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A boiler rarely fails without leaving evidence. Triage the gauge, the taps and the display before you call — unless you smell gas, in which case skip straight to the red rule below.
The red rule first: if you smell gas, this stops being a plumbing page. Leave the property, touch no switches, and call the National Gas Emergency Service on 0800 111 999 from outside. For everything less than that — no heat, falling pressure, error codes — run the checks below, then ring 020 4577 2888 to be connected with a local plumber.
Top up once yourself; call if it happens again within days. A repeat drop means the system is losing water somewhere.
Most sealed systems idle at roughly 1 to 1.5 bar when cold — your boiler's manual gives the exact range, and the gauge on the front tells you where you stand. Below about 1 bar, heating goes sluggish or the boiler locks out. Repressurising through the filling loop is a legitimate DIY job: manual open, valves opened slowly, loop closed properly afterwards.
What you're really watching for is the trend. A one-off dip after bleeding radiators is nothing. A gauge that keeps sagging week after week is a slow leak — often a weeping radiator valve or a joint hidden under a floor — and each top-up feeds it fresh water to lose. High pressure has its own story: much above 2.5 to 3 bar suggests a filling loop left cracked open or an expansion vessel fault, worth professional attention before the relief valve starts discharging down your outside wall.
Five checks, five minutes — and you'll either fix it or ring with far better information.
If it's still cold after that, ring with the model, the code on the display and what you've tried. That single sentence of triage can decide what parts come out in the van.
Very likely the condensate pipe has frozen. Thaw it with warm — never boiling — water, reset once, and lag the pipe when the weather relents.
Modern condensing boilers drain slightly acidic water through a plastic pipe, and where that pipe runs outside — down a back wall, into a gully — a hard frost off the hills can freeze it solid. The boiler senses the blockage and shuts itself down, often showing a specific code that the manual will name.
The fix is unglamorous: jugs of warm water along the frozen section, or a hot water bottle held against it, then one reset. Boiling water is off the menu — it can crack the pipe. If the same lockout returns every cold night, the long-term answer is insulating the pipe or having its run altered, both quick jobs in mild weather and miserable ones in a frost.
The code is the boiler telling you what it already protected itself against. Note it, look it up in the manual, restart once at most — then quote it when you ring.
Every manufacturer has its own dialect of codes, which is why this page won't pretend to translate them all. The pattern that matters is repetition: a boiler that locks out once after a power blip is being cautious; a boiler that locks out on the same code every day is reporting a genuine fault, and each forced restart makes the underlying component work while injured.
Write the code down before it clears — it's the single most useful thing you can say on the phone. And a boundary worth stating in plain terms: anything beyond the reset button and the filling loop on a gas appliance is work for a suitably registered gas engineer. That's a safety line, not a skill judgement.
Usually yes — topping up through the filling loop is a routine householder job if you follow your boiler's manual step by step and close the loop afterwards. What the manual can't fix is a pressure that keeps falling: that pattern means the system is losing water somewhere, and it deserves a professional look rather than endless top-ups.
Mostly it's a diagnosis, not an alarm — the code names the fault the boiler has already protected itself against. Note the code, check the manual, and try one restart if the manual allows it. Repeated lockouts on the same code mean stop restarting and get it looked at; quote the code when you ring.
A frozen condensate pipe is a common winter culprit — the small plastic pipe that runs outside can ice up and shut the boiler down. Thawing that pipe gently with warm (never boiling) water, then resetting the boiler, often restores heat. If the lockout repeats or the cause isn't obvious, have it checked.
It outranks everything else on this page. Leave the property immediately, don't operate switches or anything with a flame, and once outside at a safe distance call the National Gas Emergency Service on 0800 111 999. Gas work itself is for suitably registered engineers only — never a DIY job.
The main page — the triage board and the areas this line covers.
Go to home →The sixty-second triage for escaping water.
Read the guide →What to try, what never to pour, and when it's NI Water's sewer.
Read the guide →No invented prices — how charging works and what to ask first.
Read the guide →Pressure, timer, tripped switch, diverter valve — the checks before the call.
Read the guide →Prevention that costs pennies, thawing that doesn't end in a flood.
Read the guide →The signs, the stopcock test, and when a damp patch turns urgent.
Read the guide →Ring any hour with the model and the code, and be connected with a local plumber covering Derry and the villages around it.
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