It's below freezing, the heating has gone off, and the boiler is gurgling or showing a fault code. In a Scottish winter, the usual culprit is a frozen condensate pipe: the small plastic pipe that carries water from your boiler to a drain outside.
You can normally clear it yourself, safely, with a hot water bottle or some warm water. Here's how to do it, which fault codes to look for, when not to try, and how to stop it happening every winter.
The short answer
- Ice in the condensate pipe traps the water your boiler produces, so the boiler shuts itself down. Signs are gurgling, a lockout or fault code, and ice at the pipe end.
- Thaw it from ground level with a hot water bottle, heat pack or warm (never boiling) water, working from the open end back towards the wall.
- Reset the boiler as your user guide describes. If it locks straight back out, stop and call an engineer.
- No ladders in frost, no high-level pipes, and never open the boiler casing.
- If it freezes every winter, the lasting fix is rerouting the pipe indoors or fitting a larger, insulated external pipe no longer than 3 metres.
- Can't reach it, or it won't restart? Book a repair. The £85 call-out covers the visit and diagnosis, and any fix is quoted at a fixed price first.
What a condensate pipe is and why it freezes
A condensing boiler gets extra heat from its flue gases by cooling them until the water vapour in them turns back into liquid. Vaillant puts the output at around 2 litres an hour on average. Because this condensate is mildly corrosive to metal, it drains through a white or grey plastic pipe, usually out through the nearest wall and down to a drain or gully.
Why it freezes
Condensate trickles rather than flows, and a trickle in a cold pipe freezes quickly. The risk goes up with:
- Long external runs. Current guidance says 3 metres maximum, and many older installations are longer.
- A narrow pipe outside, often the same small pipe used indoors.
- Flat or sagging sections where water sits instead of draining.
- Exposed, windy walls, and missing or split lagging.
- The boiler switched off overnight, so the water stops moving and freezes before morning.
Why Scotland gets it worse
We get long spells of hard frost, and a lot of our housing puts condensate pipes in the worst possible spot. Tenements, ex-council blocks and four-in-a-blocks across Glasgow and Lanarkshire often have pipes running down exposed gable or rear walls, sometimes from an upper-floor kitchen. Those long, wind-blasted runs are exactly what current guidance is designed to avoid.
Signs your condensate pipe has frozen
Suspect the condensate pipe if it's freezing outside and:
- the boiler or the pipe gurgles, often as the boiler tries to light
- the boiler locks out or shows a fault code or warning light
- you've lost heating and, on a combi, hot water too
- there's ice at the open end of the pipe
Fault codes manufacturers link to a frozen condensate pipe
These come from each manufacturer's own guidance. Displays vary by model, so check your user guide as well.
- Worcester Bosch. What to look for: EA, D5 on some models, or a blue light flashing slowly (off for longer than it's on).
- Vaillant. What to look for: F28 or F29 (shown as F.28 or F.29 on some displays).
- Ideal. What to look for: L2 ignition lockout on the Logic Heat and Logic+ Heat, or text messages such as Ignition Lockout or Blocked Flue/Condensate on models like the Logic Max Combi2.
- Baxi. What to look for: E133 (some displays alternate between E1 and 33) or E28.
None of these codes only means a frozen pipe. Vaillant's F28, for example, means the boiler couldn't detect a flame when lighting, which has several possible causes. If it's mild outside, or the code comes back once the pipe is clear, photograph it and book a repair.
How to thaw a frozen condensate pipe safely
Work from the ground only. No ladders in frost and no leaning out of windows. Ideal's own user guide says not to thaw pipework above ground level. Never use boiling water and never remove the boiler casing. If you smell gas at any point, get everyone outside and call the National Gas Emergency Service on 0800 111 999.
- Check the signs. It's freezing outside and you've heard gurgling or seen a code from the table above. Photograph the display.
- Find the pipe. Look for white or grey plastic coming out of the wall near the boiler and running down to a drain. The ice usually sits at the open end, at a bend or along a flat section.
- Peel back any lagging over the part you think is frozen.
- Warm it gently with a hot water bottle, microwavable heat pack or wheat bag, or pour warm water over it from a watering can or jug. Water from the hot tap is fine, or kettle water left to stand until it's warm rather than hot.
- Work from the open end back towards the wall until water drips or runs freely. It can take several goes.
- Grit or salt the ground. The run-off will refreeze on paths and steps.
- Reset the boiler with the reset or restart button as your user guide describes, then give it a few minutes to run through its lighting sequence.
- Refit the lagging once the condensate is flowing.
What not to do
- Boiling water. It can crack the plastic pipe or its joints and scald you if it splashes.
- Repeated resets. Once or twice is fine. If it locks straight back out, stop. More resets won't shift ice you can't reach, and they can hide a different fault.
- Hitting the pipe to break the ice. Cold plastic is brittle.
- Anything inside the boiler casing. That's work for a Gas Safe registered engineer only.
If the boiler won't restart, or it keeps freezing
If water is running freely from the pipe end and the boiler still locks out, the blockage is further up, in the boiler's internal condensate trap, or it's a different fault. Stop there and book a repair. Our boiler not working checklist covers the other common causes, such as low pressure.
If the pipe refreezes in every cold snap, thawing it only treats the symptom. The pipework itself needs upgrading.
How to stop your condensate pipe freezing
The Heating and Hotwater Industry Council (HHIC) publishes the industry installation guide for condensate pipes, currently Issue 3.0 from November 2023. It works alongside BS 6798, and the boiler manufacturer's instructions always take priority. These are its main measures.
