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Top 10 most common boiler problems and how to fix them

There's never a convenient time to start experiencing a boiler problem, especially during the colder seasons of the year.

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Simply answer a few multiple choice questions about your existing boiler and we’ll give you a fixed price for a new boiler including all parts and installation[28]

Many central heating problems can occur after long periods of your boiler being idle, such as leaving the heating off throughout summer. Whether you have a conventional, system or combi boiler problem, here we've provided a guide to the most common boiler problems to help you understand the potential causes and recommend what to do if the problem does occur. Whilst there could be some simple checks you could do to identify if you have a boiler problem, in most cases we advise getting in touch with a Gas Safe engineer and not trying to fix the problem yourself to avoid any potential danger or worsening of the issue.

1. Low boiler pressure

Usually displayed on the boiler's built-in pressure gauge, check that your boiler pressure is not too low. If your boiler pressure is below 1, your central heating system is likely not able to function properly. Low pressure can be a result of various potential reasons, such as a water leak in the system, the pressure relief valve needs replacing or a cause of recently bleeding the radiators. A simple first step you can take is to look for a visible leak in the system. If a leak is found, you'll need to call out a Gas Safe engineer. If you don't find a leak in the system, try re-pressurising your boiler by following our step-by-step guide.

2. Radiators not heating up

Cold patches in radiators is a common problem and is often due to the build-up of sludge or air trapped inside the system causing an unequal distribution of heat. The resolution will depend on whether the cold patch is at the top, middle or bottom of your radiator. For example, if only the bottom of your radiator is getting hot then there is most likely trapped air and you'll need to bleed the radiator. This is a simple task and usually doesn't require an engineer. You can follow our step-by-step guide on how to bleed a radiator.

A cold patch in the middle or at the bottom of your radiator is usually caused by a build-up of sludge and rust. Find out more about how to fix a cold radiator.

3. Banging, whistling or gurgling noises from your boiler

Whilst hearing any of these noises can be disruptive to your everyday life, they are a common issue that many home owners experience. The type of noises your boiler is making can help identify the area of concern. A noisy boiler is normally caused by either air in the system, low water pressure, a faulty pump, or more seriously can be a sign of kettling which occurs because of a build-up of limescale in the system. See our article on Noisy Boiler and Pipes for more information on the potential cause of different sounds and how to resolve them.

4. Frozen condensate pipes

All new boilers are condensing which means that the small amount of waste gas that a boiler naturally produces and turns to water as is cools is drained outside by the condensate pipe. In the frequent cold conditions that we experience in the UK, it is common for the water to freeze and cause a blockage in the condensate pipe. Follow our guide on how to defrost a condensate pipe.

5. Intermittent heat and hot water

Intermittent or no heat and hot water can often be due to either broken diaphragms and airlocks, faulty motorised valves, a broken thermostat or low water levels. You can start by checking if the boiler problem is caused by fault of the boiler pressure or thermostat, as these are easily accessible.

If your pressure is low, use our step-by-step guide on how to re-pressurise your boiler.

If your thermostat is not working and having checked your manufacturer's manual are unable to find a simple solution, you can contact the manufacturer direct if the thermostat is still in warranty or call us for advice on what to do next.

Still experiencing a problem with your boiler? A broken diaphragm, airlock or valve may need to be replaced which will require a trained Gas Safe engineer.

6. Boiler switching itself off

If your boiler continues to switch itself off then the boiler pressure could be too low, there could be a blockage in the system restricting water flow or the pump could be faulty and therefore not circulating the water properly. Follow our guide to re-pressurise your boiler. If your boiler pressure is correct and does not need re-pressurising, then contact us for further assistance in diagnosing and repairing the problem.

7. Pilot light keeps switching off

The pilot light is the blue flame that always stays lit under your boiler, and if it keeps turning off then your boiler is not working. The most common reason is a faulty thermocouple which is inhibiting the gas supply reaching the boiler; this draught may blow the pilot light out. The occurrence of the pilot light turning off is less common in new boilers and harder to reignite. It is worth checking that there are no issues with your gas supply before trying to reignite a pilot light. Be sure to check that your gas stopcock is on and if your boiler is receiving no gas, or if none of your other gas appliances are working you will need to contact your gas supplier. If you feel comfortable attempting to relight the pilot light yourself ensure you follow the instructions found in your boiler manual. However, we highly recommend contacting a Gas Safe engineer to help you rectify the issue accurately and safely.

8. An unreliable thermostat

If the thermostat you have installed is losing accuracy or is sporadically turning on and off incorrectly then it could be a good time to invest in a new one. Firstly though, you can check that your thermostat is in the ‘on’ position and that it is set correctly.

Another factor to consider is that your home might be warmer than you think; as a thermostat won't allow the boiler to heat your home higher than the temperature it has been set at. If so, then try increasing the temperature slightly until you're happy with the temperature in your home.

If you’re still experiencing problems with your thermostat, it could have malfunctioned or lost accuracy over time in which case it could be time to consider a replacement. Find more information on the range of thermostats we offer.

9. Leaking and dripping

A boiler leak could be caused by many different reasons. To determine the problem, the location of the leakage area will need to be identified but we recommend leaving this to a Gas Safe registered engineer due to the potential dangers of a leaking boiler. The most common cause is a broken internal component, such as a pump seal or pressure valve. If the leak is coming from the pressure valve, your boiler pressure could be too high. If the issue is caused by the pump seal, the seal could have aged and worn out in which case it could be a sensible decision to replace this.

If you have a leak around your pipes or tank then corrosion may have occurred, or the boiler may not have been fitted correctly. Please consult an engineer who will be able to diagnose the problem and help rectify the issue. However, in some instances where the damage is beyond repair, an engineer could recommend a new boiler.

10. Kettling

If you hear unusually loud, alarming noises from your boiler and pipes, then lime scale or sludge could have built up inside the system and have caused something called kettling. When these deposits build up in your system, they block the flow of water inside the heat exchanger which will then overheat and create these noises. Kettling is more common in areas with hard water, but can affect any area in worst cases. This may be a costly issue and could also shorten your boiler's lifespan if not treated quickly. Therefore, we advise you to call our team of experts as soon as you hear any unusual noise from your central heating. An engineer will be able to drain the system and remove the unwanted substances. For more information, see noisy boilers and pipes.


If you experience persistent cold patches in your radiators, contact us on 0203 805 4581 for advice on what to do next and if necessary we'll arrange for one of our Gas Safe engineers to visit you.

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[28] Exclusions apply. See www.help-link.co.uk/fixed-price-quote-faqs/