ASHRAE 19 or in older systems. A vacuum pump that cannot remove enough air to satisfy the vacuum switch setting will not cycle as it should, but will simply continue to run. In this situation, or where higher levels of vacuum or unusual air removal problems exist, vacuum units using separate pumps to remove air and condensate are available. Exercise great care before replacing a vacuum pump with a simple condensate transfer pump. Although it may work, you could have problems in returning condensate through piping that was sized to operate with the greater vacuum-pressure differential. 50 59–2009 YE A R S JOURNAL f i g . 6 Typical low pressure steam boiler with float operated low water fuel cut-off and electric water feeder. Boiler Level Controls As solid fuels were replaced by oil or gas burners, it became apparent that a more positive means to protect the boiler from overheating due to lack of water could be designed. By law, every steam boiler must have at least one low water fuel cut-off. www.info.hotims.com/25208-41 66 ASHRAE Journal September 2009