What the condensate pipe does and why it freezes
Modern gas boilers are condensing boilers. They recover extra heat from the flue gases, and in doing so they produce water, known as condensate. This mildly acidic water drains away through a plastic pipe, usually white or grey and about the width of a thumb or wider. The pipe normally runs outside to a drain or soakaway, although some go into an internal waste pipe.
When the pipe runs outside, the slow trickle of condensate can freeze in a cold snap. Ice builds up, often at a bend or at the open end, until the pipe is blocked. The water then backs up into the boiler, a sensor detects it and the boiler shuts down. It is one of the most common reasons for a boiler failing on the coldest mornings of the year, and it is usually something you can fix yourself.
Signs your condensate pipe is frozen
- The boiler stopped working during freezing weather, often overnight or first thing in the morning.
- You can hear a gurgling or bubbling sound from the boiler or the pipe.
- The boiler tries to fire, fails and locks out, sometimes after several attempts.
- The display shows a lockout or ignition code, even though your gas supply is fine.
- Ice is visible at the end of the pipe, or the pipe feels solid and very cold.
Because the boiler cannot run safely with condensate backing up, it often shows an ignition or flame failure code rather than a specific condensate fault. The code you see depends on the make and model.
| Brand | Code often seen with a frozen condensate pipe |
|---|---|
| Ideal | L2 (ignition lockout) on many models |
| Worcester Bosch | EA 227 on many Greenstar models |
| Vaillant | F28 (ignition failure) on many ecoTEC models |
| Baxi | E133 on many models |
These codes have other causes too, such as a gas supply problem. If it is not freezing outside, work through the checks on the individual code page instead.
How to find the condensate pipe
Start at the underside of the boiler. Among the copper pipes you will see one plastic pipe, often 21.5mm or 32mm across. Follow it as far as you can. It usually goes through the wall behind or beside the boiler and comes out on the outside of the house, then runs down the wall to a drain, a gully or into the ground.
The most likely freezing points are:
- the open end of the pipe where it drips into the drain
- any bend or elbow, especially where the pipe turns downwards
- any section that runs flat along a wall rather than sloping down
- exposed sections on the side of the house that catches the wind
If the boiler is in a loft or garage, the pipe may run through an unheated space and freeze there rather than outside.
How to thaw a frozen condensate pipe
Never use boiling water. It can crack the plastic pipe and cause burns. Use warm water from the kettle that has been left to cool, or warm water from the hot tap. Take care on icy ground, and do not climb ladders to reach a pipe high on the wall. If the frozen section is out of reach, call a Gas Safe registered engineer.
- Fill a watering can, jug or kettle with warm water. Hand-hot is ideal.
- Find the frozen section. It is usually at the lowest point, a bend or the open end.
- Pour the warm water slowly over the pipe, starting at the end and working back towards the wall. Repeat several times.
- Alternatively, hold a hot water bottle or a warm wheat bag against the frozen section for a few minutes.
- Keep going until you hear or see water running out of the end of the pipe.
- Pour a little extra warm water around the drain so the ice there does not block the end again.
Do not use a blowtorch, a heat gun or any open flame near the pipe. Do not try to cut the pipe or remove it from the boiler.
Resetting the boiler afterwards
Once the pipe is clear, reset the boiler following your user manual. Most boilers have a reset button or a reset option on the display. The boiler may take a minute or two to run through its start-up checks before firing.
If it locks out again, wait a few minutes, check the pipe is still clear and try once more. Stop after 2 or 3 reset attempts. Repeated lockouts mean either the pipe is still blocked somewhere you cannot reach, or there is a different fault that needs a Gas Safe registered engineer.
While you wait for things to thaw, set the heating to come on for short periods through the night during a cold spell. Warm condensate running through the pipe regularly makes it less likely to freeze solid.
How to stop your condensate pipe freezing again
Lagging
Fitting waterproof, weather-resistant pipe insulation to the outside section of the pipe helps it hold heat. Standard foam lagging from a DIY shop can work, but it needs to be suitable for outdoor use and taped securely so rain cannot soak it.
A wider pipe
Current guidance recommends that external condensate pipes are at least 32mm in diameter. Older installations often used narrower pipe, which blocks with ice far more quickly. Upgrading the external section is a common and effective fix.
Rerouting the pipe
The best long-term fix is often to shorten the external run, make sure it falls steeply, or reroute it internally to a waste pipe such as a sink or washing machine drain. This work should be done by a Gas Safe registered engineer, because the pipe connects to the boiler and must meet the installation requirements.
An engineer can make these changes during an annual service, which is also a good time to check the condensate trap and make sure the internal part of the pipe is clear. If your boiler has frozen more than once in the same winter, mention it when you book, so the engineer can look at the whole route of the pipe and suggest the most practical fix for your home.