- Route it inside. What the guidance says: Connect to an internal soil stack (preferred) or a kitchen, bathroom or utility waste, so nothing runs outside. A condensate pump can be used where gravity won't work. Who does it: Engineer.
- Bigger external pipe. What the guidance says: At least 30mm internal diameter (typically 32mm outside diameter) for any section outside or in an unheated space. Who does it: Engineer.
- Shorter run, proper falls. What the guidance says: No more than 3 metres outside, as vertical as possible with no flat sections. At least 45mm fall per metre indoors and 3 degrees down through the wall, with the end cut at 45 degrees and a leaf guard over the gully. Who does it: Engineer.
- Weatherproof insulation. What the guidance says: At least 13mm thick on a 32mm external pipe, waterproof and UV-resistant, with the joints sealed. Who does it: You, if it's reachable from the ground.
- Trace heating or a siphon. What the guidance says: A thermostatic heating cable along the insulated pipe, or an internal siphon that releases condensate in larger, warmer batches that are less likely to freeze. Who does it: Engineer, with any wiring through our approved, separately registered electrical contractor.
Manufacturers add their own rules. Worcester Bosch, for example, says external runs on most of its boilers must be insulated and kept to 3 metres, while on certain Greenstar models and those fitted with its CondenseSure siphon, a 32mm pipe of up to 3 metres doesn't have to be insulated.
What you can do before the next cold snap
- Find your condensate pipe now, in daylight, and lag any section you can reach from the ground.
- Keep the pipe end and gully clear of leaves and debris.
- Going away? Vaillant suggests leaving the boiler running on a timer through freezing weather rather than switching it off.
- Worcester Bosch suggests turning the boiler's heating dial up to maximum during a freeze. Turn it back down once the freeze has passed.
- Ask about it at your next boiler service. HHIC guidance asks engineers to check the condensate pipe at every service or repair and mark any that need upgrading.
Tenements, flats and high-level condensate pipes
In many Glasgow tenements and ex-council flats, the boiler is in an upstairs kitchen and the condensate pipe leaves the wall high up. You can't thaw that safely. Don't use a ladder on ice and don't lean out of a window to reach it. Leave the boiler off and book a repair.
A long drop from an upper floor is usually well over the 3-metre guideline, which is why these pipes freeze so often. The better fix is normally to bring the condensate back inside to the kitchen waste or soil stack, with a condensate pump if the drain sits higher than the boiler. In most tenements the outside walls are maintained jointly by the owners, under the title deeds or the Tenement Management Scheme, so keeping pipework indoors can also save a conversation with neighbours or your factor. Replacing the boiler anyway? See our guide to the best boiler for a Glasgow tenement flat.
Renting? Tell your landlord or letting agent. Under the Repairing Standard, landlords in Scotland must keep heating and hot water installations in a reasonable state of repair and proper working order.
How Titan Heat can help
Can't reach the pipe, it won't thaw, or the boiler won't restart? Book a repair online. The £85 fixed call-out covers the visit and diagnosis. Anything further is quoted at a fixed price before work starts, and our work carries a 1-year parts and labour guarantee. Same-day emergency repairs are available, subject to engineer availability.
If your pipe freezes every winter, we can reroute or upgrade the condensate pipework to current guidance so it doesn't come back with the next frost. See typical prices in our boiler repair cost guide, or read more about our boiler repair service.
Titan Heat is Gas Safe registered (business registration 982732) and based in Blantyre, covering Glasgow, Lanarkshire, Renfrewshire, East Renfrewshire, and East and West Dunbartonshire.
Frequently asked questions
How do I know if my condensate pipe is frozen?
If it's below freezing, the boiler has stopped, and it's gurgling or showing a fault code, check the open end of the pipe outside. Ice there, or along a flat section, confirms it.
Can I pour boiling water on a frozen condensate pipe?
No. Boiling water can crack the plastic pipe or its joints and can scald you. Use warm water from the hot tap, kettle water left to cool until it's warm, or a hot water bottle.
Why is my boiler gurgling in cold weather?
Usually because condensate is backing up behind ice in the pipe, and the boiler will normally lock out soon after to protect itself. Thaw the pipe and reset. Gurgling in mild weather can still mean a condensate pipe blocked by leaves or debris, or something else such as air in the system, so it's worth getting checked.
Is a frozen condensate pipe dangerous?
Not in itself, because the boiler shuts down safely. The real risks are slipping on ice, falling from a ladder or scalding yourself. Separately, if you smell gas or have carbon monoxide symptoms such as headaches, dizziness, feeling sick, breathlessness or unusual tiredness, turn the appliance off, open doors and windows, and call 0800 111 999.
Can I just wait for my condensate pipe to thaw on its own?
Yes, once the temperature rises above freezing, and then reset the boiler. In a long Scottish cold snap that can mean days without heating or hot water, so thawing it safely from the ground is usually the better option.
Who fixes a frozen condensate pipe in a rented flat?
Report it to your landlord or letting agent. You can still thaw a pipe you can safely reach from the ground, but rerouting or upgrading the pipework is for the landlord to arrange.
Will lagging stop my condensate pipe freezing?
It helps on a short, correctly sized pipe. On a long, narrow or flat run, lagging alone often isn't enough, and rerouting the pipe indoors or fitting a larger one is the lasting fix.
