Glossary · Boilers and boiler types
What is a condensate pipe?
A condensate pipe is the plastic drain pipe that carries the acidic waste water made by a condensing boiler away to a drain. In cold weather the water in an outside section can freeze and block it. The boiler then shuts down for safety, often with a fault code and a gurgling noise.
Key points
- Every condensing boiler makes condensate, the waste water formed when flue gases cool, and the condensate pipe carries it to a drain.
- A frozen condensate pipe makes the boiler shut down as a safety measure, usually with a fault code and a gurgling sound.
- Fault codes differ by make: Vaillant uses F28 or F29, Baxi uses E133 or E28 and Worcester Bosch shows EA.
- Makers say you can thaw it with warm water or a hot water bottle, but never boiling water, then reset the boiler as the manual says.
- Insulating the pipe or moving it indoors reduces the risk. Pipe changes should be done by a Gas Safe registered engineer.
What a condensate pipe is and why it matters
A condensing boiler captures heat from its exhaust gases. As the gases cool, the water vapour in them turns into liquid. That liquid is the condensate, and it has to drain away.[5]Baxi, What is a condensing boiler and how does it work? The condensate pipe carries it from the boiler to a household drain or soakaway outside.[2]Worcester Bosch, Boiler not working in freezing temperatures?
Vaillant says the pipe is usually white PVC-U, like a washing machine waste pipe, and that a condensing boiler makes on average around 2 litres of condensate an hour.[1]Vaillant, How to thaw a frozen boiler condensate pipe Baxi says the condensate is slightly acidic, with a pH similar to black coffee, which is why it drains through a separate pipe.[5]Baxi, What is a condensing boiler and how does it work? Condensate pipes are plastic because it can be corrosive to some metals.[1]Vaillant, How to thaw a frozen boiler condensate pipe Evidence of water running through the pipe is a sign the boiler is working normally.[2]Worcester Bosch, Boiler not working in freezing temperatures? See heat exchanger for where the condensate forms.
What causes a frozen condensate pipe
If part of the pipe runs outside and the water in it freezes in prolonged cold weather, it blocks.[2]Worcester Bosch, Boiler not working in freezing temperatures? Vaillant says it usually freezes at the most exposed point, often the end of the pipe or a bend where insulation is thinnest.[1]Vaillant, How to thaw a frozen boiler condensate pipe Baxi says a dip or bend where the condensate collects is a likely spot.[3]Baxi, How to thaw a frozen boiler condensate pipe
When the pipe blocks, the boiler shuts down. Baxi says this is a failsafe that protects the boiler from water going back up the pipe and into the electrics.[3]Baxi, How to thaw a frozen boiler condensate pipe Worcester Bosch says the boiler may gurgle when starting, or repeatedly lock out, with an EA code on the display.[6]Worcester Bosch, How do I know that my condensate pipe is blocked? See frozen pipes for other cold weather problems.
| Maker | Code or sign they describe |
|---|---|
| Vaillant | F28 or F29, no heating or hot water, gurgling or bubbling |
| Baxi | E133 or E28 on most boilers, shown as E1 and 33 alternating |
| Worcester Bosch | EA code and a gurgling noise |
Codes vary by make and by model, so check your own manual. See E133 fault code, EA fault code, F28 fault code and boiler fault codes.[1]Vaillant, How to thaw a frozen boiler condensate pipe[3]Baxi, How to thaw a frozen boiler condensate pipe[6]Worcester Bosch, How do I know that my condensate pipe is blocked?
What you can check yourself
The makers describe these steps for a frozen pipe.
