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Anti-freezing drainage pipes

The winter of 2009/10 resulted in a significant increase in the number of calls from householders with condensing boilers where the condensate drainage pipe had frozen, causing temporary boiler shutdown.

In nearly all cases, this occurred where part of the condensate drainage pipe was located externally.

To minimise the risk, the following methods should be used, in order of priority:

  • Wherever possible, the condensate drainage pipe should be routed and terminated so that the condensate drains away from the boiler to a suitable internal foul water discharge point. A suitable permanent connection should be used and all other relevant guidance/boiler maker’s instructions should be followed. Access to an internal “gravity discharge” point should be considered in determining boiler location.
  • Where “gravity discharge” to an internal termination is impossible, or where very long internal runs of drainage pipe would be required, condensate should be removed using a suitable proprietary condensate pump. The pump outlet pipe should discharge to a suitable internal foul water discharge point and follow the advice in point 1.

If no other discharge method is possible, the use of an externally run condensate drainage pipe terminating at a suitable foul water discharge point, or purpose-designed soakaway, may be considered. If this method is chosen, the following measures should be taken:

  • The pipe should run internally, as far as possible before going externally, and the pipe diameter should be increased to 32 mm before it passes through the wall to the exterior. The pipe should be insulated using suitable waterproof and weather-resistant insulation.
  • The external pipe should take the shortest and least exposed route to the discharge point, and should “fall” as steeply as possible away from the boiler, with no horizontal runs in which condensate might stand.
  • The use of fittings, elbows, and so on should be kept to a minimum and any internal “burrs” on cut pipework should be removed so that the internal pipe section is as smooth as possible.
  • Where the pipe terminates over an open drain or gully, it should terminate below the grating level, but above water level, in order to minimise “wind chill” at the open end. An additional drain cover may offer further protection.
  • Where there are likely to be extremes of temperature or exposure, the use of a proprietary trace-heating system for external pipework, incorporating an external frost thermostat should be considered. If such a system is used, the requirement to use 32 mm pipe does not apply.
  • Internal pipe runs in unheated areas such as lofts, basements and garages should be treated as external runs.

Martyn Bridges is director of marketing and technical support at Worcester