Operating Sequences - Carrier Moduline 37HS Series Installation, Start-Up And Service Instructions Manual

OPERATING SEQUENCES — System powered control for
the Moduline® terminal is shown schematically in Fig. 27.
System Powered Cooling— A high-pressure signal and a low-
pressure signal, measured across the control unit distribu-
tion baffle, are transmitted through the unit control block to
the control air filter and to the volume controller. The dif-
ference between the high and low pressure is an indication
of the unit airflow. The control air filter, besides preventing
dirt from entering the control system, also acts as a manifold
to connect the volume controller to the unit.
The high and low pressure signals act on diaphragms in
the volume controller to position a seat relative to a bleed
nozzle. The relationship of the seat to the bleed nozzle de-
termines the air pressure transmitted back through the con-
trol filter to the unit bellows, which inflates to a position
necessary to provide the airflow as preset on the volume con-
troller. A change in input pressures caused by changes in duct
pressures will cause the seat to be repositioned, relative to
the bleed nozzle, and change the output pressure to the bel-
lows to maintain the unit airflow. The airflow is preset using
the maximum airflow adjusting lever.
The thermostat, wall- or diffuser-mounted, is connected
to the volume controller to provide a means of varying the
airflow delivery as room temperature varies from the ther-
mostat set point. As room temperature cools to the cooling
thermostat set point, another bleed orifice in the thermostat
opens and bleeds control air from the volume controller low
side. This causes the volume controller seat to move toward
the bleed nozzle, increasing the bellows output pressure, thus
closing the unit.
prevent the unit from closing completely. The minimum air-
flow adjustment restricts the volume controller seat from clos-
ing the bleed nozzle, thus preventing the unit from fully closing.
A hood airflow measurement is required to set minimum
airflow.
System Powered Cooling With Warm-Up— The cooling ther-
mostat (diffuser- or wall-mounted) is a direct acting thermo-
stat. With the space temperature at a nighttime condition be-
low the set point, the thermostat port is open, reducing the
pressure on the upper diaphragm (A) and causing the ele-
ment assembly (B) to rise. This closes the bleed port (C),
which diverts high pressure to the bellows, reducing the unit
flow. In warm-up, hot air flows over the immersion warm-up
switch installed in unit plenum. The switch closes, raising
the low-side pressure on the volume controller diaphragm
and moving the element away from the bleed nozzle. This
reduces the bellows pressure and permits warm duct air to
flow to the space.
System Powered Heating and Cooling— The 37HS can pro-
vide heating as well as cooling with the addition of a change-
over switch installed in the unit plenum and a wall heating/
cooling thermostat. A separate amplifier is not required.
connected to the cooling side of the thermostat, and the bel-
lows is bled at the volume controller according to the pilot
pressure from the thermostat. Hot air in the plenum shifts
the connection to the heating side of the thermostat.
Fig. 27 — 37HS Control Schematic
17
A minimum airflow can be set on the volume controller to
With cold air below 65 F in the duct, the changeover is