Carrier 37HS Ð VAV Control Arrangement with, Electric Heat Interlock, Cooling/Heating Hot Water

VAV Cooling With Electric Heat Interlock Ð (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 satis®ed), it closes a set of contacts. This allows the heating system to operate as the heating thermostat dictates. See Fig. 51.

Fig. 51 Ð VAV Control Arrangement with

Electric Heat Interlock

SYSTEM-POWERED CONTROLS WITH PNEUMATIC IN- TERFACE Ð 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) Ð (Func- tion 10.) A 37HS VAV cooling 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 ®eld-supplied hot wa- ter heat valve, the system can be con®gured 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 Ð (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.

VAV Cooling With Fire Safety Ð (Functions 11 and 12.) Code requirements may specify that air distribution terminals be open or closed during a ®re. For example, in tower construc- tion there may be a requirement that if a ®re begins on one ¯oor the terminals on the ``®re ¯oor'' must be shut off to prevent the addition of oxygen to the ®re; terminals on the ¯oors above and below the ®re ¯oor 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 ®re master control to open the ad- jacent ¯oor units. Air supply to units on the ``®re ¯oor'' is interrupted by the use of duct ®re-dampers.

50 70 90

Fig. 52 Ð Pilot Valve for Pneumatic Sequenced

Cooling/Heating (Hot Water)

Fig. 53 Ð VAV Control Arrangement with

Pneumatic Warm-Up Switch

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