How to Diagnose Boiler Issues and Problems

Boiler problems can be a total nightmare and almost always involve having to call out a plumber or heating engineer at great cost.

As pretty much all boilers use gas as a fuel (some use oil) you are not allowed to work on them on a DIY basis due to the obvious dangers as you won’t know what you’re doing, you can only work on a boiler if you have the necessary qualifications e.g. you are a Gas Safe registered engineer.

Despite this there are a few issues that you can resolve yourself, the most common of these can be found below with their corresponding fixes.

Diagnosing Problems With Your Boiler

Before you can actually fix anything you first need to diagnose the cause of the issue. Use the below scenarios to identify which relates to your particular issue.

Low Boiler Pressure

If you have ever inspected your boiler you may have noticed that it has a pressure gauge, this will either be a circular analogue gauge with a needle that points to the pressure the boiler is currently at or a digital gauge displaying the boiler pressure in bar.

The pressure reading that’s displayed indicates the current pressure of the hot water in the central heating system.

For the system to work correctly the pressure needs to be at least 1 bar and ideally around 1.5 bar but this can depend on the type of boiler (manufacturers instructions will state ideal operating pressures).

If however the system drops below 1 bar of pressure then it simply won’t work correctly and will either shut down or display an error on the LCD screen (if there is one).

Boilers and heating systems naturally loose a bit of pressure over time but if it loses a lot of pressure in a very short space of time then this indicates a sizable issue. If this is the case for you then you will need to contact a Gas Safe heating engineer, do not attempt to investigate your boiler yourself.

If your boiler has slowly lost pressure over time, this is generally due to 2 reasons:

• Leak: A very slight leak in your heating system is extremely hard to spot, especially if you have pipework running under floors and through walls. Unless there is a telltale damp patch somewhere the only way to tell is a gradual loss of pressure in the system over time
• Radiator Bleeding: If you have had to bleed one or more radiators then the air that you bled out of the system will leave quite a gap that can cause the water level and subsequently the pressure to drop

If your pressure has dropped due to a leak then you will need to call out a Gas Safe heating engineer to go through your system and find it, but in the mean time and also if you have just bled your radiators then you will need to get some pressure back into your system by adding some water.

Most central heating these days is provided by combi-boiler and to add water to it this needs to be done via the filling loop as follows:

1. Locate the filling loop. This will normally be just underneath the boiler and appear as a flexi pipe between 2 sets of copper pipes
2. First, open the valve on main water supply pipe. This will be a valve on the larger 22mm copper pipe
3. Next, slowly open the second valve and you should start to hear water rushing into the boiler
4. Keep a very close eye on the pressure gauge on the boiler and fill until the gauge reads at least 1 bar
5. Once re-pressurised turn both valves off

Leaking / Loss of Water in the System

If you have identified a leak in your heating system this will generally either be from the pipework or from the boiler itself.

If there is water leaking from any pipework joints then you can try gently tightening the joint (if it’s a compression joint) to see if this will stop the leak, but don’t tighten too much as this can make it worse.

If any pipework or soldered joints are leaking then they will need to be replaced. If you are a competent DIY’er then you can drain down your system and make the repairs yourself, see our project on repairing a burst pipe and also see our project on how to solder pipe joints for help.

If you’re not then the best option is to call in a heating engineer to look at it for you.

If there is water visually leaking from your boiler this will most likely be due to an internal fault with the boiler, the most common being either the pressure release valve or PRV or some kind of seal.

The pressure release valve or PRV prevents any pressure in the system building to the point that it explodes as obviously this could be potentially lethal.

If pressure in the system starts to build, the valve will open and release the pressure and then close once it’s dropped to a safe level.

Within your boiler there are various rubber seals to prevent leaks. Over time these seals can become brittle and worn and then fail, causing leaks.

In either case, unfortunately fixes such as these cannot be done on a DIY basis and you will need to call in a professional Gas Safe heating engineer.

Condensate Pipe Blocked or Frozen

If you have a modern condensing combi boiler then it will feature a condensate pipe that runs from the boiler to a nearby drainage point.

To make condensing combi boilers more efficient, the heat contained in water vapour that would normally be vented away is extracted and reused and the waste water from this process is then piped away to a drain.

If this condensate pipe becomes blocked or in colder weather, freezing in the pipe causes a blockage, the boiler will detect this and throw an error and stop working.

To fix this issue you will need to remove the blockage from the pipe or if it has frozen, use hot water and pour it over the pipe to thaw it out. Don’t use boiling water as this can damage the pipe!

As a long term fix for this issue ensure that you lag your condensate pipe properly. For help on how to do this see out lagging pipework project here.

Room Thermostat not Affecting Boiler

The room thermostat for your boiler monitors the temperature in your home and turns your boiler on and off according to the temperature it’s set to. If your thermostat is set to 20°C and the area your thermostat is in drops below this, the thermostat will detect this and then signal your boiler to turn on heat it back up again.

