Boilers are the beating heart of the central heating system, and we trust them to run day in, day out. Unfortunately, despite most modern boilers being exceptionally long-lasting, no boiler is indestructible, and they become liable to common problems after years of use.
While the general advice is to service your boiler every year, few people actually do that, which accelerates the development of faults and issues.
In the UK, around 4 to 6% of boilers develop faults every year. While that seems relatively low, over the course of 10 to 15 years, the chances of your boiler developing a fault are quite high.
This is a guide to the most common boiler problems.
1: Leaking boiler
If you discover a leak from your boiler, you should investigate immediately. Boiler leakage can cause damage to the electrical circuits in and around your boiler. At worst, this is a fire risk - you should immediately shut off your central heating and contact our heating engineers if you find that your boiler is leaking.
Leaks aren’t always caused by something serious. For example, worn seals, corroded pipework or faulty components cause fixable leaks. However, leaks are often signs of a broader problem that needs fixing, like excessive boiler pressure or widespread corrosion.
- Leaking boilers are a serious issue that warrants immediate attention.
- Do not disassemble the boiler yourself.
- Not all leaking boilers are a sign of a catastrophic fault.
- Can be cheap to fix, in the case of worn seals, or expensive, in the event the entire boiler needs to be replaced.
2: Low boiler pressure
If you notice that some radiators are failing to heat up in your home, check the boiler pressure gauge. Typical boiler pressure is around 1.5 bar, but different boilers may have slightly different target pressures. If your boiler pressure is significantly below 1.5 bar, then it’s worth investigating your heating system for leaks.
Furthermore, if you’ve recently bled a radiator and released too much water from the valve, this might cause the system to de-pressurise. You can re-pressure the system by closing off your central heating, allowing it to cool, and allowing water to enter the filling loop. Some boilers make this process simpler via a repressure key.
Get in touch to book an engineer if you’re unsure - repressurising a boiler isn’t a costly job, providing there are no other faults.
- Low pressure boilers will fail to heat radiators adequately.
- Most boilers operate between 1 and 2 bar. 1.5 bar is a safe bet most of the time.
- If your boiler loses pressure, check for leaks.
- You may have released too much water if you’ve recently bled a radiator.
3: No heating or hot water
If your boiler is providing no heating or hot water, there may be an issue with the boiler’s valves, diaphragms and airlocks, thermostats, hot water cylinder or other internal components. The system may have suffered a catastrophic pressure loss from a burst pipe or bad leak.
Faults inside the radiator can cause it to shut off automatically, as is the case if the boiler pressure is too high or low.
Frozen or totally blocked pipes can also turn off heating and hot water. You can check your boiler for an error code and compare that against the manufacturer’s guidance - this might reveal the fault. It’s definitely worth consulting our heating engineers if you’re unsure.
- No heating or hot water may signal a severe internal boiler fault.
- Check for boiler error codes, if applicable.
- Rule out the possibility of a catastrophic leak or frozen pipe.
- Ask a heating engineer if you’re unsure.
4: Not heating, but hot water is fine
Some combi boilers can be set to provide hot water without heating. In the summer, it’s common practice to switch off the heating mode entirely - make sure it’s switched back on in winter.
There could also be an issue with the radiators themselves, or the boiler pressure may be too low to distribute water to the radiators.
- Check your boiler settings.
- Ensure your boiler pressure is around 1.5 bar.
- Check over each individual radiator.
- Assess boiler for error codes.
5: Frozen condensate pipe
If you have a condensing boiler, you’ll have a condensing pipe that transports wastewater out of the property. If the pipe is fitted somewhere exposed to the cold, such as an outside wall or inside a garage, it may freeze during winter. This can cause the boiler to shut down automatically - some boilers will display a fault to indicate what’s happened (e.g. F28 or F29 on Vaillant boilers).
By locating this plastic pipe, typically fitted underneath your boiler, you can usually safely defrost it with hot water. This is only likely to occur in freezing temperatures. Check for frost on or around the pipe.
- Condensate pipes are vulnerable to freezing during the winter.
- The boiler might shut down if this pipe becomes blocked from freezing.
- Some boilers might display a fault to that effect (e.g. F28 or F29 on Vaillant boilers).
