Air Systems - Cargo Heat 
Introduction
The Forward, Aft, and Bulk Cargo Compartments are provided heat from the bleed air system and is, in part, controlled from the cockpit.
Two sources of heat are used to heat the cargo compartments:
Warm cabin air flows down through the side walls to the lower compartment before being recirculated or dumped overboard.
In addition, the cargo heat system supplements this cabin heat by discharging hot bleed air under the cargo compartment floor. Bleed air can be supplied by the engines, APU or a ground pneumatic source.
When a Cargo Heat Switch is ON, the shutoff valve is commanded open and the heat control valve opens at about 7°C and allows bleed air to flow under the associated cargo compartment floor. When the compartment is sufficiently heated the heat control valve closes, stopping the bleed air flow.
Normal Operation
Separate switches control a shutoff valve and a heat control valve in each compartment heat supply line. The system automatically maintains compartment temperature above approximately 7°C.
Bulk Cargo Heat Selector
A BULK CARGO HEAT selector allows a higher temperature to be selected for the bulk compartment:
NORM SCHEMATIC
With the selector in the NORM position, the bulk compartment operates the same as the forward and aft compartments, with the heat valve allowing bleed air to flow into the bulk cargo compartment to keep the compartment temperature above 7°C.VENT SCHEMATIC
With the selector in the VENT position, the bulk compartment is maintained (via the Heat Control Valve) above approximately 18°C, and a vent fan draws cabin discharge air into the compartment to provide air circulation. If an overheat occurs, the Ventilation fan runs with the selector in either the NORM or VENT position.
Non-Normal Operation
The cargo heat switch OVHT light illuminates and the EICAS advisory message FWD, AFT or BULK CARGO OVHT displays when the cargo compartment temperature is above the standard control range. The compartment shutoff valve closes automatically if a compartment temperature exceeds approximately 32°C (90°F). When the compartment temperature decreases below the overheat temperature setting, the OVHT light extinguishes, the EICAS message is removed, and the compartment shutoff valve re-opens. If heating is called for, the heat control valve opens.
If an overheat condition is reached, there is a possibility the system will cycle on and off at approximately 32°C (90°F). The decision to allow cargo heat to cycle may be based on the cargo in the compartment. The higher heat setting will not damage the airplane structure. Cargo compartments may reach temperatures above the overheat setting when the airplane is on the ground during hot weather operations.