Once the thermostats detects that the room is back up to the correct temperature again it will then tell the boiler to turn off.

If your thermostat develops a fault and will no longer communicate with your boiler then the above process won’t happen and in pretty much all cases your heating won’t turn on.

In the majority of cases when a standard room thermostat stops working, this is due to the fact they have just come to the end of their life and need replacing. This can be done on a DIY basis, but if you are unsure then call in a Gas Safe heating engineer.

In recent years with the drive to be more energy efficient smart thermostats have come on to the market. These clever devices connect to the internet and allow you to turn your heating on, off, up and down when you’re not even there!

As smart thermostats are wireless devices, most are portable and due to this run off of batteries, so if yours has developed a fault and is not communicating with your boiler, first check the batteries and ensure they are charged. If not replace them or plug in the mains adaptor that should have come with your thermostat to recharge them.

Once charged and the thermostat has power it should then spring into life again and start communicating with your boiler.

Additionally, as we have mentioned, smart thermostats need an internet connection to work so if they become disconnected then they will stop working. If your thermostat is no longer connected to your home router then reconnect it and it should then start communicating with your boiler again.

Heating / Hot Water not Working at all

In extreme instances, boilers can suffer a full shut down resulting in no heat or hot water at all. Although these occurrences are rare they do happen and when they do it can be very frustrating!

If this happens to you, work through the following checklist to diagnose the fault and resolve it:

• Boiler Errors: If you have a modern combi boiler then it should have an LCD screen on the front of it. If there is an issue with the boiler itself, then it will be displaying an error message or code on the screen. To resolve this you will need to contact a Gas Safe boiler engineer
• Power: Does the boiler have power and is turned on? If not check the plug socket it’s connected to and make sure it’s on (if not, turn it on) and then check the consumer unit and ensure nothings tripped. If it has then you will need to call an electrician
• Condensate Pipe: Is it particularly cold or does your condensate pipe have a blockage? See the condensate section above for tips on resolving this issue
• Fuel: Is your boiler getting fuel? If your gas supplier has cut supply or they are having an issue then the required gas won’t be reaching your boiler. Contact your supplier and check. If your boiler uses oil then check your oil tank. If it has run out or there is a blockage in the supply pipe you will need to call a registered Oftec engineer
• Low Pressure: If your boiler is having pressure issues then in most cases it will not function correctly until this is resolved. See the section above on “Low Boiler Pressure” for tips on how to resolve this
• Room Thermostat: If your room thermostat is having issues or if it is set too high, then your heating won’t work correctly. See the “Room Thermostat not Affecting Boiler” section above for tips on how to resolve these issues
• Pilot Light: If you have an old boiler then it will feature a pilot light that provides an ignition point for the gas the boiler uses. If this goes out then your boiler won’t be able to heat anything. See the section below on “No Pilot Light on Boiler or Won’t Stay Lit” to find out how to resolve this

If your boiler is not producing heat and therefore not running your heating or hot water and none of the above points have resolved or narrowed down the cause then you will need to call a registered Gas Safe engineer, do not be tempted to try and repair your boiler yourself!

Additionally we also have a project guide on central heating faults that may also help to resolve issues with your heating and hot water.

Hot Water and no Heating

If you have an issue where your boiler is producing hot water but you have no heating then this can be due to a few issues.

As we have discussed in the points above, if the pressure has dropped in your heating system then it won’t run correctly. To get it up and running again you will need to re-pressurise it by adding water via the filling loop. See the section above on low pressure for tips on how to do this.

As we have also discussed above, if your thermostat is set too high for the temperature of the room or it has a fault then it won’t tell the boiler to switch the heating on. Again, see the section above on room thermostats for information on resolving these issues.

If some or all of your radiators are cold or parts of them are only lukewarm then a potential cause of these issues is air locks in the system, lack of system balancing or rust and sludge buildup.

To solve any air locks you will need to bleed your radiators (see our bleeding radiators project here), to solve issues with balancing you will need to either turn down the hotter radiators or balance your system (see our balancing heating systems project here) and for sludge buildup you will need to flush your system (see our power flushing heating systems project here).

One final potential cause could be that your diverter valve could be faulty and needs to be replaced.

If none of these seem to be a factor then there may be something more serious going on in the internals of your boiler such as a fault with a diverter valve, motorised valve, pump or another of the internal components so this will need to be checked by a Gas Safe (gas)/Oftec (oil) engineer.

“Kettle” Noise Coming From Boiler

Kettling is a phenomenon that is more often than not experienced in hard water areas and is where limescale buldsup around your boilers heat exchanger (the bit in the boiler that heats the water).

