Cambridge Air Solutions.
94
M-Series Technical Manual
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
energized on a “call for blower” from
the Remote Control Station.
2) The LTC (Low Temperature Cutout) circuit
functions 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
ventilation mode; (b) the gas valve fails to remain
energized 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
functions 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 function
limits the initial heater firing for the first 15 seconds
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 (SW1, SW2, SW3)
The service switches are mounted in the
electrical control enclosure. In the
“LOCAL”
or “OFF” position, the service technician
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 (STS)
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 (T1, T2)
Heaters are furnished with a dual
voltage transformer unless the
primary voltage is 115 Volts. The
transformer furnished depends on
the primary voltage (208, 230, 460
or 575 Volts). This transformer
provides 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.
