Combi boilers are highly efficient boilers that heat water for both your central heating and your hot water taps. They work by taking in mains water as needed and heating it quickly, meaning you do not need a large hot water storage cylinder. As effective and efficient as combi boilers are, they can still malfunction. Below are some of the most common problems - with solutions.


A fall in pressure

Your boiler's pressure may fall to an unsatisfactory level - around 0.4 bar or lower - which will mean the unit won't start up. An error code is likely to be displayed. To remedy this, locate the filling loop (probably underneath the boiler) then turn the tap in an anti-clockwise direction. Turn the tap clockwise once the pressure reaches about 1.5 bar.


Low pressure in your boiler may be down to the issues listed below.


Radiator leaks

Have you spotted water leaking from your heating system? First check the radiator valves, looking under tap heads. Make sure compression fittings are tight and there's no water coming out. Use your hands to thoroughly check for any leaks.


No radiator leaks?

Look out for dark brown patches on the ceiling - which may indicate a leak elsewhere.


If you still think there may be a leak, floorboards may have to be lifted to investigate further - a costly, inconvenient and time-consuming task.


Automatic air vents

Automatic air vents are designed to vent air from the system. But these can be problematic if the boiler has not got enough inhibitor in it, or if the pump setting is too high for the speed. The inhibitor stops the water from reacting with the inside of the radiators and the interior of the heating system components. Without any inhibitor, water comes into direct contact with the inside of the radiator, causing rust and sludge to build up in your heating system. Hydrogen may also be generated, which can move through your heating system, eventually being released by the automatic air vent. However, this means your boiler’s pressure drops. The key here is to ensure there is adequate inhibitor in your heating system.


Pump speed

If your pump speed is set too high and water does not move away from the heating system fast enough, a phenomenon called ‘cavitation’ occurs. This is when the pump impellers divide the water and air, creating hydrogen. This is solved by turning down the pump speed, while maintaining a sufficient pump speed around your entire system. Once again, any hydrogen will be released by the air vent and your system's pressure levels will drop.


Expansion vessel problems

The expansion vessel gives water somewhere to expand without pressurising the heating system above the safe limit. If the rubber diaphragm ruptures, water will enter and air will seep into the heating system, which will be removed by the automatic air vent. Failure of the diaphragm will result in an area of no expansion. At first this will cause the system pressure to rise, causing the pressure relief valve to open.


Find out if you have an issue with your expansion vessel by locating a bicycle-style valve on the boiler and depressing it. This should release all the air inside. If water also escapes, you know you need a new expansion vessel. If there's no water, there may have simply been a loss of charge in the straighter valve. Use a bicycle pump to re-pressurise it - and use a pressure gauge to ensure it's pumped up to 1 - 1.5 bar.


Many combi boilers only have expansion vessels sufficient for the boiler itself, but not for any expansion from the radiators. If you’re having a new boiler installed, ask the plumber to fit a remote expansion vessel - most likely in the airing cupboard. This should reduce the chances of an issue with your expansion vessel or pressure release valve - by half.


Pressure relief valve

A pressure release valve is simply a rubber valve with a set spring. When water pressure pushes that spring to a certain point, the valve dumps water from the system. The water will be led away from the property via a pipe leading outside.


Over time, the spring in the valve will wear out; more than half becoming faulty within a 10 year period.


The simplest way to see if your system is losing pressure in this way is to check the external 15mm pipe and see if it's wet. The presence of water here means you probably need to replace your pressure release valve.


Other combi boiler problems


Thermostat batteries are flat

If your heating goes off unexpectedly, check to see if your thermostat batteries have run flat, in which case a flat battery icon will be displayed. Most thermostats require AA size batteries. When removing old batteries try to replace them with new ones in under a minute - and your settings should be saved. If you take longer, you'll probably have to re-program your system.


Water supply

Your combi boiler and heating system will not work if the water supply has been inadvertently switched off, or if water is not coming into the property due to maintenance works by the water supplier.


Power cut causing system reset

If there is a power cut your system may revert to its factory settings when the power comes back on, so you may need to re-program it.


Reset the boiler

If a problem is not related to the above, it might be solved by pressing the restart button on your boiler.


Timer issues related to daylight savings

Your timer may not be designed to take account of daylight savings time, which may lead to the boiler coming on earlier or later than expected. Adjust your timer if this is the case.


Central heating working but no hot water

Combi boilers contain your central heating system's primary heat exchanger. When you turn on the hot water tap, the exchanger diverts water to a secondary heat exchanger via a diverter valve. If this is faulty, you may have heating but no running hot water.


No gas

A gas-fed combi boiler will feature a Flame Supervision Device or FSD, which will switch off the boiler if it cannot detect gas. If this happens, check gas is reaching the boiler - for example by checking the gas hobs. Never open or modify gas pipes yourself - always call a Gas Safe Engineer.


Frozen condensate pipe

Your combi boiler may stop working if your condensate pipe is frozen. This is located somewhere on the outside of your property. Use hot (but not boiling) water to thaw the pipe out. To prevent the issue in the future, ask an engineer to install type-O insulation on your condensate pipe.


Please check your boiler’s manual before attempting any advice given above. We also always recommend seeking the services of a professional central heating engineer if you are not totally confident carrying out any repairs/maintenance yourself.