Over time, the lime deposits in hard water collect on the heat exchanger and also around the inside of your heating system, clogging everything up and also contributing to sludge buildup in your system.

When it is allowed to buildup on these important components it eventually restricts the flow of water causing it to sit too long and eventually boil making it sound like a boiling kettle.

If you are experiencing issues such as these then it’s important to get them resolved as soon as possible as they can severely damage your boiler and your system as a whole.

In terms of a fix, you will need to get your system fully flushed out to remove all of the deposits and if the heat exchanger is too badly coated then this may also need to be replaced, but this should only be done by a qualified Gas Safe or Oftec engineer.

Noises Coming From Boiler

Noises from your heating system are common as you may have noticed if you have one in your home. When the boiler fires up it makes a noise, when your pipework heats up and cools down it makes a noise and so on, so some noises are just part and parcel with the operation of a central heating.

However in some cases other noises can occur that may not be as innocent, some of these are as follows:

Gurgling

When listening to your pipework and radiators and you hear a gurgling sound this is usually the sign of air in your system.

Air locks can cause issues with radiators not warming up correctly and also damage internal elements of your boiler so they need to be removed. This can be done by bleeding your radiators.

Humming

As mentioned above, when your boiler starts up it will make a humming kind of noise with some more noisy than others. If you have a silent running boiler and it starts to sound louder and louder over time then this may be the sign that an internal component is failing.

Typically the pump in your boiler is the item that makes a humming noise so if this is getting worse it may be a sign that the pump is failing.

If this is the case then you will need to get a qualified Gas Safe heating engineer to take a look.

Buzzing

Buzzing noises are normally related to electrical components, so if your boiler is making a buzzing sound this may be the sign of something failing

Again you will need to call out a qualified heating engineer to rectify the issue.

Clanking or Banging

Clanking and banging, as you may imagine, is not a good sign and is almost sure to be a fairly major internal issue and the sign of a failing component, so you will need to get it checked by a Gas Safe or Oftec (if you have oil-fired central heating) engineer.

However in some instances, it could be the sign of an object that’s come loose such as a pipe and is banging around so it’s worth checking your pipework over just to be sure.

Whistling

A whistling noise in your boiler will probably be due to “kettling” and caused by buildup around the heat exchanger.

See the section above on “Kettle Noise Coming From Boiler” for full information and a fix.

Faults With Diverter Valves

Modern combi boilers heat water on demand e.g. there is no immersion tank or hot water store, the boilers heats the water as it’s required.

The diverter valve in your boiler controls where this hot water is required to go, either to your taps or shower or to the radiators.

If your boilers diverter valve is having issues then you will most likely experience one of the following issues:

• Hot water only slightly lukewarm
• Hot taps only working if central heating on
• Hot taps work but no central heating

If you are experiencing any of these then you will need to call out a heating engineer to confirm and replace the faulty valve.

Boiler Constantly Turning off

In some instances you may find that your boiler won’t stay turned on and just keeps shutting itself off.

This could simply be down to the thermostat setting and the fact the heating coming on and heating the room up to temperature and the thermostat is then shutting the boiler off again. Turn the thermostat up or down and see if this has an effect.

Another cause for your boiler shutting off constantly is that you may have pressure issues. Check your boiler pressure gauge and ensure that it’s above the required 1 bar pressure. If not, add some more water to the system via the filling loop.

One other cause might be the fact that there is a huge amount of rust and sludge buildup in the system and it is simply preventing the correct flow of water around the system and thus your boiler is shutting off. If this is the case then you will need to power flush your entire system.

Likewise if you have huge amounts of air in your heating system this can prevent water flowing correctly and again the boiler will shut down because of this. Bleed each radiator and see how much, if any, air comes out.

Finally if none of the above seem to be the cause it may be that the pump or other internal component such as a valve has failed or is failing and water cannot be pumped correctly. If this is the case this will need to be rectified by a heating engineer.

No Pilot Light on Boiler or Won’t Stay Lit

Some older boilers have a pilot light. This is a small permanently lit flame that is used to light the boilers main burner when it’s needed to heat water.

If this pilot light won’t stay lit then the main burner won’t ignite when hot water is required.

There are several reasons why a pilot light won’t stay lit and these can range from a faulty thermocouple, a safety device that shuts off the boilers gas valve when the pilot light goes out.

Other causes can be the pilot light system itself failing and an excess of buildup causing the flame to go out.

It is certainly possible to re-light the pilot light in your boiler yourself, your boiler’s instruction manual will contain instructions on how to do so, but if it keeps going out or you’re not confident in doing so you will need to call out a Gas Safe engineer.

Boiler problems and issues can be a real pain, especially if you have a busy house and you’re left with no heating or hot water.

Some boiler issues can be easily overcome yourself however this can be fairly daunting to do if you’ve never done it so in these instances always call out a qualified Gas Safe heating engineer.