Eco Happy (Solar Expert)

Published March 9, 2024

The reasons that a boiler performs poorly or breaks down are varied. They range from frozen pipes and pressure problems to component failure and a gas supply cut. In this definitive guide, we’ll spotlight over 12 causes for a boiler not working and discuss the best solutions.

The good news is there are many boiler troubleshooting fixes you can handle yourself; you don’t always need to call a Gas Safe engineer and incur a cost. Read on for Eco Happy‘s valuable insights into boiler behaviour.

1. The Boiler Is Switched Off

We have to start with this basic troubleshooting chestnut – check that your boiler is turned on! Right now, there is a homeowner somewhere in the UK reporting that their boiler has mysteriously stopped working. And the reason? It isn’t switched on.

A local power cut is often the reason the boiler has ‘mysteriously’ switched off. Or it might have been turned off to unfreeze the condensate pipe or for a maintenance check.

Switching the boiler back on again will see normal service resumed quickly enough. You may need to check your settings and reapply your choices since power cuts sometimes disrupt the timings you’ve programmed.


Sometimes a boiler that’s not firing will benefit from a simple reset. Similar to a computer reboot, a boiler reset involves restarting the boiler to clear any faults or issues glitching the system.

The reset button should be located somewhere on the front control panel. Your boiler manual will indicate its location and explain any steps you need to follow to reset the system. If a reset doesn’t work, you could try switching the boiler off and on again.

If you find yourself having to reset your boiler often, you should have it checked by a qualified engineer as there is likely an underlying problem.

2. Power Supply Problems

If you lose electricity for any reason, don’t expect your electric boiler to be serving you hot water until the connection is restored (unless you have renewable power or a backup). Gas, LPG, and oil boilers are also affected. Almost all boilers need electricity to ignite. Boiler motors are typically electric and modern thermostats and controls are electronic too.

Losing power could be due to:

  • A power cut in your area.
  • A tripped circuit breaker or a blown fuse in the house.
  • A faulty switch or a loose wire in your boiler.


  • Check if your neighbours have electricity. If the outage affects the neighbourhood, hope your electricity company is on the case to restore power promptly.
  • Check your fuse box to see if any switches are tripped or if any fuses are blown. Reset tripped switches or replace the blown fuse.
  • Check your boiler for signs of damage or loose wires. Contact a qualified heating engineer to repair and reconnect.
  • Try resetting the boiler if it doesn’t start again when the power returns.

3. Gas Supply Problems

The gas supply to your gas boiler may be compromised for several reasons:

  • The gas company has switched off the supply because of an emergency, such as a power line collapse.
  • The outlet gas pressure may be too low.
  • A frozen meter sometimes obstructs the flow of gas, reducing the gas reaching your boiler.
  • If a gas pipe or filter isn’t installed properly, the gas may be contaminated with air and other impurities. This will prevent it from burning properly.
  • A faulty gas valve won’t allow enough gas for combustion.


  • Check you didn’t turn the gas supply off. 
  • See if other gas appliances in the home are working. If there is clearly no gas supply to the property, you’ll need to contact your supplier.
  • Check your gas meter to make sure it is not damaged. If the meter is frozen, you should thaw it carefully with warm (but not boiling) water or compress it with a hot towel.
  • Consider insulating your gas meter to keep it from freezing. A good way to do this is to lag it.
  • You may need to have a qualified engineer check the gas valve and confirm the boiler is securely and properly installed.

4. Thermostat Down

The modern thermostat is a fancy boiler control panel that allows you to manage all aspects of your central heating system – from setting desired temperatures to programming heating schedules and changing settings via your app as you head off for a weekend away.

If the thermostat isn’t working properly and doesn’t signal your boiler to turn on as required, your boiler effectively won’t work.

Typical thermostat niggles include:

  • Flat batteries in a battery-operated device.
  • No power to a wired thermostat.
  • Wi-Fi connectivity issues that prevent the thermostat from communicating with the boiler.
  • The thermostat might be damaged or faulty.


  • Ensure your thermostat has working batteries or is properly connected to power and that your Wi-Fi connection is sorted and sound.
  • It’s also worth double-checking your timer settings to see if the correct schedule is set and that it isn’t in ‘holiday’ or ‘standby’ mode.
  • In the event the thermostat is damaged, it will need to be repaired or replaced by a heating technician.

Note: You shouldn’t place your thermostat too close to a heat source such as a radiator or a fire because this can ‘confuse’ the thermostat. The heat source affects the ambient temperature the thermostat reads. If it feels it has quickly reached its set temperature, the thermostat may turn the heat on and off more frequently than necessary, causing the heating system to short cycle.

5. Frozen Condensate Pipe

The condensate pipe is a pipe that carries water containing impurities generated in the combustion process to a drain outside your house. In cold winter weather, the water inside the pipe may freeze and block the pipe, stopping the excess water from escaping. This prevents your boiler from working properly and should prompt an error message.

A frozen condensate pipe may cause your boiler to shut down.