First confirm it is frozen. Look at the outside section for ice or frost, usually at the end or at a bend.[1]Vaillant, How to thaw a frozen boiler condensate pipe Baxi says to switch the boiler off at the control panel and turn off the power at the wall before you start.[4]Baxi, How to prevent a boiler condensate pipe from freezing
Then thaw it. Worcester Bosch says to pour warm water along the pipe, not boiling water.[2]Worcester Bosch, Boiler not working in freezing temperatures? Vaillant suggests a watering can or a hot water bottle against the pipe, and warns that boiling kettle water can crack the pipe and cause costly damage.[1]Vaillant, How to thaw a frozen boiler condensate pipe Baxi says a microwaveable heat pack or a cloth soaked in hot water also works, and that poured water may freeze on the ground and make it slippery.[3]Baxi, How to thaw a frozen boiler condensate pipe
Finally reset. Check your boiler manual for how to clear the fault and restart it.[3]Baxi, How to thaw a frozen boiler condensate pipe Vaillant says to check the manual for how to reset the boiler correctly.[1]Vaillant, How to thaw a frozen boiler condensate pipe Reset it once. If it still does not work, Baxi says to call a Gas Safe engineer.[3]Baxi, How to thaw a frozen boiler condensate pipe
When to call an engineer
Call a Gas Safe registered engineer if the boiler will not restart after thawing, the fault returns, or you see any leak or damage. Baxi advises that any remedial work should be carried out by a Gas Safe registered engineer.[4]Baxi, How to prevent a boiler condensate pipe from freezing HSE says anyone employed to work on gas appliances in a home must be Gas Safe registered.[7]Health and Safety Executive, Domestic gas: frequently asked questions Do not open the boiler casing or dismantle the pipe yourself.
For a lasting fix, Baxi says you can ask your Gas Safe registered installer to run the pipe internally, fit a wider pipe, lag it, add a trace heater, or combine these.[4]Baxi, How to prevent a boiler condensate pipe from freezing Lagging with waterproof and weatherproof material is the simplest step.[4]Baxi, How to prevent a boiler condensate pipe from freezing Worcester Bosch also suggests insulating the external pipework and fitting a drain cover to protect the pipe end.[2]Worcester Bosch, Boiler not working in freezing temperatures? Our boiler repair team deals with frozen condensate call-outs.
Example
Lena in Leeds wakes on a freezing morning to no heating and a fault code on her Vaillant boiler. The code is F28. She checks the outside condensate pipe, finds frost at the end and pours warm water along it from a watering can, not boiling water.[1]Vaillant, How to thaw a frozen boiler condensate pipe She then resets the boiler as her manual says. It restarts. She books an engineer to lag the pipe before the next cold snap.[4]Baxi, How to prevent a boiler condensate pipe from freezing
What people get wrong
The most common mistake is pouring boiling water from a kettle on the pipe. Vaillant says it can damage or crack the pipe, leaving the boiler out of action until it is repaired.
Vaillant warns that boiling water can damage or crack the pipe.[1]Vaillant, How to thaw a frozen boiler condensate pipe A second mistake is to treat the thaw as the cure. Thawing clears this morning’s blockage, but the pipe will freeze again in the next cold spell unless it is insulated or moved. A third is to read a code from another make: an E133 on a Baxi is not an F28 on a Vaillant.[1]Vaillant, How to thaw a frozen boiler condensate pipe[3]Baxi, How to thaw a frozen boiler condensate pipe
The bottom line
A condensate pipe drains the acidic waste water from a condensing boiler. If the outside part freezes, the boiler shuts down for safety. You can thaw it with warm water, never boiling, and reset once as the manual describes. For lagging, rerouting or any gas work, use a Gas Safe registered engineer.
Common questions
How do I know if my condensate pipe is frozen?
The boiler shuts down, often with a fault code, and you may hear gurgling. Check for ice on the outside section of the pipe, usually at the end or at a bend. Codes differ by make, so check your boiler manual.
Can I thaw a frozen condensate pipe myself?
Baxi, Vaillant and Worcester Bosch all say yes, using warm water or a hot water bottle, never boiling water. Then reset the boiler as the manual says. If it still does not work, call a Gas Safe registered engineer.
How can I stop the condensate pipe freezing?
Insulate the outside section with waterproof lagging. Baxi says a Gas Safe registered engineer can also reroute the pipe inside, fit a wider pipe or add a trace heater. Keeping the heating on in cold weather also helps.
Why does a condensing boiler make condensate?
A condensing boiler recovers extra heat from the flue gases. As they cool, water vapour turns into liquid. Baxi says this condensate is slightly acidic, with a pH similar to black coffee, so it drains through a separate plastic pipe.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Vaillant. How to thaw a frozen boiler condensate pipe. Checked 11 November 2026. ↩
- Worcester Bosch. Boiler not working in freezing temperatures?. Checked 11 November 2026. ↩
- Baxi. How to thaw a frozen boiler condensate pipe. Checked 11 November 2026. ↩
- Baxi. How to prevent a boiler condensate pipe from freezing. Checked 11 November 2026. ↩
- Baxi. What is a condensing boiler and how does it work?. Checked 11 November 2026. ↩
- Worcester Bosch. How do I know that my condensate pipe is blocked?. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