No hot water from your combi boiler can be caused by low pressure, a fault code, supply problems or internal boiler components. Here are the common causes, safe checks and signs that you need a Gas Safe engineer.
Turning on a hot tap and getting nothing but cold water can be frustrating, especially when your boiler appeared to be working normally just hours earlier.
A combi boiler provides heating and hot water directly from one unit, so there is no separate hot water cylinder to reheat. When you open a hot tap, the boiler should detect the demand and begin producing hot water almost immediately.
If that does not happen, the symptoms around the rest of your heating system can provide useful clues.
For example, having working radiators but no hot water is a different problem from having neither heating nor hot water.
Understanding that difference can help narrow down the likely cause.
Check Whether Your Heating Is Still Working
Common Reasons a Combi Boiler Has No Hot Water
What You Can Safely Check Yourself
Heating Works but There Is No Hot Water
Why Hot Water Goes Hot and Cold
Why Hot Water Is Only Lukewarm
Should You Reset Your Boiler?
When to Call a Gas Safe Engineer
Preventing Future Hot Water Problems
Boiler Repairs & Diagnostics in London
Frequently Asked Questions
Before assuming that the entire boiler has stopped working, check whether your central heating still operates.
Turn the heating on and see whether the radiators begin warming up normally.
There are two useful possibilities.
Heating works but there is no hot water:
The boiler can still operate, but something may be preventing it from responding correctly when a hot tap is opened.
Neither heating nor hot water works:
The problem may involve boiler pressure, power, gas supply, a fault code, lockout or another issue affecting the whole boiler.
Also check the boiler display before changing any settings.
A warning symbol or fault code can provide important information about why the boiler has stopped producing hot water.
There are several reasons why a combi boiler may stop producing hot water.
Some involve simple external checks. Others require the boiler to be inspected by a Gas Safe registered engineer.
Low system pressure is one of the first things worth checking.
Look at the pressure gauge or digital pressure reading on your boiler.
The correct operating range varies between manufacturers, so always check your boiler manual rather than relying on one pressure figure for every model.
If the pressure has fallen significantly below the manufacturer’s recommended range, the boiler may stop operating correctly or enter a fault condition.
A one-off pressure adjustment may solve the immediate problem if the manufacturer permits the homeowner to repressurise the system.
However, pressure that repeatedly falls should be investigated.
If this is happening to your system, see our guide Why Is My Boiler Losing Pressure? Causes & Solutions.
A combi boiler needs electricity, an appropriate gas supply and mains water to operate normally.
Check whether there has been an interruption to any of these services.
You can safely check:
Whether the boiler display has power
Whether other electrical appliances are working
Whether cold water is available from the taps
Whether there is a known gas supply interruption
Do not attempt to dismantle, modify or repair gas pipework yourself.
If the boiler has lost both heating and hot water after a power interruption, check its display when power returns.
It may require a reset according to the manufacturer’s instructions.
Modern boilers monitor their operation and can shut themselves down when a fault is detected.
This is commonly known as boiler lockout.
Look for a fault code, warning light or symbol on the display.
Write the code down before resetting anything.
Boiler fault codes vary between manufacturers and models, so check the boiler manual or manufacturer’s information for the meaning of your specific code.
A fault that returns after the boiler has been reset is usually a sign that the underlying problem still needs attention.
If your boiler has stopped working more generally, see Boiler Not Working? Common Boiler Problems Explained.
The diverter valve plays an important role inside many combi boilers.
It helps direct heat according to whether your home requires central heating or hot water.
A diverter valve problem becomes particularly relevant when your radiators heat normally but the hot taps remain cold or only become lukewarm.
The valve may fail to move correctly when a hot water demand is created.
Another possible symptom is hot water that only works properly while the central heating is running.
The diverter valve is an internal boiler component.
Diagnosis, cleaning, repair or replacement should be carried out by an appropriately qualified heating engineer rather than by removing the boiler casing yourself.
A combi boiler needs to recognise when somebody opens a hot tap.
Depending on the boiler design, a flow-related sensor or component helps detect this demand.
If the boiler does not correctly recognise that water is being requested, it may fail to fire for hot water even though other parts of the system appear normal.
One clue can be that opening a hot tap produces no obvious response from the boiler.
Diagnosing an internal sensor problem requires professional inspection.
A combi boiler transfers heat so that fresh mains water can be heated as it passes through the appliance.
Over time, scale or debris can contribute to restrictions in parts of the system, particularly in hard-water areas.
Possible symptoms can include:
Poor hot water performance
Lukewarm water
Temperature fluctuations
Reduced hot water flow
Unusual boiler noises
The exact cause cannot reliably be identified from these symptoms alone.
A heating engineer can inspect the boiler and determine whether a heat exchanger or another component is responsible.
During very cold weather, the external section of a condensing boiler’s condensate pipe can sometimes freeze.
If condensate cannot drain correctly, some boilers will shut down and display a fault code.
This usually affects the operation of the boiler rather than only one hot tap.
If you suspect a frozen condensate pipe, follow the specific guidance supplied by your boiler manufacturer.
Never use boiling water on pipework.
If you cannot safely identify the condensate pipe or the problem keeps returning, contact a heating engineer.
Before arranging a repair, there are several external checks you can make without opening the boiler.
Look for:
Fault codes
Warning symbols
Low-pressure warnings
Lockout notifications
Loss of power
Make a note of any code displayed.
Read the pressure gauge or digital display and compare it with the operating range stated in the boiler manual.
If pressure continually falls, the underlying reason should be investigated rather than repeatedly topping up the system.
