Your boiler has displayed a fault, stopped working, or is behaving strangely — and now you’re not sure whether to pick up the phone and call 999 or simply wait until morning.
The answer depends entirely on what kind of fault you’re dealing with. Some boiler problems are genuine emergencies that require you to leave your home immediately. Others are urgent but not dangerous. And some can safely wait for a weekday appointment.
This guide cuts through the uncertainty so you can act confidently and safely.
Not sure what’s wrong? Talk to a local Gas Safe engineer — free, no obligation.
At a Glance: Boiler Fault Urgency Guide
Use this table to find your situation quickly.
| Fault Type | Urgency | Action |
|---|---|---|
| Gas smell indoors | 🔴 Emergency | Leave immediately, call 0800 111 999 |
| Carbon monoxide symptoms | 🔴 Emergency | Leave immediately, call 999 |
| Water flooding from boiler | 🔴 Emergency | Turn off water supply, call engineer now |
| Yellow/orange flame (not blue) | 🔴 Emergency | Stop using boiler, call 0800 111 999 |
| Black soot around casing | 🔴 Emergency | Stop using boiler, call engineer now |
| No heating in winter | 🟡 Urgent | Call a Gas Safe engineer today |
| Recurring fault code | 🟡 Urgent | Call an engineer — do not ignore |
| Pressure below 0.5 bar | 🟡 Urgent | Repressurise; call if it drops again |
| Boiler locked out, no fault code | 🟡 Urgent | Try reset; call if it fails or recurs |
| No hot water only (summer) | 🟢 Can wait | Book an appointment within a few days |
| Minor fault code, no other symptoms | 🟢 Can wait | Check manual, try reset, monitor |
🔴 This Is a Genuine Emergency — Act Now
You Can Smell Gas
A gas smell inside your home is always a serious emergency, regardless of whether your boiler is the source. Natural gas is odourless by default — the smell (like rotten eggs or sulphur) is added deliberately so you can detect leaks.
Do this immediately:
- Do not operate any switches, light switches, or electrical devices
- Do not smoke or use a naked flame
- Open doors and windows to ventilate
- Turn off the gas supply at the meter if you know where it is
- Leave the property and call the National Gas Emergency Service: 0800 111 999 (free, 24 hours)
Do not re-enter until you’ve been told it is safe.
Carbon Monoxide Symptoms
Carbon monoxide (CO) is produced when gas does not burn completely. It is colourless and odourless — you cannot detect it without an alarm. Symptoms include headaches, nausea, dizziness, breathlessness, and confusion. If multiple people in the household feel unwell simultaneously, CO poisoning is a strong possibility.
Do this immediately:
- Move everyone (including pets) outside into fresh air
- Call 999
- Do not re-enter the property
- Contact a Gas Safe engineer to inspect the boiler before you return
Fit a CO alarm if you do not already have one — they cost under £20 and can save your life. Current UK building regulations require them in new builds; retrofitting is highly recommended.
Water Leaking Heavily from the Boiler
A small drip from a pressure relief valve is not immediately dangerous, but any significant water leak from the boiler casing, pipes, or heat exchanger needs urgent attention. Water near electrics is a safety hazard, and an unchecked leak can cause structural damage quickly.
Turn off your water supply at the stopcock and call an engineer the same day.
🟡 Urgent — Call a Gas Safe Engineer Today
These situations are not life-threatening, but waiting several days will likely make things worse (or more expensive).
No Heating in Cold Weather
A home with no heating when temperatures are low is a health risk — particularly for elderly residents, young children, and anyone with a health condition. If your boiler is not producing any heat, call an engineer the same day. If you can’t get one out today, use electric heaters as temporary cover.
Renters have legal rights here — see the tenant rights section below.
A Fault Code That Keeps Coming Back
Modern boilers display fault codes when something is wrong. Many codes are resolved with a simple reset (hold the reset button for 3–5 seconds, then wait 10 minutes). But if the same code returns repeatedly, this is the boiler telling you something is genuinely broken.
Do not keep resetting and ignoring it. Recurring codes — especially those related to ignition failure, gas pressure, or flue issues — indicate a fault that will deteriorate. An engineer can read the code, identify the root cause, and fix it properly.
Pressure Has Dropped Below 0.5 Bar
A boiler operating below the minimum pressure will lock out automatically. This is a safety feature, not a malfunction. You can repressurise most modern boilers yourself using the filling loop — your boiler manual will show you how, and it takes about two minutes.
However: if the pressure drops again within a day or two after repressurising, there is a leak somewhere in the system. That needs an engineer to diagnose and repair — it won’t resolve itself.
Need a quote for a pressure or leak repair? Get one here — no obligation.
🟢 This Can Wait — Book Within a Few Days
Not every boiler fault needs a same-day response.
