Insulate a boiler condensate pipe

Wrap your boiler's condensate pipe in foam insulation to stop it freezing and shutting down your heating.

BCP-01Find and check the condensate pipeLook for a thin white or grey plastic pipe leaving the bottom of your boiler. Follow it to where it leaves the building — often dropping outside to a gully, drain or soakaway. Check it for existing cracks, splits or leaks before you start, as insulating a damaged pipe won't fix that.

BCP-02Buy the right pipe insulationMeasure the pipe's outside diameter — most condensate pipes are 21.5mm or 32mm plastic. Buy foam pipe insulation ('pipe lagging') to match, with a wall thickness of at least 25mm for any part of the run that's outside. For sections that stay indoors, thinner 9-13mm foam is fine.

RULES

3 each · Foam pipe insulation, 1m length (match pipe size)

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1 roll · Self-adhesive pipe insulation tape

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1 roll · Self-amalgamating weatherproofing tape

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KNOW THE RULES

There's no law requiring this for a DIY job, but boiler manufacturers and UK Building Regulations guidance (Approved Document L) recommend insulating any part of a condensate pipe that runs outside, to stop it freezing and cutting your boiler out in cold weather.

BCP-03Turn the boiler off before you startSwitch the boiler off at its control panel or at the wall switch that feeds it. This stops it firing and producing condensate while you're working on the pipe. Leave it off until you've finished and tested everything.

BCP-04Clean and dry the pipeWipe the full length of the pipe with a dry cloth to remove dust, cobwebs and grease so the tape sticks properly. If any old insulation is perished or wet, strip it off and let the pipe dry fully before continuing.

1 each · Cloth or rag

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BCP-05Cut the insulation to lengthMeasure the pipe run and cut the foam insulation with a sharp knife, mitring the ends at corners so the sleeves meet neatly. Most foam pipe lagging has a slit down one side — open this to fit it around the pipe rather than threading it on.

1 each · Utility knife

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BCP-06Fit the insulation and seal the seamOpen the foam sleeve along its pre-cut slit and press it around the pipe so it closes up with no gaps. Run self-adhesive pipe insulation tape along the whole seam and over every joint between sections, so there's no bare pipe or cold air can get in.

BCP-07Weatherproof any section that runs outsideStandard foam insulation breaks down quickly in sun and rain. Wrap self-amalgamating or PVC weatherproofing tape around any part of the pipe that's outdoors, overlapping each turn by about half its width. Work from the top down so water sheds off the wrap instead of running into it.

BCP-08Secure the pipe and check the fallPush the insulated pipe back onto its existing clips, or use adhesive-backed clips if you need extra support — no drilling should be needed for this job. Check the pipe still slopes downhill all the way to the drain or gully, as condensate needs to flow away by gravity, not sit in a dip.

4 each · Adhesive-backed plastic pipe clips

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BCP-09Turn the boiler back on and testSwitch the boiler back on and run the heating or hot water for a few minutes so it produces some condensate. Check the whole insulated run for drips or leaks, and confirm water is draining away freely at the far end.

BCP-10Tidy upSweep up any foam offcuts and tape scraps and bin them. Wipe your tools clean and keep any leftover insulation and tape for next time.

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