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System-Powered Controls with Pneumatic
Interface
PNEUMATIC SEQUENCED HEATING/COOLING (HOT
WATER) — By using a 20 psi pneumatic source, one ther-
mostat can control both heating and Moduline cooling in the
conditioned space. The pneumatic circuit is interfaced with
the system-powered circuit by use of a pilot valve. In cool-
ing, the Moduline airflow is controlled by the duct-powered
volume controller just as in all system-powered control. As
the schematic diagram shows, a DA pneumatic wall ther-
mostat working through the pilot valve, replacing the system-
powered thermostat, modulates the pilot valve and thus the
volume controller low side pressure.
Figure 75 shows the piping diagram. Figure 76 shows the
sequence diagram for a direct acting thermostat with a nor-
mally open pilot valve. With the system shut down and the
space temperature low, at start-up the thermostat branch pres-
sure is low, and the normally open water valve (or other heat-
ing means) is open.As the space temperature rises, the branch
pressure rises, modulating the water valve to a closed po-
sition. The pilot valve is normally open, with set point above
water valve spring rate, thus maintaining a low pressure in
the low side of the volume controller, closing the bleed valve
in the controller and raising the bellows pressure to the duct
pressure level; this results in unit shutoff.
If the space temperature continues to rise, the NO pilot
valve will modulate toward closed, raising the low side pres-
sure, opening the bleed, reducing the bellows pressure, and
opening the Moduline unit air valve. The airflow will rise
until the volume controller setting is reached.
Note that the spring rates of the hot water valve and the
pilot valve must be properly matched to accomplish the ac-
tion described above.
Aheating/cooling sequence can also be accomplished with
a reverse acting circuit (Fig. 77 and 78). At start-up, the tem-
perature is low, the branch pressure is high, and normally
closed water valve is wide open. As the space temperature
rises, the branch pressure falls and the water valve closes,
stopping the heating supply. The pilot valve is NC and thus
at branch pressures above its setting will open, volume con-
troller low pressure will be reduced, the bellows pressure
will rise and the unit will shut off.
LEGEND FOR FIG. 75 - 78
DA — Direct Acting NC — Normally Closed
HW — Hot Water NO — Normally Open
M—Main RA — Reverse Acting
Fig. 75 — Piping Diagram, Heating/Cooling
Sequence, DA Thermostat
Fig. 76 — Sequence Diagram — DA Thermostat
Fig. 77 — Piping Diagram, Heating/Cooling
Sequence, RA Thermostat
Fig. 78 — Sequence Diagram, RA Thermostat
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