPORTANT
The pressure drop across the burner profile damper
that was adjusted in Step 5 (page 30) must not fall
below 0.51 inches WC for natural gas heaters without
filters or 0.55 inches WC for natural gas heaters with
a filter section or filters in the rainhood. The pressure
drop must not fall below 0.75 inches WC for LP gas
heaters without filters or 0.79 inches WC for LP gas
heaters with a filter section or filters in the rainhood. If
the pressure drop is below these values, it is an indica-
tion of low airflow or burner overfiring. To correct this
condition, either adjust the manifold pressure regulator
to reduce the manifold pressure or increase the blower
speed, provided the motor full load amps are not ex-
ceeded.
k. Turn the blower and burner service
switches to the
“
OFF
”
position, remove
the manometer from the manual shut-off
valve, re-install the 1/8" NPT plug and
remove the jumper (if used).
o
8. GAS VALVE LEAK CHECK
All heaters should be evaluated for the gas tight-
ness of the gas valve seat. Heaters with separate
redundant valves are equipped with a leak test facil-
ity to assist in checking this seal. A momentary switch
and a gas port for measuring pressure between
valves are provided as the leak test hardware.
Refer to the Individual Component Description
Section for more information regarding the leak
test switch. The procedures for the gas valve leak
check are as follows:
a.
Connect a 0 to 10 inches water column
(
inch
WC) manometer to the 1/8" NPT
tapped fitting on the manual shut-off
valve located just prior to the burner for
the purpose of monitoring an increase in
pressure. Verify that the manometer is
properly zeroed.
b.1. On single redundant valve applications
(heaters rated less than 400,000 Btu/hr),
close the manual burner shut-off valve
and wait 30 seconds to read the manom-
eter. If the reading is greater than 0
inch
WC, replace the gas valve and retest. If the
reading is 0 inch WC, remove the manom-
eter and reinstall the pipe plug.
b.2. On heaters with separate redundant
valves, close the manual burner shut-
off valve, hold the momentary leak test
