BS 5864:2004 pdf free.Installation and maintenance of gas-fired ducted air heaters of rated input not exceeding 70 kW net (2nd and 3rd family gases) - Specification. BS 5864 specifies the selection, installation and maintenance (see Note 1) of flued, gas-fired, ducted-air heaters of rated input not exceeding 70 kW, based on net calorific value, burning 2nd or 3rd family gases...