A condensing boiler that cannot drain its condensate shuts down to protect itself. We will get you running again, and then deal with the reason – which is almost always a pipe that is too small, too long, poorly insulated or badly fallen.
First, try this yourself
It is free, it takes ten minutes, and it works more often than not. If it does, you do not need us today.
- Find the external section. A small plastic pipe, usually white or grey, emerging low from the boiler and running down an outside wall to a drain or gully.
- Find the likely freezing point. Usually the lowest point, an elbow, a long horizontal run, or the open end.
- Apply warm water – not boiling. Pour it over the pipe along its length, concentrating on bends and the end. A hot water bottle or warm cloths wrapped round it also works and is gentler.
- Be patient. Several applications may be needed. Boiling water can crack cold plastic and, on a genuinely cold day, refreezes into a worse blockage.
- Reset the boiler once the pipe is clear.
- Watch that it drains – you should see condensate discharging from the end as the boiler runs.
It is working exactly as designed. A condensing boiler that cannot dispose of its condensate safely refuses to run. Once the pipe is clear it will restart and behave normally. Knowing that is worth a great deal at seven on a January morning.
Why it happens to some boilers and not others
The difference is almost never the appliance. It is where the pipe goes.
| Condition | Effect |
|---|---|
| Condensate terminates internally | Cannot freeze. This is why some houses never have the problem |
| Long external run | Every metre outside is a metre that can freeze |
| Smaller pipe size used outside | Guidance specifies a larger minimum for external runs. A great many installations used the smaller size throughout |
| Poor or no insulation | Ordinary foam lagging degrades in sunlight and is largely useless after a couple of years |
| Shallow fall or a sag | Standing water in the pipe is what freezes. A continuous generous fall drains instead |
| Open pipe end in cold air | The first thing to ice up |
| Acidic sludge narrowing the bore | Condensate systems accumulate deposits over years, reducing the effective diameter |
Most installations that freeze repeatedly have two or three of those at once – a small pipe, run a long way outside, with poor fall and no effective insulation.
The permanent fixes, in order of preference
- Reroute internallyBy far the best answer. A condensate pipe terminating into an internal soil stack or waste has nothing to freeze. Where the building allows it, this removes the problem permanently.
- Increase the pipe diameterA larger bore holds more water for the same length and takes considerably longer to freeze. Guidance specifies a larger minimum size for external runs precisely for this reason.
- Shorten the external runBring the pipe through the wall as close to the termination as possible.
- Insulate properlyExternal-grade waterproof insulation, continuous, with no gaps at bends or the termination – which is exactly where the gaps usually are.
- Improve the fallSo the pipe drains rather than holding standing water. Sags and near-horizontal sections are where ice forms.
- Terminate sensiblyInto a gully below the grating rather than above it, or with a proper termination rather than an open pipe end in cold air.
- Trace heatingA last resort where nothing else is possible. It works, it needs a power supply, and it can fail.
We also clean the condensate trap while we are there, because a partially blocked trap narrows the effective bore and makes freezing more likely – and it causes several other fault codes that look like completely different problems.
Why it is worth fixing properly
A boiler that shuts down every cold snap is not something to live with. Each occurrence means a house with no heating on the coldest day of the year, frequently with somebody vulnerable in it, and frequently when nobody is home to thaw it.
- It always happens at the worst time. That is not bad luck – it is definitionally when it is coldest.
- Repeated freeze-thaw damages the pipe, and eventually it splits.
- Condensate backing up into the boiler is acidic and it is not doing the appliance any good.
- An empty property – a second home, a holiday let – can be down for days without anybody knowing.
- It is a one-off job. Internal rerouting, or upsizing and shortening, deals with it permanently for a modest cost.
The full explanation of the mechanism, and what to do in the moment, is in frozen condensate pipes: why it happens.
Thawed it yourself this winter? That is the moment to get it rerouted, not the next cold snap.
It affects oil and gas equally
Any condensing appliance produces condensate. Modern oil boilers are condensing, modern gas boilers are condensing, and the problem and the fixes are identical for both.
There is one Norfolk-specific wrinkle worth knowing: a large share of the properties we attend for this are rural, off-grid and oil-fired, with external boilers. An external oil boiler has its condensate outside by definition, which is why they account for a disproportionate share of cold-weather call-outs – see internal, external or boiler house.
If your boiler is external and its condensate has ever frozen, the rerouting options are more limited but upsizing, shortening, insulating and improving the fall all still apply, and together they make a real difference.
Questions we get asked about this
Can I thaw it myself?
Usually, yes – warm water over the external pipe, concentrating on bends and the end, then reset the boiler. It works more often than not and it costs nothing.
Can I use boiling water?
Warm is better. Boiling can crack cold plastic and, on a genuinely cold day, refreezes into a worse blockage. Warm, patient and repeated is more reliable.
Will my boiler be damaged?
No. It shut down precisely to avoid damage. Once the pipe is clear it runs normally.
Why does it only happen to my boiler?
Almost certainly the pipe route. A boiler with an internal condensate termination will never do this; one with a long, thin, poorly insulated external run will do it every winter.
Is it a fault with the installation?
Frequently, in the sense that a long external run in the smaller pipe size with no effective insulation is not good practice, even where it met the requirements at the time. It is fixable.
How much disruption is rerouting?
Less than people expect where an internal termination is available – typically a short run to an existing waste or soil stack. Where it must stay external, upsizing and shortening is a straightforward visit.
What happens when you call
Call and describe it
You speak to an engineer, not a call centre. Most jobs can be narrowed down on the phone.
We look at it properly
A real diagnosis before a price, and a straight answer if it is not worth doing.
A written quote
Itemised, so you can see what you are paying for.
The job, and the paperwork
Certificates where the work needs them, and what will need attention next.
Where we do this work
Across Norwich and the whole of Norfolk. If you are on the edge of the county, call and ask — we will tell you honestly whether we are the right people.
- Norwich
- Wymondham
- Dereham
- Attleborough
- Aylsham
- North Walsham
- Cromer
- Diss
- Loddon
- Long Stratton
- Watton
- King's Lynn
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.