SMOKE DETECTORS
Photocell detectors detect smoke within the system. They actuate the blower motor controls and other de- vices to perform the following:
• Shut off the entire system
• Shut down the supply blower, close outside air, and return air dampers and runs
• Run supply-air blower, open outside-air dampers, close return-air dampers, and stop return- air blower or exhaust fans
• Run supply-air blower, open outside-air dampers, close return-air dampers, and run return-air blower or exhaust fan
Actuation occurs when smoke within the unit exceeds a density that is sufficient to obscure light by a fac- tor of 2 or 4 percent per foot. A key switch is used for testing. Two detectors are used. One is located in the return air section. The other is located in the blower section downstream from the air filters.
FIRESTATS
Firestats are furnished as standard equipment. Manual reset types are mounted in the return air and the sup- ply air stream. They will shut off the unit completely when either firestat detects excessive air temperatures.
On this type of unit, the blowers are turned by one motor with a shaft connecting the two fans. There are three small fan motors and blades mounted to exhaust air from the unit. There are four condenser fans. The evaporator coil is slanted. There are also two condenser coils mounted at angles and two compressors. The path for the return air is through the filters and evaporator coil back to the supply-air ductwork.
RETURN-AIR SYSTEMS
Return-air systems are generally one of two types: the ducted return-air system or the open plenum return- air system (“sandwich space”). (See Figure 5.14.) The ducted return-air system duct is lined with insulation, which greatly reduces noise.
The open-plenum system eliminates the cost of return-air returns or ducts and is extremely flexible. In a building with relocatable interior walls, it is much easier to change the location of a ceiling grille than reroute a ducted return system.
FIGURE 5.14 Return-air system for the rooftop unit. (Lennox)