Condensate pipe

The condensate pipe is a pipe through which a condensing boiler discharges waste water from the condensing process into the sewer. As metal pipework cannot be used, it is often easily identifiable as the only plastic pipe connected to the boiler.

Before it is discharged into the sewer via the condensate pipe, waste water – or condensate as it’s known – is collected in the condensate trap. Condensate pipes are generally considered to be a component of the boiler’s flue. Any work on them should only be carried out by a Gas Safe engineer.

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Condensate pipe overview

Water vapour from the combustion process is channelled through the boiler until so much heat is drawn out of it, it condenses back into water. (In a non-condensing boiler, this process is absent. The hot gases are simply expelled out of the flue and the heat is wasted.)

The condensate, i.e., the water produced from the condensation process, is collected in a sump before entering the condensate trap. The condensate trap prevents toxic fumes from being expelled into the sewer. It does this by holding a small portion of water which acts as a physical barrier between the boiler and the sewer. This is comparable to how the water in the U-bend of a sink stops bad smells from travelling up the sewer and into the home. A standard condensing boiler will produce 2-3 litres of condensate per hour of operation. This figure will of course vary depending on the model and capacity of the boiler. Per BS 6798, the provision must be made for this waste water to be discharged into either an internal soil stack or waste pipe, or into an external soil stack, gully, or soak-away.

In many boilers, the condensate trap contains a small siphon. The trap collects condensate until it fills up to a level where the siphon is activated. The trap then empties itself automatically. Condensate flows through the pipe in bursts of a few hundred millilitres at a time. This may be heard trickling through the pipe. The discharge of small amounts of water through the condensate pipe is preferable to a steady flow. This is because a constant drip is more liable to freeze. Plus, short bursts of warm condensate will thaw any ice that has started to form in the pipe.


The condensate pipe must be plastic – usually solvent-weld fittings – and at least 22 mm in diameter. Copper or steel pipework cannot be used. This is because, at a pH of 3-4, the condensate is slightly acidic and would corrode it.

Condensate pipes may run outside or through unheated buildings, such as garages. Outside condensate pipes should not be longer than 3 metres. They must also be insulated with waterproof lagging in order to prevent them from freezing. Where it is undesirable to insulate the pipe for aesthetic reasons, 32 mm external piping will greatly reduce the risk of freezing. The upsizing of 22 mm to 32 mm pipe should ideally be made within the property. This is so that water cannot freeze in a 22 mm section that is outside or within a wall cavity.

The condensate pipe must have a fall of at least 1:100. This gradient is required in order to prevent waste water from other appliances connected to the sewer (such as a washing machine) inadvertently entering the boiler’s combustion chamber. The gradient should also prevent water from sitting in the pipe and possibly freezing.

The condensate pipe must also have as few bends as possible. This is in order to prevent the condensate from getting trapped in the pipe.


One of the most common condensate pipe problems is that the water freezes inside it, blocking the pipe. The boiler’s built-in sensors will detect that it cannot discharge the condensate. This will trigger a fault code and prevent the boiler from lighting. Frozen condensate pipes can easily be remedied with warm water or a hot water bottle. For more information on this topic, check: Frozen condensate pipe: how to fix.

Other obstructions in the condensate pipe may include sewage. This may be the result of a poor connection location to the waste water system, or of a poor waste water system in general. In some very bad cases, the sewage may travel far back enough up the condensate pipe to overwhelm the boiler internally.