Cambridge Engineering, Inc.
68
M-Series Technical Manual
HIGH TEMPERATURE LIMIT
The high temperature limit opens when
the discharge air temperature exceeds
150˚F. This limit must be manually reset.
INCLINED MANOMETER
An inclined manometer with a range of
-0.1 to 1.0 inch WC is provided on each
heater with manual profile damper
adjustment to indicate the pressure drop
across the burner profile plate. The
pressure drop should be set without the
burner operating (blower only) at 0.68
inch WC for natural gas applications
and 0.90 inch WC for LP applications. If the heater is sup-
plied with a filter section or filters in the rainhood, increase
the manometer reading to 0.72 inch WC for natural gas and
to 0.94 inch WC for LP gas heaters.
DIFFERENTIAL PRESSURE SWITCH/GAUGE
A differential pressure switch/gauge is
available as part of the Automatic
Profile Adjust system to control the
pressure drop across the burner profile
plate. The differential pressure switch
drives a Profile Damper Adjust Motor
to maintain approximately 0.68 inch
WC for natural gas applications, or approximately 0.90
inch WC for LP applications.
IGNITER
The hot surface igniter is the ignition
source for lighting the gas in the burn-
er. It is made of silicon carbide which
is very fragile. Care should be used in
handling. It operates on 24 Volts and
the current ranges from 1.3 to 1.7 amps. It will reach
temperatures in excess of 2400˚F during the ignition
trial. It is furnished with a sleeve for shock mounting and
sealing the igniter in the mounting tube.
INTERIOR LIGHT(S)
Available for the heater cabinet, electrical
control enclosure,
and/or gas train enclo-
sure. Light switches are located inside the
heater’s electrical enclosure. Furnished with safety
guards. 100 watts maximum, each. A separate electrical
supply source is required.
LEAK TEST FACILITY
The leak test facility is provided on
all heaters over 400,000 Btu/hr and
consists of a momentary switch for
the first safety shut-off valve in the
gas train and a gauge port between
the first and second safety shut-off
valve. By holding the gas valve
momentary switch closed, the first
gas valve is energized which allows
gas pressure to build on the seat of the second gas valve.
The gauge port between valves is used to determine if
the first gas valve seat is properly sealed.
MODULATING VALVE
The modulating valve responds
to a 4 to 24 Volt DC signal
from the amplifier to modulate
the flow of gas to the burner. On
the Maxitrol M511 or M611
valve, the minimum fire adjusting
screw is located on the far side
of the valve under the dust cover. On the Maxitrol
MR212 valve, the minimum fire adjusting screw is located
under the large dust cover. The MR212 modulating valve also
serves as the manifold pressure regulator to control the
burner manifold pressure. The MR212 is rated for 5
PSIG.
COMBINATION VALVE
The combination valve serves as a mani-
fold pressure regulator and redundant gas
shut-off valve. This control is typically
used for gas capacities below 400,000 Btu/hr.
The valve operates on 24 Volts AC power.
The combination valve is rated for a maxi-
mum gas supply pressure of 14 inches WC.
MANIFOLD PRESSURE REGULATOR
The manifold pressure regulator con-
trols the burner manifold pressure.
When the MR212 valve is used, this