Try more than one hot tap.
If one tap or shower has a problem while hot water works normally elsewhere, the fault may be with that particular outlet rather than the boiler.
If every hot tap is cold, the boiler or hot water system becomes a more likely source of the problem.
Turn the central heating on.
Knowing whether the radiators still work can provide an important diagnostic clue.
If heating works normally but all hot taps remain cold, tell your heating engineer.
Make sure the boiler has power and that its display is operating normally.
Check whether there has recently been a power cut or whether an electrical circuit has tripped.
Without removing the boiler casing, look for water around:
The boiler
Visible pipe connections
Radiators
Radiator valves
Accessible heating pipes
A visible leak should be investigated, particularly if it is accompanied by falling boiler pressure.
If a fault code is displayed, check the manufacturer’s instructions.
Only carry out resets or homeowner checks specifically permitted in the manual.
Do not remove the boiler casing or attempt to repair internal gas, combustion or electrical components.
This is one of the most useful symptoms when diagnosing a combi boiler problem.
If the radiators become hot normally, the boiler clearly still has the ability to produce heat.
The problem may instead relate to the way the boiler detects or responds to a demand for domestic hot water.
Possible causes can include:
Diverter valve problems
Hot water flow detection faults
Heat exchanger issues
Sensors or controls
Other internal boiler components
A common pattern is turning on a hot tap while the boiler either does nothing or briefly responds without providing properly heated water.
This situation usually requires professional diagnosis rather than repeated boiler resets.
Hot water that repeatedly changes between hot and cold can be caused by several different problems.
Possible causes include:
Restricted water flow
Scale or debris
Heat exchanger problems
Sensors
Flow detection problems
Boiler modulation or control faults
An issue with a particular shower or tap
First determine whether the problem occurs at every hot water outlet.
If only one shower changes temperature while the other taps work normally, the shower itself may need attention.
If temperature fluctuations occur throughout the property, the boiler should be investigated.
A combi boiler producing some hot water but never reaching a comfortable temperature can point towards a different problem from complete hot water failure.
Possible causes include a restriction, heat exchanger issue, diverter valve problem, sensor fault or an unusually high water flow rate.
Check whether reducing the hot tap flow changes the temperature.
Also compare several hot water outlets.
If every tap produces only lukewarm water despite normal boiler settings, professional diagnosis may be needed.
Do not increase internal boiler settings beyond the manufacturer’s recommended limits in an attempt to compensate for a fault.
A boiler reset can sometimes restore operation after a temporary fault.
However, resetting should be carried out according to the instructions for your particular boiler model.
Before resetting:
Note any fault code
Check the pressure
Check that the boiler has power
Look for obvious leaks
Read the manufacturer’s instructions
If the boiler starts normally after one reset and continues operating correctly, the interruption may have been temporary.
If the same fault returns repeatedly, continuing to reset the boiler is not a proper repair.
The cause should be diagnosed.
Professional help is advisable when:
There is still no hot water after basic checks
Heating works but hot water does not
A boiler fault code repeatedly returns
The boiler repeatedly enters lockout
Pressure continues to fall
Water is leaking from the boiler
Hot water continually changes temperature
Water remains lukewarm
The boiler makes unusual noises
You suspect an internal component fault
You should never remove the casing of a gas boiler to investigate internal components yourself.
If you smell gas or suspect a gas or carbon monoxide emergency, do not continue troubleshooting the boiler. Contact the National Gas Emergency Service immediately on 0800 111 999.
Not every boiler fault can be prevented, particularly as components wear with age.
Regular maintenance can nevertheless help identify developing issues before you are unexpectedly left without hot water.
Useful steps include:
Have the boiler serviced regularly
Investigate recurring pressure loss
Deal with leaks promptly
Do not ignore repeated fault codes
Have unusual boiler noises investigated
Keep manufacturer documentation available
Address persistent changes in hot water temperature
Have heating system problems checked before they worsen
A boiler that continues working despite small recurring symptoms can eventually develop a more disruptive fault.
Early investigation can often prevent a minor problem from becoming a complete loss of heating or hot water.
If your combi boiler is no longer producing hot water, TES Heating can diagnose the boiler and connected heating system to identify the likely cause.
Depending on the symptoms, checks may include:
Boiler operation
System pressure
Fault codes
Hot water demand
Heating performance
Controls and sensors
Circulation
Visible leaks
Internal boiler components where appropriate
This is particularly useful when the heating still works but the hot water does not, because the symptoms can help narrow down which part of the boiler requires attention.
TES Heating provides boiler diagnostics, plumbing and heating services across London, with gas-related boiler work carried out by Gas Safe registered engineers.
See our Plumbing & Heating Services London for more information.
If the radiators heat normally but the taps remain cold, possible causes include a diverter valve problem, hot water flow detection fault, heat exchanger issue or another internal component. A Gas Safe engineer can diagnose the exact cause.
Low pressure can prevent some boilers from operating correctly or cause them to enter a fault condition. Check the pressure against the range stated in your boiler manufacturer’s instructions.
Fluctuating hot water can be associated with water flow, scale, heat exchanger problems, sensors or other boiler components. Check whether the problem occurs at every hot water outlet or only one tap or shower.
No. A single reset carried out according to the manufacturer’s instructions may clear a temporary fault, but a fault that repeatedly returns requires investigation.
Basic external checks such as reading the pressure, checking fault codes and checking the power supply can be carried out safely. Internal inspection or repair of a gas boiler should be carried out by an appropriately qualified Gas Safe registered engineer.
