Route condensate safely: plastic pipe, a sealed trap, 19 mm internal pipe falling 45 mm per metre, and a short, upsized, insulated external run.
Condensate from a condensing boiler is mildly acidic and leaves as a slow trickle. The pipe that carries it must be the right size, fall steadily and be protected from frost, or it can freeze, block and lock the boiler out.
Why the route is planned
A condensing boiler cools its flue gases below their dew point, so the water vapour they carry condenses. Combustion gases dissolve in that water and make it mildly acidic. Use the specified corrosion-resistant pipework, commonly suitable plastic, rated for the appliance’s discharge temperature and connected to a suitable drain. Because it leaves the boiler slowly and in small quantities, an exposed pipe can freeze solid, and a blocked condensate path makes the boiler lock out.
A condensing boiler gets its extra efficiency from condensing the water vapour in its flue gases. Turning vapour back into liquid releases latent heat, which the boiler passes into the heating water; a non-condensing boiler lets the vapour, and the heat it carries, leave through the flue. The condensate is the by-product of that recovered heat.
The trap
The condensate trap holds a water seal that stops flue gases escaping through the condensate outlet while it drains the condensate. It does not neutralise the acidity or pump the condensate away. Fill it before the boiler is first fired.
The required seal depends on the connection. For condensate joining downstream of a sink or basin trap, the HHIC layout has a separate visible air break and 75 mm trap. A boiler's integral seal of at least 75 mm removes the need for those extra parts. The guide's separate external drain/gully layout instead uses a 38 mm minimum. So 75 mm is not the minimum for every condensate trap: use the detail for the actual route and follow the appliance instructions.
Internal pipework (preferred)
- Take the pipe through heated space to a suitable waste connection wherever possible, with any required trap or air break.
- The preferred termination is an internal soil and vent stack, joined at least 450 mm above the invert at its foot (up to three storeys); a kitchen or bathroom waste is the alternative. Where gravity cannot reach an internal point, use a condensate pump.
- Use at least 19 mm internal diameter.
- Fall at least 45 mm per metre (about 2.5°) by the shortest practical route. Industry diagrams use 52 mm per metre for extra margin.
- A pipe in an unheated loft, basement or garage still needs frost protection. Being indoors does not make it freeze-proof.
- A 19 mm bore is typically 22 mm outside diameter pipe.
External pipework (only where unavoidable)
- Increase to at least 30 mm internal diameter (typically 32 mm outside diameter), making the increase before the pipe leaves the building.
- Keep the external run short, within the usual 3 m maximum, and keep the required fall. Slope it down at least 3° as it passes through the wall, with no horizontal sections outside.
- End it below the gully grid but above the water, cut at 45° with the insulation run to its tip, and fit a leaf guard.
- Use weatherproof, UV-resistant insulation. On the typical 32 mm outside-diameter pipe, make it at least 13 mm thick, sealing its joints and the wall penetration against moisture.
- Upsizing and insulating work together. The larger bore gives ice somewhere to form without blocking the pipe, and the insulation keeps the pipe above freezing for longer. Neither alone is enough.
Good routing and insulation reduce the freezing risk. They cannot guarantee it will drain in a long, severe cold spell. Follow the appliance instructions for the whole route.
When the appliance sets a different requirement
Use the condensate instructions for the exact appliance alongside the general routing guidance. The October 2023 Ideal Logic+ Combi2 manual requires horizontal condensate runs to fall at least 52 mm per metre away from the boiler. For that installation, 52 mm is the stated minimum, not an optional margin above 45 mm. It uses at least 19 mm internal diameter indoors and at least 30 mm internal diameter outside, with the increase before the wall and suitable weatherproof insulation.
Its integral siphonic trap has a 75 mm water seal. Fill that seal before the first operation and again after maintenance, as the boiler instructions require. Do not rely on the next service to refill a trap that was emptied during other work.
Trap figures are also specific to the arrangement. The 2021 Vaillant ecoTEC plus VU/VUW ..6/5-5 manual requires a 200 mm or deeper water seal for a third-party condensate siphon. That requirement is for the accessory that manual describes, not every integral boiler trap. A dry or inadequate seal can let combustion products escape, however freely the drain runs.
After cleaning that Vaillant boiler's own condensate trap, refill its lower section to approximately 10 mm below the upper edge before refitting it. That filling position is separate from the 200 mm water-seal requirement for a third-party siphon. One is a reassembly instruction for a particular part; the other specifies the depth of an accessory's gas seal.
Common mistakes
- Confusing internal diameter with outside diameter when choosing pipe.
- Assuming a pipe inside the building cannot freeze.
- Leaving a long, small-bore external run uninsulated.
- Servicing a boiler without checking where its condensate ends up. Check the termination, advise the responsible person on moving it inside or upgrading it, and label the pipe.