If your boiler is showing a fault code but you still have hot water and heating, it may be displaying a precautionary warning rather than a critical failure. Check your boiler manual, try a reset, and monitor over 24 hours. If it doesn’t recur, it may have been a temporary sensor reading.
If you’ve lost hot water but not heating, and it’s during warmer months, you have more flexibility to book a convenient appointment rather than paying emergency rates.
That said: do not let any fault code sit for weeks. What starts as a minor warning can escalate.
Should I Turn My Boiler Off If It Has a Fault?
If you smell gas or suspect carbon monoxide: yes. Turn it off, leave the building, and call 0800 111 999.
For every other type of fault, you generally do not need to switch the boiler off. In fact, doing so can make diagnosis harder — fault codes often reset when power is removed, which means the engineer arrives to a boiler with no visible error, making it more difficult (and potentially more expensive) to identify the underlying problem.
Leave the boiler on unless the engineer advises otherwise when you call.
Will My Boiler Explode if the Pressure Is Too Low?
No — and this is a question people search for constantly, so it’s worth a direct answer.
Modern boilers are built with multiple safety mechanisms. If the pressure drops below a safe threshold (typically below 0.5–0.8 bar depending on the model), the boiler will lock out automatically rather than operate unsafely. It will display a fault code and stop firing.
A low-pressure boiler will not explode. It simply will not work.
What you should watch for is the opposite: excessively high pressure (above 3 bar). This can trigger the pressure relief valve to discharge water — that’s what the valve is for. If your pressure is consistently running above 2.5 bar, call an engineer.
How Long Can I Legally Be Left Without Heating?
This section is primarily relevant to renters.
Under the Landlord and Tenant Act 1985, landlords in England and Wales are legally required to keep heating and hot water in working order throughout the tenancy. This is not optional — it is a statutory obligation.
There is no specific number of days set out in legislation, but courts and local councils interpret “reasonable time” as:
- Winter (October–March): 24 hours for no heating is typically considered the threshold
- Summer: a few days may be considered reasonable depending on circumstances
- No hot water: usually 24–48 hours maximum at any time of year
If your landlord is not responding:
- Put your request in writing (text or email — keep a copy)
- Contact your local council’s Environmental Health department
- In severe cases, you may be entitled to withhold rent or arrange repairs yourself and deduct costs — get legal advice before doing so
Homeowners dealing with a boiler breakdown have no such recourse, of course — but if you have a home emergency cover policy, check whether it includes boiler faults.
Frequently Asked Questions
Is a boiler fault an emergency?
It depends on the fault. Gas smells, carbon monoxide symptoms, or significant water leaks are genuine emergencies — call the National Gas Emergency Service (0800 111 999) immediately and leave the property. A fault code with no heat or a pressure drop is urgent but not a 999 situation; call a Gas Safe engineer the same day.
What should I do if my boiler shows a fault code?
First, check your boiler manual — many fault codes are resolved with a simple reset. Press the reset button and wait 5–10 minutes. If the fault code returns, or if you see codes related to gas pressure or ignition failure, call a Gas Safe registered engineer. Do not attempt to bypass or ignore a recurring fault code.
Will my boiler explode if the pressure is too low?
No. A boiler with low pressure will shut itself down as a safety measure — it will not explode. The minimum safe operating pressure is typically 1 bar. If your pressure gauge reads below 0.5 bar, the boiler will likely lock out. You can repressurise it yourself using the filling loop; if pressure keeps dropping, call an engineer as it suggests a leak.
Should I turn my boiler off if it has a fault?
If you smell gas or suspect carbon monoxide, yes — turn off the boiler, open windows, leave the property, and call 0800 111 999. For all other faults (fault codes, pressure issues, no heat), you do not need to turn the boiler off unless the engineer advises it. Turning it off unnecessarily may reset a fault code that would help the engineer diagnose the problem.
How can I tell if a boiler is dangerous?
Warning signs of a dangerous boiler include: a yellow or orange flame instead of blue, black soot or scorch marks around the boiler, unusual smells (sulphur or rotten eggs suggests a gas leak), symptoms of carbon monoxide poisoning in anyone in the household (headache, nausea, dizziness), or water leaking from the casing. Any of these signs require immediate action — call 0800 111 999 and leave the building.
How long can I legally be left without heating or hot water?
In England and Wales, landlords must keep heating and hot water in working order under the Landlord and Tenant Act 1985. There is no fixed number of days specified in law, but a repair must be carried out within a ‘reasonable’ timeframe — typically interpreted as 24 hours in winter for no heating, longer in summer. If your landlord does not act, contact your local council’s environmental health department.
Ready to Get an Engineer Out?
Whatever your boiler is doing, a Gas Safe registered engineer will be able to diagnose the fault, give you a transparent fixed-price quote, and get things working again — often on the same day.
For more on what a repair is likely to cost before anyone arrives, see our boiler repair cost guide.
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