🕑 8 minutes read

Common Boiler Problems

A fully functioning gas boiler is an essential item in every household especially when the cold weather is upon us. A boiler operating effectively during the winter months satisfies one of our basic physiological needs for survival, which is warmth (assuming there’s sufficient water, food, and shelter). If winter is nearing and your boiler has been serviced regularly it will be well placed to operate for extended periods and keep everyone warm and cozy.

If you’ve missed your Annual Boiler Service there could be an underlying fault just waiting to happen? When it does, it always happens when you least expect or want it – in winter! If your boiler is 10 – 15 years old chances are that it is not as efficient as a new more energy-efficient one. That’s not to say your boiler needs changing, definitely not, as it still caters superbly to your heating and hot water needs.

However, the running costs might be a tad expensive? Added to this is the pressure from the boiler manufacturers who advise that a less efficient boiler should be replaced with one that is, could be a strong influencer, even though there’s nothing mechanically wrong with it. Here are some of the common boiler faults we have experienced.

Pump failure

The function of a central heating pump is to circulate hot water around the system through the radiators and hot water cylinder (unless you have a combi boiler that does away with the cylinder). The lifespan of a pump can be years. However, you will notice certain irregularities with its performance when it’s on its way out:

  • When the pump is on but not running.
  • The pump is running but not pumping - the impeller is jammed/corroded/broken.
  • Noises of squeaking of grumbling coming from the bearings.
  • Rattling sounds etc.

A healthy pump is silent when running, so when you recognize any of the above it’s an indication that the pump has or is about to fail. The system cannot operate without a functioning pump and needs to be replaced ASAP

Circuit Board

The circuit board is another crucial part of modern boilers today. Their performance can be impaired by heat exposure, dust, moisture, or power surges (which are common) weak soldered joints, cracked joints, etc. Without a functioning circuit board, it is almost impossible to detect any other faults within a boiler. Once a circuit board has been replaced the boiler can be fired up and a diagnostic check is undertaken.

It is a misconception that replacing the circuit board will cure all problems – IT WON’T! It will flag up other parts that have malfunctioned and need to be replaced. The cost of any replacement part/s will be in addition to the cost of the circuit board. Sorry. An error code may appear in the window of the boiler indicating the cause of the problem. The display could be ‘blinking’, which again indicates that something could be amiss with the electronics of the boiler i.e. the circuit board.

Pump seals

Any part incorporating a seal will, in the fullness of time, wear of perishing. Reassembling apart and using the ‘old’ seals presents a high risk of leaks. For example, when a magnetic filter is serviced the seals should always be replaced with new ones or you will risk a leak if the old ones are recycled. Any leaking or dripping beneath the boiler is a sign of a lost connection or worn seal.

Faulty boiler fan

This is another important part of the boiler. Its function is to force all harmful gasses out of the property through the flue. If the fan is unable to perform its job the circuit board will stop the boiler from turning on. If you hear a buzzing, whirring, or whining noise coming from the boiler it’s a concern and could be the pump bearings, a broken burner, or a vibrating fan.

Unfortunately, these are not jobs for the keen DIYer rather a qualified professional boiler repair engineer. Call serviceteam Ltd if you need professional help on 0203 918 6911.

Motorised valve

This particular part is responsible for directing the water in the central heating systems. The device (when instructed) will give hot water or heating separately of both at the same time depending on the circumstance.

For example, in the middle of a hot summer, central heating is not required, but hot water is. By adjusting the thermostat you can pre-set the thermostat, which in turn governs the motorized valve. There are a few checks you can undertake to trace a fault back to the motorized valve:

  • You may have heat only but no hot water or vice versa
  • You may have neither
  • During the summer when the heating is off, you may notice a couple of radiators warming up - this is a sign that the valve is letting by.
  • In all of the above, the motorized valve will need replacing or repairing (if possible).

A motorized valve has moving parts and subject to wear and tear as it is working constantly and will not, unfortunately, last forever. To replace a motorized valve will require a drain down of the system.

Leaks or drips

One of the most common issues with boilers is water leakage. Although the reasons may vary, it is safe to say it does not cause any hazardous problems.

An easy way to identify leakage or dripping is to look around the pipe joints and the tank for any signs of water or limescale deposits. It may be due to a perished or damaged seal or a loose connection?


Kettling is a phenomenon that usually occurs in hard water. In layman’s terms, it’s similar to the noise made when water boils inside a kettle. When a kettle comes to the boil it is quite loud and can sometimes drown out conversation. There are ways you can stop kettling but it’s not for the DIYer rather the qualified gas engineer. Kettling is caused by water pausing for longer over the heat exchanger and boiling it. The easiest way to counteract kettling is with a solution of Boiler Noise Reducer. It stops a noisy boiler and reduces pipework vibration.

