Top 13 Common Boiler problems and what to do about them


Thirteen is an unlucky number…for some! With that in mind let’s look briefly at some of Top 13 Common Boiler problems, and where to go to begin troubleshooting and fixing your problem if you don’t have boiler cover insurance

Problem 1: Boilers that turn themselves off

Is your boiler switching itself off every few seconds or every few minutes? If so, most technicians will point out that boilers are designed with a number of fail-safes that begin to work in particular situations. This is what they call boiler lockout.

If your boiler seems to be switching itself off, it’s likely due to one of the following reasons:

  1. The boiler is losing pressure.
  2. The boiler is low on pressure.
  3. The thermostat is not working
  4. The pump is leaking or broken.
  5. Valves are shut preventing water into the system.

Fortunately, the repair is an option for each, rather than replacement. Diagnostic checks for each must be conducted. The first two checks can usually be competently performed by the everyday person, though the last three will probably need someone trained and certified.

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Problem 2: Boiler has a pressure loss

Correct boiler pressure is vital for normal functioning. While this is not an in-depth guide to the causes behind boilers losing pressure.

Always remember a boiler cannot lose pressure without any leaks. Leaks, however small, do cause pressure loss. If the needle on the pressure gauge consistently drops overnight, it is likely to be one of the following reasons:

  1. The pressure relief valve (PRV) is not working correctly.
  2. The auto air vent is leaking pressure.
  3. There are leaks in your home overall. This could be on towel rails, radiators, radiator valves or the pipe themselves
  4. The radiators are full of air.
  5. The expansion vessel is broken
  6. Old joints have rusting and are leaking.
  7. The pressure gauge on the boiler isn’t working correctly.

If the boiler pressure loss is only when the heating is on, this a hallmark of a small leak. The added pressure caused by increased heat places an extra burden on the leaking component. If the boiler appears to lose pressure even when the heating and boiler is off, this is an indication of larger leak.

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Problem 3: Leaky Boilers

Most of the time, boiler leaks are negligible and will not be likely to cause a great deal of damage. However, even small leaks can result in the corrosion of parts leading to costly repairs. This is the case if the water gets into the boiler PCB which can be up to £500 to fix. If this is the case, sometimes the cost-optimal option is for a complete boiler replacement.

Most boiler problems that lead to leaks have been listed below:

  1. Expansion vessel diaphragm has worn away.
  2. Cracked heat exchanger.
  3. The heating pump is insecurely fastened.
  4. The heating pump has lost its seals or gaskets.
  5. Boiler leaks when it rains, because water is entering via the flue.
  6. The filling loop is leaking.
  7. Pipework connected to the boiler is soldered/installed incorrectly.
  8. The pressure release valve has stopped working
  9. Corrosion of internal parts which are now leaking.


Problem 4: Noisy Boilers

As parts degrade, some boilers develop faults that result in the sound of  banging or gurgling noises for those who can have live with them.

The main reason that noises happen are central heating pumps — caused by a build-up of air which results in an airlock. This can be easily rectified. An airlock can be removed from a central heating pump which is housed where a pump is found, behind the boiler casing. Here will be a tiny bleed screw that is designed to let air out. Once the air stops hissing and trickle of water comes from the bleed screw the airlock should no longer be a problem.

There are other causes of noisy boilers including limescale build-up, central heating sludge, low water flow rate, or covered, blocked or faulty heat exchangers. We recommend researching your specific sound in more detail.

There are many everyday actions you can perform to increase the longevity of your boiler and reduce boiler noises that include tapping, banging and humming:

  1. Add a central heating inhibitor, a scale reducer in hard water areas and a boiler filter;
  2. A heating engineer can perform a hot-flush, or power flush on your heating system;
  3. Check the boiler for visible leaks with the naked eye. Also check that operating pressure and water flow rate is in the normal range;
  4. Remove airlocks in the boiler’s heating pump;
  5. Inspect the heat exchanger for limescale build up;
  6. Test the ignition leads and probe to see if they are working properly;
  7. Assess whether the boiler pump is seized

Problem 5: Fire up my Boiler, it makes a loud bang

If your boiler makes a sound, not unlike a small explosion chances are it is a small explosion. Listen just before your boiler starts up. When you turn the heating on, the boiler will try to ignite. If it fails to ignite after the third attempt, it will likely lockout. However, if the boiler happens to fire on the second or third time round, there will be excess gas in the chamber. The ignition is now lighting too much gas. The solution is usually a new ignition burner as they usually wear out and break over time.


