M-Series Technical Manual
69
Cambridge Engineering, Inc.
modulating valve also serves as the pressure regulator.
The maximum gas supply pressure rating is normally
determined by the exposed pressure rating of the regulat-
ing device. The RV61 and RV81 are rated at 1 PSIG;
and the MR212 is rated at 5 PSIG. (For make-up air
heaters rated at 400,000 Btu/hr or below, see the combi-
nation valve above.)
MOTOR STARTER, OVERLOAD,
& AUXILIARY CONTACT
The motor starter assembly consists of a
motor starter, overload relay and auxiliary
contact. The overload relay protects the
motor from excessive current or single phas-
ing. If the overload relay trips, it must be
reset manually. The auxiliary contacts
are used in the gas valve safety circuit as
an indication the blower is operating, and
as an optional exhaust fan contact for interlocking other
equipment with the operation of the heater.
MULTI-FUNCTIONAL PC BOARD
The multi-functional PC board provides five
(5) separate functions for heaters with the
Maxitrol Series 14 or Series 44 controls. The
functions are as follows:
1) Blower Relay (CR1) which is ener-
gized on a “call for blower” from the
Remote Control Station.
2) The LTC (Low Temperature Cutout) circuit func-
tions to turn off the blower in approximately 3 1/2
minutes if any of the following occurs: (a) The
inlet temperature drops below the selectable LTC
setpoint (40˚F, 45˚F, 50˚F, or 55˚F) in the ventila-
tion mode; (b) the gas valve fails to remain ener-
gized during a heating cycle or (c) the high airflow
switch contacts indicate the burner profile plate
pressure drop is too high.
3) The EAT (Entering Air Thermostat) circuit func-
tions automatically to turn off the burner when the
outdoor temperature reaches the selectable EAT
setpoint (45˚F, 50˚F, 55˚F, 57.5˚F, 60˚F, 62˚F,
64˚F, 66˚F, 68˚F or 70˚F).
4) The PT (Purge Timer) circuit function is preset
at the factory to provide four air changes within
the heater cabinet prior to an ignition attempt
(normally set at 4 or 8 seconds). If inlet ducting
is attached to the heater, the delay time can be
increased to 8, 16, or 32 seconds, as applicable.
5) The patented LFS (Low Fire Start) circuit func-
tion limits the initial heater firing for the first 15 sec-
onds of a heating cycle to obtain proper ignition.
The voltage to the modulating valve is adjusted
between 9 and 13 Volts DC by an adjustable
potentiometer on the multi-functional PC board.
SERVICE SWITCH
The service switches are mounted in the
electrical control enclosure. In the
“LOCAL”
or “OFF” position, the service techni-
cian has local control of the heater. These
switches must be placed in the “REMOTE”
position for normal control from the Remote Control
Station.
SPACE TEMPERATURE SELECTOR
The space temperature selector is part of the
Maxitrol Series 44 control system. The space
temperature selector senses the space temperature.
A 3˚F drift above the set temperature will cause
the heater to modulate to the MIN setting on the
amplifier and a 3˚F drift below the set temperature will cause
the heater to modulate to the MAX setting.
TRANSFORMER
Heaters are furnished with a dual
voltage transformer unless the pri-
mary voltage is 115 Volts. The
transformer furnished depends on
the primary voltage (208, 230, 460
or 575 Volts). This transformer pro-
vides secondary control voltages of
24 and 115 Volts. Heaters supplied
for 115 Volts are furnished with a 24 Volt secondary
transformer only. Secondary fusing is provided in all
Class I transformer circuits to protect the downstream
components from short circuit. Fuse sizing is as follows:
FNM-6.25 for 150 va 24 Volt and FNM-1.6 for 150 va
115 Volt. Consult the heater wiring diagram to identify
the proper fusing for the heater in question. Do not
increase the fuse rating over that which is specified.
