If your radiators are toasty but the hot tap runs stone cold, it usually points to a problem inside your combi boiler. The good news is your boiler has power and is firing, but a fault is stopping it from heating your domestic hot water properly.
How a combi boiler should heat your water
A combi boiler heats water for your taps and your radiators, but not at the same time. When you open a hot tap, the boiler should pause central heating and divert its full effort to your domestic hot water.
It does this using internal parts such as the diverter valve, sensors and thermostats. If any of these stop working as they should, you can end up with good heating but lukewarm or cold water at the taps.
Quick safety checks before you call an engineer
There are a few simple checks you can do safely without opening the boiler casing. These can help you rule out a setting issue or give useful information to the engineer you call.
Check the controls: Make sure the boiler is set to heat both heating and hot water, and that no eco or holiday mode is blocking hot water.
Try a reset once: Follow the manual to perform one reset only. If the same problem returns, do not keep resetting.
Look for error codes: Note down any fault code or flashing lights and when they appear.
Test more than one tap: Check both kitchen and bathroom taps to see if the problem is everywhere.
Compare with your radiators: Turn on the heating and make sure radiators still get hot as normal.
While the boiler is running, listen for any new noises such as kettling (a kettle-like boiling sound), banging or whistling. Mention these to your engineer, as they can be clues to what is going wrong inside the boiler.
Common combi faults when heating works but hot water does not
If your controls and settings look right, the fault is likely inside the boiler itself. These are some of the issues heating engineers regularly see in this situation.
Diverter valve problems
The diverter valve is the traffic controller inside a combi boiler, deciding whether heat goes to your radiators or your taps. When you open a hot tap, the diverter should swing over to prioritise hot water.
If the valve sticks, seizes or its motor fails, the boiler may still happily heat radiators but send little or no heat to your hot water. This is not a DIY fix, as getting to the valve means opening the boiler casing and often draining the system.
Plate heat exchanger blocked with scale or sludge
Your hot water is heated by a component called a plate heat exchanger. It transfers heat from the boiler’s side of the system to the water that flows to your taps, without the two water supplies mixing.
Over time, limescale from hard water, or sludge from the heating system, can clog the slim channels inside the exchanger. When that happens, you might get water that starts warm then quickly cools, or never gets properly hot at all.
Flow sensor or flow switch faults
A flow sensor (or flow switch) tells the boiler that a hot tap has been opened. If it does not register the flow correctly, the boiler may not fire up for hot water even though your heating still works.
Sometimes this sensor sticks, fails electrically, or is affected by debris or limescale. Diagnosis and replacement should be left to a Gas Safe registered engineer.
DHW thermostat or temperature sensor issues
The domestic hot water (DHW) thermostat sets and monitors the temperature of your hot water. If it reads incorrectly or fails, the boiler may shut off too early or not fire properly for hot water.
You might notice unstable hot water temperatures, or water that is only lukewarm, while the radiators remain fine. Again, testing involves electrical checks and access to internal parts, so it is not something to tackle yourself.
System pressure and control settings
On many combi boilers, low system pressure will show on the gauge at the front and may cause intermittent hot water problems. Some models will lock out and display a fault code if the pressure is too low.
You can usually check the pressure gauge safely and compare it with the recommended range in your manual. Repressurising can be straightforward, but if you are unsure or the pressure keeps dropping, it is better to speak to an engineer rather than keep topping it up.
Hard water, limescale and Dorset homes
Many areas of Dorset have hard water, which means there is more calcium and magnesium in the supply. When this is heated inside your boiler, it forms limescale deposits on internal parts.
The plate heat exchanger and flow sensors are especially vulnerable. Because the hot water side has narrow channels and higher temperatures, limescale tends to affect hot water performance before you notice issues with your radiators.
To reduce limescale problems in the first place, you can consider:
Fitting a water softener or scale reducer on your incoming water supply
Booking regular boiler servicing to check and clean key components
Using manufacturer-approved inhibitor chemicals in the heating system
A properly specified water softener can protect your boiler, taps and appliances, as well as helping hot water components last longer and stay more efficient.
What you should not attempt yourself
It is tempting to take the cover off and have a look inside, especially if you have watched online videos. However, in most modern boilers, the casing is classed as a safety cover that should only be removed by a qualified engineer.
Do not try to strip or replace diverter valves, sensors, heat exchangers or gas components yourself. Apart from safety risks, you could also void your boiler warranty or cause further damage that makes the repair more costly.
Frequently asked questions
Is it the diverter valve?
A sticking diverter valve is a very common cause of heating working but no hot water. However, it is not the only possibility, and the symptoms can overlap with issues like a blocked plate heat exchanger or a faulty flow sensor.
The only way to be confident is for an engineer to test the boiler while it is trying to produce hot water. They can then confirm whether the diverter valve needs repair, freeing off or replacement.
Will a reset fix it?
A single reset can clear a one-off glitch, especially after a power cut or control fault. If you reset the boiler and the same hot water problem comes straight back, it is a sign of an underlying fault.
Repeatedly resetting the boiler will not fix worn parts or limescale build-up, and in some cases may cause the boiler to lock out. If one reset does not help, it is time to book a proper diagnosis.
How urgent is it?
If you have no hot water at all, it is usually classed as an urgent, but not immediate safety, issue. You may be able to cope for a short time with kettles and electric showers (if separately supplied), but it is inconvenient.
If you notice burning smells, constant leaking, banging noises or signs of scorching, switch the boiler off and call for help straight away. Otherwise, arrange a prompt visit so the fault does not worsen or lead to a full breakdown.
Next steps and getting professional help
If you have gone through the basic checks and still have no hot water but working heating, the next step is a diagnostic visit from a Gas Safe engineer. They can test the diverter valve, sensors and plate heat exchanger, and carry out any parts replacement safely.
To book a boiler diagnostic and repair, contact Wood & Williams on 01202028037 or request a visit through our boiler repair service page. If you live in a hard water area and want to protect your boiler for the long term, ask us about water softener installation at the same time.