18
OPERATING SEQUENCES
System powered control for the Moduline
®
terminal is shown
schematically in Fig. 29.
System Powered Cooling
A high-pressure signal and a low-pressure signal, measured
across the control unit distribution baffle, are transmitted
through the unit control block to the control air filter and to
the volume controller. The difference 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 control-
ler to the unit.
The high-pressure and low-pressure signals act on dia-
phragms in the volume controller to position a seat relative to
a bleed nozzle. The relationship of the seat to the bleed nozzle
determines the air pressure transmitted back through the con-
trol filter to the unit bellows, which inflates to a position nec-
essary to provide the airflow as preset on the volume control-
ler. 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 bellows to
maintain the unit airflow. The airflow is pre-set using the
maximum airflow adjusting lever.
The thermostat, wall-mounted or diffuser-mounted, is con-
nected 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.
A minimum airflow can be set on the volume controller to
prevent the unit from closing completely. The minimum air-
flow adjustment restricts the volume controller seat from
closing the bleed nozzle, thus preventing the unit from fully
closing. A hood airflow measurement is required to set mini-
mum airflow.
System Powered Cooling With Warm-Up
The cooling thermostat (diffuser-mounted or wall-mounted)
is a direct acting thermostat. With the space temperature at a
nighttime condition below the set point, the thermostat port is
open, reducing the pressure on the upper diaphragm (A) and
causing the element assembly (B) to rise. This closes the
bleed port (C), which diverts high pressure to the bellows, re-
ducing the unit flow. In warm-up, hot air flows over the im-
mersion 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 noz-
zle. This reduces the bellows pressure and permits warm duct
air to flow to the space.
System Powered Heating and Cooling
The 37HS can provide heating as well as cooling with the ad-
dition of a changeover switch installed in the unit plenum and
a wall heating/cooling thermostat. A separate amplifier is not
required.
With cold air below 65°F in the duct, the changeover is con-
nected to the cooling side of the thermostat, and the bellows
is bled at the volume controller according to the pilot pressure
from the thermostat. Hot air in the plenum shifts the connec-
tion to the heating side of the thermostat.
Fig. 29 — 37HS Control Schematic
