What is the maximum horizontal run for a condensate pipe?

For 21.5mm pipe: maximum 3 metres horizontal. For 32mm pipe: maximum 6 metres. For 40mm pipe: maximum 10 metres. These limits assume a minimum fall of 2.5 degrees (approximately 44mm per metre). Longer runs require a larger diameter pipe.

What is the minimum fall for a condensate pipe?

The minimum fall is 2.5 degrees, which equates to approximately 44mm per metre of horizontal run. This ensures condensate flows freely by gravity without pooling, which could freeze in cold weather or cause the boiler to lock out.

Can a condensate pipe run externally?

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.

What is a condensate soakaway?

Where no suitable drain is available, BS 6798 allows condensate to discharge into a purpose-built soakaway. This should be minimum 300 × 300 × 300mm, filled with limestone chippings (to neutralise the acidic condensate, pH 3-4), and located at least 500mm from any building.

Why does my condensate pipe freeze?

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.

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Condensate Pipe Calculator — Boiler Drainage

Calculate maximum condensate pipe runs, minimum fall, external insulation requirements, and termination options per BS 6798 and Building Regulations.

Horizontal Run Length

Total horizontal pipe run

Pipe DiameterCondensate pipe outer diameter

Boiler Output

Boiler rated output

Termination TypeWhere condensate pipe discharges

External Run

Does the pipe run outside the building?

How We Calculate This

This calculator helps plan condensate pipe installations for condensing boilers in accordance with BS 6798 and manufacturer guidance. It checks run lengths, fall requirements, and external installation compliance.

Key rules

Maximum run: 3m (21.5mm), 6m (32mm), 10m (40mm)
Minimum fall: 2.5° (~44mm/m)
External runs: Minimum 32mm, insulated, keep short
Condensate rate: ~0.15 L/kWh at full fire (BS 6798 / BRE DER) — e.g. 4.5 L/hr for a 30 kW boiler
Termination: Soil stack, trapped gulley, or limestone soakaway