Solutions – dealing with a frozen condensate pipe

  1. Turn off your boiler at its switch.
  2. Locate the condensate pipe and look for the section with ice or snow on it. Pour warm water over the frozen part. Holding a hot water bottle or wrapping hot towels around the pipe will also work.
  3. Once you have thoroughly thawed the pipe, turn on your boiler and reset it.
  4. Going forward, you may want to insulate the pipe in winter with lagging to reduce the risk of freezing. Be sure to check the pipe regularly and remove any snow or ice from it.

6. Pilot Light Not Burning Bright

While modern boilers have an electronic ignition system rather than a pilot light, a traditional pilot light still burns brightly in millions of old gas boilers across the country. If this light is out, your boiler’s ability to heat is snuffed out too.

The pilot light is a small flame that burns continuously and ignites the main burner to activate the central heating system. If the pilot light goes out, the boiler will not receive the signal to start heating. This can happen for several reasons, such as:

  • A strong draught that extinguishes it.
  • A dirty or damaged thermocouple – a device that senses the heat from the pilot light and sends a signal to the gas valve. This signal keeps the gas valve open and allows gas to flow to the burner and pilot light.
  • A faulty gas valve.
  • Low gas pressure.

If the pilot light goes out, the lack of signal from the thermocouple shuts off the gas valve to prevent leaks.


  • Relight the pilot light.
  • Check and, if necessary, replace the thermocouple.
  • Repair or replace a damaged gas valve.

Note: The above fixes should be done by a Gas Safe registered engineer (relighting the pilot light involves a few straightforward steps you could do yourself but you have to be careful. If the boiler is under warranty you want to be sure your DIY doesn’t void the warranty).

  • You can troubleshoot by checking the gas supply and the gas pressure and repressurising the boiler, if necessary.

7. Low Boiler Pressure

Low boiler pressure is a major cause of poor performance and can result in a boiler not working. When the pressure is too low, water flow is reduced and the boiler cannot heat enough water to serve the central heating needs. Expect an error code and possibly the boiler to shut down to prevent damage.

Operating at low pressure often or for long periods puts strain on seals, pipes and fittings, and contributes to corrosion. Sometimes, low pressure is caused by leaks in the boiler or heating system. It, in turn, can cause further leaks.

Low boiler pressure can also lead to inefficient combustion in the boiler, potentially producing dangerous carbon monoxide (CO) gas.


Boiler pressure should be between around 1.5 and 2 bar when the heating is on and between 1 and 1.5 bar when the boiler is off. Check your pressure gauge or digital display. If the pressure is below these levels, you need to increase it by adding more water to the system. You do this by using the filling loop – a flexible hose connecting the boiler to the cold water pipe. Follow these steps:

  1. Turn off the boiler and wait for it to cool down.
  2. Locate the filling loop which will have one or possibly two valves that you should open.
  3. Open the valves slowly and keep an eye on your pressure gauge. Listen out to hear water filling the system.
  4. When the pressure increases to the required level (between 1 and 1.5 bar), close the valves and remove the filling loop.
  5. Turn on the boiler and see that it works normally.

Repressurising your boiler is normally something you can do yourself if you’re comfortable with the process. There is no need to call a Gas Safe engineer or heating plumber.

If the low pressure is caused by a leak or a broken component, you should call an engineer out. A small leak repair that is easily accessible is not too expensive. Expect the cost to mount if the leak is under the floorboards.

8. High Boiler Pressure

High pressure is another problem that should never be ignored. In extreme cases, it can lead to the boiler rupturing.

High boiler pressure means the pressure is heading to 2.75 bar and over. A traditional boiler pressure gauge will show the needle in the red zone – a clear warning sign. Regular or continual high pressure damages the boiler’s parts and wastes energy. Consequences include leaks and cracks in the unit or pipes and increased running boiler repairs.

The problem is worsened if the boiler’s pressure relief valve is damaged. The pressure relief valve is a critical safety device that releases excess pressure by letting water and steam out of the system to prevent leaks and rupturing.

A damaged valve often makes unusual sounds such as hissing, popping, or whistling. If this safety valve is faulty and unable to lower the pressure, the entire heating system may shut off.


The usual first fix for high pressure is to bleed the radiators. Bleeding your radiators lowers the pressure by releasing air and some water from the system. Here’s how to bleed your radiators:

  1. Turn off the boiler and wait for your central heating system to cool down.
  2. Start with the radiator furthest from the boiler and use a radiator key or flat-head screwdriver to open the valve slightly.
  3. Have a bowl or towel ready to catch the water that comes out of the valve. Once water rises and exits the valve, close it and move on to the next radiator.
  4. After bleeding all your radiators, check that the pressure gauge shows between 1 and 1.5 bars. If it is too low, you may need to add some water using the filling loop.

If a radiator bleed doesn’t resolve the problem, you’ll want a professional to check that the relief valve isn’t damaged, loose, or leaking.

9. Diverter Valve Problem

Do you have central heating but no hot water? Or hot water but no heating? This sounds like the classic stuck diverter valve problem.

The diverter valve is the switching mechanism in a combi boiler that directs the flow of hot water to different parts of the central heating system, depending on demand. A working diverter valve ensures that you enjoy both a warm shower and toasty radiators from one compact combi unit.

