Condensate Pipe Calculator — Boiler Drainage

For 21.5mm pipe the firmly-cited limit is 3 metres horizontal (HHIC / manufacturer guidance). 32mm (≈6 metres) and 40mm (≈10 metres) gravity-run figures are typical trade sizing convention rather than a verbatim BS 6798 limit, so always check the boiler manufacturer’s instructions. These limits assume a minimum fall of about 45mm per metre (2.58 degrees). Longer runs require a larger diameter pipe.

HHIC and BS 6798 quote a minimum fall of about 45mm per metre (2.58 degrees) of horizontal run; some manufacturers (e.g. Worcester Bosch) specify 3 degrees (52mm per metre), so follow the boiler manual. Adequate fall ensures condensate flows freely by gravity without pooling, which could freeze in cold weather or cause the boiler to lock out.

External condensate runs should be avoided where possible. If unavoidable, use minimum 32mm pipe, keep the external run as short as possible (ideally under 3 metres), insulate with minimum 19mm waterproof insulation, and consider trace heating in exposed locations.

Where no suitable drain is available, BS 6798 allows condensate to discharge into a purpose-built soakaway. A typical detail (e.g. McAlpine SOAK1GR, sold as BS 6798:2014 compliant) is a hole about 400mm deep × 300mm diameter, filled with 10mm limestone chippings to neutralise the acidic condensate (pH 3-4), with the pipe cut at 45° and inserted at least 25mm in, located at least 500mm from any building or footing.

Condensate freezing is the most common cause of boiler breakdown in winter. The acidic condensate (pH 3–4) has a slightly lower freezing point than water but still freezes in sustained sub-zero temperatures. Prevention includes insulation, trace heating, minimising external runs, and ensuring adequate fall. See our Condensing Efficiency Calculator to check your boiler is running in condensing mode.