- You can defrost the pipe yourself.
6: Radiator faults
Faults with radiators are not always necessarily associated with your boiler. For example, radiators that only heat up partially may require bleeding, or there might be an issue with the TRV valves on the flow pipe. Another possibility is a build-up of sludge in the system, or pinhole leaks resulting from corrosion.
However, if most of your radiators stop working simultaneously, it’s worth assessing the boiler. A leaking or de-pressurised boiler will likely cause radiator issues. If applicable, check the boiler for any error codes that highlight the fault.
- Radiator issues are not always associated with the boiler.
- Check for faults in and around the radiator, including the TRV and lockshield valves.
- Assess your boiler for error codes.
- Check boiler pressure.
7: Boiler not responding to thermostat
The thermostat links your boiler with your home’s internal temperature. By sensing the air temperature and communicating with your boiler via radio, the thermostat instructs the boiler whether to switch on or off to maintain the set temperature.
It can be pretty frustrating when your central heating fails to respond to the thermostat. It might be that your thermostat has to be set too high to bring your heating up to temperature, or that the heating fails to kick in when the timer kicks in.
Firstly, check the obvious; is your timer synced with the actual time? (especially if the clocks went forward or back recently)? Next, check the batteries in the thermostat, and if it’s a new thermostat, ensure it’s in range of the boiler.
Another potential issue is when there’s a secondary heater placed near the thermostat. For example, if you have a fan heater in your hallway where your thermostat is installed, your hallway temperature may be higher than the rest of your home, causing the thermostat to switch your heating off prematurely.
- Check your timer to ensure it’s in sync with the actual time.
- Ensure your new thermostat is set up correctly.
- Change the thermostat batteries.
- Make sure there are no other heat sources in close proximity to the thermostat.
8: Gurgling, banging or whistling noises coming from your boiler
Boilers are usually quiet but aren’t always totally silent. Small noises coming from the boiler are usually nothing to worry about. However, a more pronounced gurgle, hiss, whoosh, bang or whistle may signal an issue.
Common noises include:
- Drone noises - often a sign of pump issues.
- Gurgling noises - typically a sign of trapped air in the boiler or pump
- Humming noises - may mean the water pressure is too high.
- Whooshing noises - occur when there are blockages or flow issues.
9: Pilot light keeps switching off
Many older conventional system boilers have pilot lights that ignite a larger burner. You’ll be able to see the pilot light in your boiler if it has one.
If the pilot light goes out, gas will still likely flow to the pilot light, which can be dangerous. If you’re experiencing boiler faults and notice that your pilot light is off, you should get it checked out immediately.
The pilot light’s components (e.g. the thermocouple) may become worn over time. In addition, if your boiler is in the garage and there’s a strong draft on a windy night, this can also blow off the pilot light.
- Not all boilers have pilot lights.
- The pilot light will wear over time.
- Strong winds or drafts can blow out the light.
- Don’t attempt to reignite the pilot light yourself.
10: Boiler won't turn on (no power)
Modern boilers require electricity to run and will switch off if your mains power goes off, e.g. in the event of a powercut. If your boiler turns off, check your mains power and breakers.
Additionally, if your boiler suffers from a serious fault, it might automatically shut down and refuse to turn on again. There may also be a blockage or a pressure issue. If your mains power is working fine, but your boiler isn’t turning on, contact our heating engineers as soon as possible.
- Modern boilers usually require mains power.
- Check your breakers if your boiler won’t switch on.
- See if your boiler displays an error code.
Believe it or not, some people can't turn their heating off! What a problem to have.
Summary: Most common boiler problems
While any boiler fault can seem ominous, many aren’t particularly serious. The key is early intervention. By fixing the issue nice and early, you can often avoid more complex, long-lasting damage and ongoing problems. We have just spoken about 10 problems, but there are many more, these could include your boiler controls not working, the pilot light goes out or even a leaking boiler.
As always, contact our heating engineers if you’re unsure or need a new boiler. Remember, rummaging around your gas boiler is a criminal offence unless you’re Gas Safe registered! Often, it’s simply not worth the risk of fixing DIY unless you have serious knowledge and qualifications.