A problem arises when a faulty valve can’t divert heat efficiently or gets stuck in either heating or hot water mode. The result is that you sit without hot water or radiator heat, depending on how it is stuck.

This is obviously not a problem in conventional heat-only boilers that store hot water in a separate cylinder.


You may well need to replace the diverter valve or the cartridge inside it. This is not a simple DIY job as it involves working with gas and water pipes. It’s best to call in a qualified heating engineer or plumber/electrician.

10. Printed Circuit Board Malfunction

The Printed Circuit Board (PCB) is the boiler’s central control unit. It is the brain that drives and regulates the boiler’s safe, effective functioning. A malfunctioning PCB will throw up all kinds of grief, including:

  • No power or display on the boiler panel.
  • Intermittent or erratic operation.
  • A burning smell or smoke from the boiler.
  • Boiler error codes; boiler lockout.
  • Loss of heating or hot water.


It may be possible to repair your boiler PCB but more often than not a replacement is needed. Replacing a PCB requires professional installation and testing due to its sensitivity and critical function. Depending on model and make, the cost of replacement and installation can easily exceed £500.

In some cases, it’s worth considering investing in a new boiler instead, especially if your boiler is out of warranty and has been giving you problems over and above the PCB issue.

11. Damaged Heat Exchanger

Gas and oil boilers feature a heat exchanger. This is the system that transfers the heat produced by burning fuel (gas, oil, LPG) to the water circulating within the boiler. The heat exchanger is a highly critical component in the process. If it’s not operating as it should, the system suffers.

The heat exchanger will accumulate debris or sludge over time if not well maintained. Limescale also builds up on the heat exchanger, especially in hard water areas. Banging or whistling noises are usually telltale signs that your exchanger is clogged up.

Common consequences of a bad exchanger are:

  • Loss of heating or hot water.
  • Leaking water or steam from the boiler or pipes.
  • Banging, clanging, or hissing boiler noises.
  • Higher energy bills due to inefficient heat delivery.
  • There is even a risk of carbon monoxide (CO) poisoning. A working heat exchanger contains the combustion gases, including carbon monoxide, and vents them out of the home via the flue. A damaged heat exchanger may allow carbon monoxide to escape into the house.


  • Sludge and limescale buildup on a heat exchanger can be fixed by power flushing the system or descaling the boiler. It’s best to leave these flushes to an expert.
  • When your heat exchanger is badly damaged, corroded, or cracked, replacement is usually the only option. This isn’t cheap and the cost generally ranges from £400 to over £1,000 with installation. Here, you may need to think about a brand-new boiler rather than simply a new part.

12. Other Key Boiler Components

Besides the heat exchanger, the following major components will also seriously compromise performance if they are damaged or faulty.

  • Burner: The burner ignites the fuel to generate heat in the boiler. A problematic burner means your boiler is not firing on all cylinders.
  • Pump: The pump circulates water throughout the system. Expect cold spots and poor heating if the pump isn’t doing its job. A bad pump may make grinding or vibrating noises.
  • Motor: The motor powers the pump and other moving components and ensures the boiler’s smooth functioning. A bad motor which may also make grinding noises could shut down the pump and other working parts.
  • Expansion vessel: The expansion vessel absorbs excess pressure in the system, preventing damage due to pressure strain. When this safety vessel isn’t working, excessive pressure may build up placing tremendous strain on the entire setup.

Replacement of any of these parts will cost a few hundred pounds (over £500 for a burner) and installation should be handled by someone certified.


Can trapped air cause my boiler to stop working?

Trapped air in a boiler system can lead to complications such as reduced heating efficiency, uneven heating in radiators, gurgling or banging noises, and unhealthy fluctuations in pressure. While trapped air doesn’t usually shut down the boiler, it will negatively affect the overall performance of the central heating system if not addressed. Bleeding the radiators is the go-to solution to release air.

How can I prevent common boiler problems in the future?

Regular boiler maintenance – such as annual servicing by a qualified technician, bleeding radiators to remove trapped air, avoiding a blockage of your condensate pipe, and monitoring and managing your boiler’s pressure – will help avoid common issues and deliver a happy heating experience.

What could be causing my boiler to short cycle?

Short cycling refers to the boiler turning on and off frequently and rapidly. Short cycling may result from a faulty thermostat, inadequate water flow, or a damaged pressure sensor. Have a technician inspect the affected components and repair them if necessary.


You will appreciate that there are numerous potential boiler problems, and just as many ways to resolve them. Some fixes are simple (yes, as simple as turning the boiler back on!). Familiarising yourself with your boiler manual and boiler error codes is always a good idea. Some problems involve a complex assessment of crucial parts by a Gas Safe registered engineer or a certified electrician.

You may be faced with a pricey decision. Do I replace my heat exchanger or should I invest in a new boiler? Getting advice from your engineer and other heating experts is key. If you need personalised guidance or quotes, Eco Happy is here to help. Contact Eco Happy to chat about solutions that are right for you.

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