Where is a boiler condensate pipe allowed to discharge?

Condensate is the water produced when a condensing boiler recovers heat from its flue gases, and it leaves at a rate of a few litres a day. Where it goes matters for three separate reasons: it must not freeze, it must not create a slip hazard or damage, and it must not reach a watercourse untreated. The regulations and the manufacturers' instructions set out a clear order of preference, and the common bad installations are all shortcuts around it.

Safety

Step by step

1. Prefer an internal soil stack.The best termination. The pipe runs entirely in warm space to an internal soil and vent stack, connecting with an air gap or an appropriate fitting. It never freezes, it is out of the weather, and the discharge goes to the foul drain where it belongs.
2. Accept an internal waste connection.Into the waste pipe of a sink, basin, bath or washing machine, connecting on the house side of that fitting's trap so the trap still seals against drain smells. Also entirely internal and also frost-proof. The connection must not be into a fitting that is rarely used, or the trap dries out.
3. Use an external gully as the next best.The pipe leaves the building and discharges into an existing foul gully, entering below the grating but above the water line. Below the grating stops splashing and freezing; above the water line stops the pipe end sitting in standing water. This is a very common and acceptable arrangement, provided the external run is properly sized and insulated.
4. Use a purpose-made soakaway as a last resort.Where there is no gully within reach, a small purpose-made condensate soakaway — a perforated pipe or a proprietary unit in a gravel-filled pit, sited away from foundations — is the recognised solution. Not a hole in the border with a stone in it.
5. Rule out the wrong terminations.Not into a rainwater downpipe that discharges to a surface water drain or a watercourse, because condensate is acidic and the surface water system runs untreated to rivers. Not onto a path, drive or step, where it makes ice. Not into a downpipe that then runs across a wall in the cold.
6. Check the septic tank case.Where the property is on a septic tank or treatment plant, check the manufacturer's guidance. Small volumes of condensate are usually tolerable but some treatment plants are sensitive to it, and it is worth confirming rather than assuming.
7. Get the sizing and fall right on any external part.At least 32mm outside diameter, the shortest practical run, continuous fall, no dips, properly clipped, weatherproof insulation, and a plain cut end rather than an elbow. Every one of those is a freezing prevention measure.
8. Check what you have.Follow your condensate pipe from the boiler and see where it ends. A long external run, a small-bore pipe, an unlagged one, or a termination onto a path are all worth correcting — and are all things to raise at the next service or with the original installer.

Common mistakes

• Connecting into a rainwater downpipe — Surface water drains commonly discharge untreated to a watercourse. Acidic condensate does not belong there, and it is a drainage misconnection as well as a freezing risk.
• Terminating over a path or a step — It freezes into a sheet of ice exactly where people walk, and the acidity attacks the surface over time.
• Connecting downstream of a trap — Removes the trap's protection and lets drain smells travel back up the condensate pipe into the room the boiler is in.
• Using indoor foam lagging outside — It degrades within a season in UV and weather, so the pipe that looks insulated is effectively bare by its second winter.

Questions people ask