Mechanical Exhaust; Use Of Hvac System - Honeywell NOTIFIER SCS Series Operation And Installation Manual

Concepts of Smoke Control

Mechanical Exhaust

Zoned smoke control using mechanical exhaust is very similar to the smoke shaft method and similarly is done in conjunction with pressur-
ization of nonsmoke zones. The driving force of smoke movement for the smoke shaft method is buoyancy. However, when the flow of
smoke through the smoke shaft is aided by mechanical fans, the method is known as mechanical exhaust. Mechanical exhaust of the smoke
zone can be accomplished through a dedicated exhaust system or by using components of the HVAC system. As with the smoke shaft
method, the location of exhaust inlets should be located near the ceiling and as far from exit passageways as possible.

Use of HVAC System

In many instances, the HVAC system, shown in Figure 2.21a, can be used to perform zoned smoke control. The steps necessary to achieve
zoned smoke control are listed below:
• Close the damper in the supply duct for the smoke zone.
• Close the dampers in the return duct for all nonsmoke zones.
• If the system has an air recirculation duct, close the recirculation damper.
• Turn on the supply fan and the return fan.
The result of following these steps during a fire condition is illustrated in Figure 2.21b.
Return
Duct
A major concern when using an HVAC system for smoke control is the location of the exhaust outlets and the air intakes. To minimize the
probability of smoke feedback into the supply intake, the exhaust outlet must be located as far from the supply intake as possible.
Another factor to take into consideration when using an HVAC system for smoke control is the air recirculation vent. Occasionally, HVAC
building systems are designed with the capability to recirculate air through the use of dampers and by connecting the supply and return ducts
as shown in Figure 2.22a. During normal operation, the damper in the recirculation duct is partially or completely open to allow air from the
building to mix with outside air. For zoned smoke control operation the damper in the air recirculation vent must be tightly closed, as shown
in Figure 2.22b, to prevent smoke feedback into the supply air.
Exhaust
Air
Outside
Air
28
Outside
Air
Supply
Duct
Figure 2.21a
Figure 2.21 Zoned Smoke Control Using the HVAC System
Normal Operation and Smoke Control Operation
Return
Ducts
Mixing Damper
Supply
Figure 2.22a Normal Mode
Figure 2.22 HVAC System with Recirculation Capability
in Normal Mode and Smoke Control Mode
Fire
Floor
Return
Duct
From
To
Outside
Ducts
Air
Figure 2.22b Smoke Control Mode
Smoke Control System Types
Outside
Air
Supply
Duct
Figure 2.21b
From
Return
Ducts
Mixing Damper
Closed
To
Supply
Ducts
SCS Series Manual — P/N 15712:L 7/18/16