The latest boiler that is getting popular is compact boiler which is known for its compact design. For direct vent application, it is designed as a sealed unit. In this type of boiler combustion air is taken from the outside directly. Moreover the products of combustion are exhausted through the same vent pipe. The boiler consist of advanced control system, an electronic ignition, coupled with a redundant gas valve. The air flow is monitored by the air pressure switch. The boiler shuts down if insufficient air for proper boiler operation is detected. There is a flame roll out switch which protects the boiler against flame roll out. Overheating protection of the boiler is provided by a high limit control.