Fernox has a decent range of chemicals from inhibitors to boiler cleaners and is well worth trying this cheap method first before attempting any expensive repairs. With a central heating system (not a combi system) there will be a central heating header tank in the loft, this is the small one and not the large tank, which is the cold water supply. Isolate the water to the property. Then drain down the system and slacken the bleed valves on each of the radiators.

When the header tank is empty, pour in the complete contents of the chemical. Tighten the bleed valves and turn on the water at the isolation stop cock to refill the system When the system is filled turn on the boiler and bleed each radiator in turn. Leave it running for a couple of hours to circulate the system after which the boiler should be much quieter

Pilot light

Occasionally the pilot flame will go out. If your boiler is fitted to an external wall and there is a draught it can extinguish the pilot flame. It is a straightforward process to ignite it. Just follow the manufacturer’s instructions.

If the pilot still won’t ignite or goes out when the gas is on, after you release the gas control knob (after 20 to 30 seconds) then it points to a thermocouple problem. Contact serviceteam Ltd and a qualified gas engineer will attend to rectify the problem. Again, it’s not a job for the keen DIYer only a gas registered engineer.

Frozen Condensate Pipe

This pipe drains any excess water produced from the condensing process in your boiler. The water produced is somewhat acidic (as it’s a byproduct of the condensed waste gasses) and needs to be exhausted through a PVC or an ABS (Acrylonitrile butadiene styrene) pipe. A typical problem, especially in winter, is when the pipe freezes.

When this happens the ice forms a plug and the condensate byproduct backfills the waste to the boiler. The blockage is detected by a sensor in the boiler and shuts it down (for safety reasons).


To prevent freezing a condensate trace heater can be installed. There a few external kits available that provide protection for your condensate pipe. The heater is connected to your boiler with the cable running the length of the external condensate pipe. The cable is fixed in position along the length of the pipe and insulated.

When the external temperature drops below a certain temperature the system activates and warms the pipe stopping it from freezing. When the temperature rises it deactivates. This will ensure no interruptions to your heating during the winter months.

Faulty thermostat

The thermostat is an important part of controlling heat and hot water. When set correctly to meet the needs of the family, it should avoid any energy going to waste. For example, shutting off the heating when your away for the weekend. Or turning on the heating in the winter when the property is empty for a few days to prevent pipes from freezing. Most of this is done remotely now thanks to an App on your mobile phone. First, some simple checks to make sure the thermostat is working correctly.

If your thermostat is battery-operated make sure the batteries are fully charged. Once the batteries are connected it will attempt to link up with its wireless receiver. A thermostat works on a pre-set temperature which the thermostat actually senses. If it doesn’t seem to be coming on just raise your desired temperature to just above the actual temperature – your central heating should then come on. A thermostat’s effectiveness is based on the way you set it up.

New thermostats can be pre-set for individual days and individual temperatures. In effect, you could program your temperature preference for 7 days.

When a room overheats

This is when a thermostat cannot determine when to turn off the heat and the room gets as hot as a sauna. This problem leaves you with only two options – turn the heating off or leave it full on? The thermostat’s job is to detect the temperature in a room and decide if it needs to tell the boiler to fire up to give more heat or switch off when the pre-set temperature has been reached.

A quick test to check if the fault is with the thermostat:
Does the boiler go off when you dial down the boiler thermostat, if so the room thermostat could be the problem? Likewise, if the boiler doesn’t respond when using the room thermostat to adjust the heating, it points to a problem with the thermostat. The thermostat has stopped messaging the motorized valve telling it to heat up or shut off. In this case, you will need to have it replaced.


Even uniform heat output from the radiators is a sign they are functioning correctly. However, over time sludge can build in the system causing cold spots resulting in uneven heat output. This indicates that the circulating water is restricted from heating radiators uniformly. In some severe cases, the bottom of the radiator is hot but the top cold. Your boiler has to work harder, stay on longer to produce less than adequate heat from the radiators.

If you are experiencing high energy bills compared to bills in the past (energy price rises considered) then it could point to sludge in the system, which must be removed. A PowerFlush is a cure for the above problems to keep your system clean and sludge-free. Also, after a PowerFlush the installation of a magnetic filter will ‘grab’ any metallic debris from entering the boiler, which could cause further problems. Call serviceteam Ltd for more information.