Problem 6: Hot Water fails to work but the heating does (or the other way around)


The diverter valve is usually the cause here. This component which allocates hot water to your taps/showers, as well as your heating will divert the most water towards taps and showers. Once these are off, the valve redirects all hot water to the heating.

As all physical components are known to do, the valve is known to get rigid over time, partly because of debris and sludge accumulation. When this happens, it will either stick on water or the heating side

If it sticks on the heating side, the result will only be tepid water at best. And, if it sticks on the hot water side, the result will be tepid heating at best.


There are many ways to troubleshoot a diverter valve, which you can find online. However, as the unit has already seen a lot of wear, it is absolutely a sage option to replace it in full.

Problem 7: No Heating or Hot Water

Most central heating systems will use a boiler that has a motoric control valve, otherwise known as mid-position or Y-plan valve. Valves are prone to get stuck and stop working with time. If you have no hot water or heating, then this could be the source of the trouble. Have a trained plumber check that the valves are working as they should.

Problem 8: The Boiler seems to work but the Radiators won’t heat up?

If your boiler is igniting then that’s at least one indication that things are working as they should. However, if radiators are not heating up at all, there’s a few things that worth looking for.

Firstly, old pumps are likely to seize up. When this happens, hot water will not be directed to any radiators, taps or showers. Giving the pump a gentle tap can sometimes be enough to make it again. However, it’s likely that it will need replacement soon enough. Before replacing the heating pump, it is worth bleeding all radiators and towel rails since pockets of air all around heating system can stop normal heating functioning.

If they radiators are hot at the bottom and cold at the bottom, there’s a good chance heating sludge is the issue. This will require intensive cleaning or replacement by a specialist.

Problem 9: Water is burning

If water from the taps is too hot, then this is likely to be a thermostat issue. The thermostat measures the water as much cooler than it is. If your central heating system uses a hot water tank or cylinder, the cylinder thermostat is likely to be what is at issue

A new thermostat will fix this. However, if the water is coming out far too hot from your shower, the thermostat probably is not at fault. Cartridges in showers are prone to failing. If they have failed while on a hot setting, it is likely that it is stuck here. Replacement of the shower cartridge will be in order.

Problem 10: Boiler lockouts and condensate issues


If the issue is boiler problem is lockouts, the boiler manual should be able to tell you what the meaning of the fault code on the PCB display is all about.

Many boiler lockouts are to with condensate blockages. All boilers are now almost always condensing. This means catch all the acidic water and waste gas that the boiler produces. This is then diverted to the draining outside, so no harmful gases are inside.

However, condensate pipe that takes the water and gases out is often narrow. This means it will often freeze in the winter prone to freezing or develop small blockages at other times of the year.

A frozen pipe is not hard to unblock. Just pour over hot water over the pipe until it thaws. If it is just a small blockage, simple removal of the pipe with a flush out of hot water through out, and then reinstallation, should do it.

If this all fails, all plumbing merchants stock condensate pipes. A replacement should do it.

Problem 11: Radiators are Cold at The Bottom, Middle or Top

Sludge build-up in the central heating system will damage all parts including pumps, heat exchangers and the radiators. If the radiator is cool but the pipes are not, even after bleeding, sludge is likely the issue

There are a few solutions:

  1. Hot flush (run chemicals through the system at normal flow rate and pressure).
  2. Power flush (a pressurised hot flush).
  3. Replacement of radiators that have sludge build-up.

Once the radiators have been fixed, it’s important to install a boiler sludge filter (Magnaclean and Fernox are two respected brands of sludge filtration).

Problem 12: Boiler Controls are Not Working Correctly

This can be a thermostat, mechanical timer, or frost thermostat. Boiler controls are likely to be the issue if:

  1. The boiler turns on and off when it is not meant to.
  2. The temperature seems hotter or colder that is indicated by the reading on the thermostat.
  3. Thermostat is not clicking in or out
  4. Thermostat or mechanical timer makes a buzzing sound.
  5. Radiator valves are not increasing or decreasing temperature according to their setting.
  6. Water from taps is too hot.

In many cases, if there is not wireless or internet thermostat/programmer it is more cost-optimal to opt for whole replacement the boiler controls.

Problem 13: Pilot light is on and off

If your boiler problem is a pilot light that keeps going out, phone a Gas Engineer as soon as possible. In some cases, a faulty pilot light could leave you in mortal danger if your gas is leaking.

The gas supply could be at fault. If a gas valve has been turned off in a property because it was vacant, there would be no gas to the boiler. Carbon would build up over time and would block the pilot light.

However, if it is windy outside, gusts could be entering the flue. This could be causing the pilot light to go out. Finally, the thermocouple might be faulty.


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