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System-Powered Controls with Electric
Interface
VAV COOLING WITH ELECTRIC WARM-UP — Another
approach to warm-up is the addition of an electric warm-up
valve; the operating principle is the same as system powered
warm-up.A building with its conditioning system shut down
during unoccupied hours must be brought close to operating
temperature before the occupants arrive. Heated air supplied
to the space from a central unit can quickly restore operating
conditions. But because the VAV thermostat has closed the
unit, the result of the lack of load, the system powered con-
trol needs to be overcome.
An electric warm-up valve located on the Moduline unit
between the volume controller and the thermostat is wired to
the primary air source machine room. The valve can be
activated by a simple manual or timer switch or it can
be connected to the warm-up terminal of the central station
unit, if such is provided. Thus, when heated air be-
gins to flow in the distribution duct, the bellows is bled down
by the action of the electric warm-up valve and heated air
can flow into the conditioned space (Fig. 68).
In the cooling mode (Fig. 69), air flows freely between
the volume controller and the thermostat and control func-
tion is not affected. When warm-up is activated, the nor-
mally open port closes and the control circuit is closed after
the volume controller. As with system powered warm-up,
this causes the pressure to rise in the low pressure chamber
of the volume controller and opens the valve that bleeds air
from the bellows pressure chamber. The increased bleed rate
lowers the bellows pressure, opening the unit damper, and
allowing the flow of primary air to increase up to the con-
stant volume set point of the volume controller.
Fig. 67 — Variable Volume Controls — Minimum Flow: Thermostat Partially Open,
Controller Partially Open, Unit Delivering Minimum Flow
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