Pressure Loss

Central heating boilers have a standard pressure of 1 to 2 bars. Usually, the system loses pressure and can happen for two reasons; water loss or damage to the pressure relief valve. The reasons might be a leak in the pipes (visible or hidden) or a slight leak on a radiator? If the boiler has been left unused for a while it can also be the cause of lost pressure?

It is a simple process to pressurize the boiler via the filling loop or key depending on the boiler make and model. Follow the simple guidelines and the pressure will be topped up in no time.

If the boiler needs topping up too often, it could point to one of the above problems. If this is the case then contact a service provider using Checkatrade or WHICH Trusted Trader to select a reputable supplier.

Boiler Noises

They can be somewhat unsettling if continuous. Noises range from sounding like a drum, gurgling, buzzing or clunking etc. Often helpful to determine the fault. A bearing in its death throes may give a squealing or growling noise or both. The boiler fan usually shows the above signs. If it continues it will eventually seize. The fan not being able to remove dangerous gases via the flue will shut the boiler down.

Pipework can rattle/vibrate when pipe clips come loose or haven’t been fitted on installation. Gurgling can be related to air in the pipework or radiators. With too much air in a central heating system, it’s likely to cause a blockage or barrier to the water trying to circulate the system. This leads to water being stuck and overheating hence the gurgling sounds. An easy way to resolve this problem is by bleeding the radiators.

If the airlock is in the boiler look for the drain valve (below), there should be one nearby. Turn off the boiler and make sure there’s nothing running. Attach a hose to the drain valve and run it outside the property. Place a container under the drain cock and slacken the square-headed nut to the left of the valve. A bubbly stream of water should follow, this is the air exiting the system accompanied by a hissing sound.

Do not tighten the nut until the water stream is bubble-free and the hissing sound has stopped. When you are sure the air is out of the system tighten the nut on the body of the drain valve. Fire up the boiler and test. You may have to repeat the process if it continues. You may experience a humming noise coming from the circulating pump? This could be down to the pump settings. Sometimes the pump is set too fast for your home’s requirements.

If so, it will you will experience vibrations (humming). If there is a speed range setting on your circulation pump that turns it down a notch, that should cure the humming?

Repair costs for your Boiler problems

Prices vary depending on the replacement part. Ranging from £90 to £700. All the work related to the replacement parts below should be completed by a qualified tradesperson.

The examples below are just a guide and nothing more and will vary from supplier to supplier.

Boiler Component Approximate Cost
Central Heating Pump £200
Circuit Board £250
Pump Seal £300
Boiler Fan £100 - £200
Motorized Valve £300
Water Pump £220
Ignition £110
Thermocouple £90
Combustion Fan £105
Overheat Thermostat £95
Air Vent £100
Diverter Valves £190
Gas Valve £220
Radiator Balancing £100 - £200
Radiator Bleeding £80 - £200
Kettling £400
Boiler Noises £80 - £150
Broken Pump £100 - £300
Heat Exchanger £300 - £600
Heating but no hot water £90 - £150
In summary

Now we have investigated some of the issues and repair costs we may have an economic decision to make? Is the repair and replacement part/s cost near to the cost of replacing the boiler? Has the boiler given you many years of good service but is breaking down more often, which is a sign that there could be more breakdowns to follow?

It’s a difficult one to gauge, but looking forward it might be time to consider replacing the boiler? Before you do, just do a couple of calculations. If you choose to install an ‘A’ rated boiler it could save 25/30% on your energy bills as it can be 90%+ efficient. A boiler 15-20 + years old will be less efficient than a modern one (naturally).

You can do a simple calculation as to how much your average monthly/annual energy costs are, with your current boiler? Find out the efficiency factor of your existing boiler and compare the efficiency rating of a new ‘A’ rated boiler.

The difference between the two is the saving you will make on your energy costs. Current boiler (rated F or G) 60-70% efficient – new boiler 90 – 95% efficient. Saving can be between 30/35% and 20/25%

  • Gas usage. Low: 8,000 KWh = £330.40 PA
  • Gas usage. Medium: 12,000 KWh = £495.60 PA
  • Gas usage. High: 17,000 KWh Cost = £702.10 PA

(above are British Gas examples)

  • Saving of 30/35% on £330.40 = £99.12 - £115.64 PA on current boiler
  • Saving of 30/35% on £495.60 = £148.60 - £173.46 PA on current boiler
  • Saving of 30/35% on £702.10 = £210.63 - £245.74 PA on the current boiler

(saving @ 20/25% will be much lower). Boilers do vary in price depending on KWh make model etc. The average price of a boiler is £1,500 + VAT upwards. The return on your investment will take time before the savings are realized.