During prolonged spells of extremely cold weather can result in a significant increase in
instances where the condensate drainage pipe from condensing (high efficiency) boilers can
freeze – usually when the pipe had been installed externally for some of its length.
Blockage due to freezing stops the condensate flowing to waste and causes the boiler to shut
This note gives brief guidance for householders on what to do in this situation.
Shutdown due to freezing and blockage of the condensate discharge pipe will usually be
indicated by a “fault code” on the boiler’s digital display, or by some other alarm signal (check
the boiler operating manual for advice on this). It is important to confirm that freezing is the
cause of the problem before taking any of the remedial action suggested below.
NB – the boiler manufacturer or the service company you use may have a helpline or
website giving guidance on this.
Ideally, remedial action should be carried out by a competent installer or service engineer –
who will also be able to advise you on ways to help eliminate recurrence in future. Where it is
not convenient to await such action, there are a number of methods which householders may
wish to try in order to thaw the pipe and free the blockage – provided they feel competent to
1. Locate the blockage.
It is likely that the pipe is frozen at the most exposed point external to the building or where
there is some obstruction to flow. This could be the open end of the pipe, at a bend or elbow,
or where is a dip in the pipe in which condensate can collect. The location of the blockage
should be identified as closely as possible before taking further action.
2. Thaw the frozen pipe.
The pipe can be thawed by applying a hot water bottle, a microwaveable heating pack (the
sort used for muscular aches and pains) or cloths soaked in warm water to the exterior of the
pipe, close to the point of blockage. Warm water can also be poured onto the pipe from a
watering can or similar container.
Do not use boiling water.
Note: You should not attempt to thaw a condensate drain pipe which cannot easily be
reached from ground level. Be aware that any water used can quickly freeze if it falls on
pathways – causing a possible slip hazard.
3. Re-set/re-start the boiler.
Once the blockage has cleared consult the boiler operating instruction, or check the
manufacturer’s website, for guidance on any action needed to clear the fault code/alarm and
re-start the boiler. If this does not succeed you should call in a competent engineer to assess
the situation and take further action if required.
4. Temporary remedial actions:
If the pipe is successfully thawed and the boiler can be re-started then the following temporary
remedial actions may help prevent re-freezing if the severe weather continues.
(a) If the external pipe is not lagged as recommended (see (5) below) then you should try to
insulate the pipe with suitable water-proof and weather-proof lagging, in order to prevent refreezing.
(b) During the cold spell it may help to run the heating system with the boiler thermostat (NB
not the room thermostat) set to maximum. Turn back to the normal setting once the cold spell
(c) It may also help to temporarily set the central heating timer/room thermostat controls to
“continuous” (24hr) mode, setting the room thermostat overnight to around 15o C. Again,
return to normal settings once the cold spell is over.
5. Longer term actions:
British Standards, Building Regulations and boiler manufacturers’ installation instructions allow
condensate discharge pipes to run either internally or externally (or a combination of these).
All the above documents give recommendations on how to run the pipe and how/when use
insulation in order to reduce the possibility of freezing. However this may not have been
sufficient to prevent freezing in extreme conditions of the type recently experienced.
Should you wish to take action in order to prevent recurrence of pipe freezing by relocating
the condensate drainage pipe then more detailed guidance is available from your installer or