A: At a minimum there are two crucial gas pressure switches in a valve train, one for low pressure and one for high pressure. The low gas pressure switch ensures the minimum gas pressure necessary to operate is present. The high gas pressure switch ensures an excessive pressure is not present. Both switches must be proven safe to permit operation.
Additional to these gas switches will be an air pressure switch. This device is used to ensure airflow is present to support burner operation. Some systems have supplementary pressure switches, such as a valve proving pressure switch. Switches such as these are typically used to enhance safety or provide other safety aspects specific to that application’s needs.