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VAV Cooling With Electric Heat Interlock — (Function 8.)
In cases where VAV cooling terminals are used in conjunc-
tion with a separate heating system, such as perimeter heat-
ing, it is necessary to prevent the heating equipment from
turning on before the cooling system turns off. The addition
of a differential pressure switch to the unit controls makes
this possible. When the switch detects that control pressures
are approaching a shutoff condition (cooling load satisfied),
it closes a set of contacts. This allows the heating system to
operate as the heating thermostat dictates. See Fig. 51.
SYSTEM-POWERED CONTROLS WITH PNEUMATIC IN-
TERFACE — The Moduline unit is designed as a stand-
alone, reactive air terminal in which the only source of en-
ergy for control purposes is the distribution air itself. There
are applications where separate system energy affords ad-
ditional control functions not possible with system power.
This section covers the use of 20 psi pneumatic energy and
standard and custom pneumatic devices for the application
of Moduline terminals in a conditioning system.
Pneumatic Sequenced Cooling/Heating (Hot Water) — (Func-
tion 10.) A37HS VAVcooling system can be sequenced with
a hot water heating system through the use of a pilot valve
and a single proportional pneumatic thermostat (one-pipe or
2-pipe). See Fig. 52. By properly matching the operating pres-
sure ranges of the pilot valve and the field-supplied hot wa-
ter heat valve, the system can be configured to automatically
change over from cooling to heating and back again based
on the thermostat branch line pressure. The cooling and heat-
ing functions can be separated by a deadband or they can be
overlapped, depending upon pressure ranges selected. The
system may be designed to use either of the following
combinations:
• pilot valve (NO), hot water valve (NO) and thermostat (DA,
one- or 2-pipe)
• pilot valve (NC), hot water valve (NC), and thermostat (RA,
one- or 2-pipe)
VAV Cooling With Pneumatic Warm-Up — (Functions 11
and 12.) Pneumatic warm-up is accomplished by using a re-
motely operated pneumatic switch in place of the system-
powered warm-up switch. The pneumatic switch is installed
in-line between the volume controller and the thermostat (unit
mounted or wall mounted) and must be closed during
warm-up. It may be either normally open or normally closed,
but must match the pneumatic line pressure available. See
Fig. 53.
VAVCooling With Fire Safety — (Functions 11 and 12.) Code
requirements may specify that air distribution terminals be
open or closed during a fire. For example, in tower construc-
tion there may be a requirement that if a fire begins on one
floor the terminals on the ‘‘fire floor’’ must be shut off to
prevent the addition of oxygen to the fire; terminals on the
floors above and below the fire floor must be wide open to
ventilate the space. These actions must occur regardless of
the space temperature and the position of the individual duct-
powered thermostats (unit mounted or wall mounted). This
application is effectively the same as the pneumatic warm-up
previously described. In this case, the pneumatic switch is
remotely operated by the fire master control to open the ad-
jacent floor units. Air supply to units on the ‘‘fire floor’’ is
interrupted by the use of duct fire-dampers.
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70
90
Fig. 52 — Pilot Valve for Pneumatic Sequenced
Cooling/Heating (Hot Water)
Fig. 53 — VAV Control Arrangement with
Pneumatic Warm-Up Switch
Fig. 51 — VAV Control Arrangement with
Electric Heat Interlock
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