Mark

Most lower cargo compartments will have a heating system.

On the 767, hot engine bleed air is fed into a long tube running the length of the cargo compartment floor. The tube has holes to disperse the heated air.

A temperature switch controls a valve that either applies or stops the hot air flow.

The airline I work for has two aircraft that will allow cold pack air to go to the forward lower compartment. This is to keep the bay cold(er) while on the ground in hot airport environments.