Pressure classes

Pressure classes are set out in Table B2 and expressed as either 'N', 'P' or 'H' followed by either '1' or '2'. N relates in general to natural draught chimneys i.e. operating under negative pressure where the value 1 or 2 allows for a different class of product; metal chimneys to BS EN 1856: 1 have the class N1. In the UK the value N2 should be assigned as a minimum to masonry chimneys. P and H relate to chimneys which operate under positive pressure e.g. for fan assisted applications and diesel generators respectively. The pressure designation depends on the gas-tightness it achieves, the lower number being the more onerous, the higher allowed leakage for positive pressure application being intended to external installations.