As common heating/cooling unit found in residential applications is the packaged unit. The heat source for this type of unit can be either a gas heat system, electric heating elements, or a reverse cycle refrigeration system. With a refrigeration/gas heat combination unit, the heating section is made up of gas burners, a heat exchanger and gas valve, and an ignition system. In both the heating and cooling modes, the air flow pattern is the same. This type of unit is often referred to as a gas pack. When a refrigeration system is used in conjunction with electric heat, the heating elements are located in the path of air flow to the conditioned space. Heat pumps, in addition to using the reverse refrigeration cycle to accomplish the heating mode, also use resistance heating elements as a supplementary heating source. Packaged air conditioning systems can be either roof-mounted or pad-mounted. The split system is another type of system commonly found in residential applications. The condensing unit, located outside, contains the compressor and condenser, while the indoor portion of the system contains the air handling system and the indoor coil. A common indoor coil is the "A" coil. Gas furnaces are designed as either down flow or up flow types. The up flow gas furnace is popular in standard construction, while the down flow furnace is commonly found in mobile or manufactured homes. Two methods of ignition used in gas furnaces are the standing pilot system and the electronic ignition system. The standing pilot system uses a thermocouple to keep the gas valve ready to allow gas flow to the main burner, and the electronic ignition system ignites the pilot upon a call for heat from the thermostat. Three types of electronic ignition systems are the glow coil, spark, and hot surface ignitor. In electric furnaces, up to five resistance heating elements are used to provide heat. A sequencing device is used to bring the elements on one at a time.