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.

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Condensate pipe overview

Water vapour from the combustion process is channelled through the boiler (along with other gases from the combustion chamber) until so much heat is drawn out of it that 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. Just like how the water in the U-bend of a sink stops bad smells from travelling up the sewer into the home, the condensate trap uses a portion of condensate water to prevent toxic fumes from being expelled into the sewer. A standard condensing boiler will produce 2-3 litres of condensate per hour of operation, although of course this figure may vary depending on the model and capacity of 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. Condensate is collected in the trap until it fills up to a level where the siphon is activated and the trap empties itself automatically. Condensate flows through the pipe in bursts of a few hundred millilitres at a time, which may be heard trickling through the pipe. The siphonic discharge of small amounts of water through the condensate pipe is preferable to a steady flow, as 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 metal-based piping would be more susceptible to corrosion.

Condensate pipes that are outside or run through an unheated outbuilding, such as a garage, must be insulated with waterproof lagging in order to prevent them from freezing and should not be longer than 3 metres. 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 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, and will consequently prevent the boiler from lighting, leaving the household without heating or hot water. If a frozen condensate pipe is suspected, this can easily be remedied by pouring warm water on the exposed section of pipe, or by placing a hot water bottle on it. Click here to read more information on this topic, including how to prevent it from freezing in future.

Other obstructions in the condensate pipe may include sewage, which has travelled up the pipe due to a poorly configured waste water system in the building. In some very bad cases, the sewage may travel far back enough up the condensate pipe to overwhelm the boiler internally.

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