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VAVCOOLING WITH ELECTRIC HEAT INTERLOCK —
Aconditioned space may contain a Moduline system for cool-
ing and a separate heating system such as baseboard electric
or hot water or an overhead fan coil. The electric interlock
switch enables the cooling and heating systems to operate as
the load dictates with overlap.
The electric interlock switch compares the bellows pres-
sure with the low pressure, as a practical means of detecting
the operating level of the unit. When the unit is delivering,
the bellows pressure is significantly lower than the low pres-
sure and the electric interlock (differential pressure) switch
is open, interrupting the power supply to the heating system.
When the unit is shut off, the pressures are approximately
equal and the switch closes, allowing the heating system to
be energized. It is field installed with the low pressure port
piped to the auxiliary bellows pressure stub of the filter. (See
Fig. 74.) The high pressure port is connected by a ‘‘T’’ into
the line between the volume regulator and the thermostat.
Fig. 73 — Heating/Cooling Unit — Heating, Hot Air in Duct, Electric Changeover in Heating,
Thermostat Closed, Controller Bleeding, Unit Heating
Fig. 74 — Connection for Electric Interlock
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