a module for a bus rooftop air conditioner is self contained in that it has all the necessary components including a compressor , if desired , which when supplied with electrical power , can provide conditioned air to the passenger compartment of a bus . in addition , an electrically powered heater is provided in the air flowstream such that heated air can also be supplied to the passenger compartment when desired . multiple units provide for incremental capacity requirements to be met as well as limp home capabilities .