If your central heating works perfectly but you have no hot water at the taps, it is usually a sign of a specific boiler or controls fault rather than a complete breakdown. Understanding the likely causes can help you stay safe, avoid guesswork, and explain the issue clearly to an engineer.
Quick safe checks before calling an engineer
Before assuming the boiler has failed, there are a few simple, safe checks you can do as a homeowner. These do not involve opening the boiler or touching gas components, and can sometimes resolve an obvious setting issue.
Use this quick checklist:
Programmer and timers: Make sure hot water is set to "on" or "continuous" and the clock is correct.
Hot water temperature dial: Check the boiler or cylinder control is set high enough to give hot water, not just lukewarm.
Mains water pressure: Confirm cold taps run at normal pressure and any external stop tap is fully open.
Boiler reset: If the boiler shows a fault light, press the reset once only, following the manual.
Visible leaks: Look around the boiler, pipework and cylinder for obvious drips, staining or puddles.
If the boiler fails to restart, repeatedly locks out, or you notice leaks, burning smells or unusual noises, switch it off and contact a Gas Safe engineer.
Heating works but no hot water on a combi boiler
With a combi boiler, your hot water is produced on demand when you open a tap. If the radiators heat up but hot water does not, the fault is often inside the boiler rather than in the radiators or pipework.
Diverter valve stuck or faulty
The diverter valve is a key combi boiler component that directs heat either to the radiators or to your hot water. When it sticks in the heating position, you can get lovely warm radiators but only cold or lukewarm water at the taps.
Common symptoms include radiators getting hot when you only run a hot tap, or hot water that starts warm then quickly runs cold. This is not usually something you can fix yourself and typically needs an engineer to free or replace the valve.
Plate heat exchanger scaling up
The plate heat exchanger transfers heat from the boiler water to your tap water. In hard water areas, limescale and sludge can build up inside this small but important part.
If it becomes restricted, the boiler might fire up but struggle to transfer enough heat, giving you weak or fluctuating hot water. An engineer can clean or replace the plate heat exchanger and may recommend a system flush or scale protection to reduce the risk of it happening again.
Flow switch or flow turbine issues
Combi boilers have a flow switch or turbine that senses when you open a hot tap. If this device sticks or fails, the boiler may not realise you are asking for hot water and simply stays in heating mode.
You might see normal boiler pressure and no error codes, but the boiler does not ignite when you run a tap. This almost always needs professional diagnosis with the boiler casing removed, which must only be done by a Gas Safe engineer.
No hot water on system or regular boiler setups
If you have a separate hot water cylinder, the boiler heats water that is stored for later use. In this type of system the heating can work while the cylinder side fails, leaving you with warm radiators and a cold tank of water.
Thermostats, programmers and motorised valves
On cylinder systems, wiring and controls play a bigger role. If the hot water programmer channel is off, time settings are wrong, or the cylinder thermostat has been turned too low, the boiler may not be asked to heat your stored water at all.
Motorised valves can also stick so that heat is only sent to the radiators. If you hear the boiler and pump running with heating demand but nothing when only hot water is selected, a motorised valve or controls fault is a strong possibility.
DHW sensors and thermistors
Both combi and system boilers rely on temperature sensors (often called thermistors) to monitor hot water temperatures. When these fail or read incorrectly, the boiler can shut down hot water production early or not start it at all.
Signs include hot water that is either very short-lived, scalding, or varies in temperature without any change at the tap. These are relatively common faults that require testing and replacement by an engineer.
Limescale restricting flow and performance
Limescale is a frequent cause of hot water problems, especially in areas with hard water. It can build up in pipes, around cylinder coils, inside the boiler heat exchanger and on taps and shower heads.
Reduced flow at hot taps, noisy kettling sounds from the boiler, and lukewarm water are all clues. An engineer may recommend descaling, fitting a scale reducer, or servicing the hot water components to restore proper flow and temperature.
When it is an emergency and when it is a routine repair
Not every loss of hot water counts as an emergency, but safety concerns must always come first. Switch the boiler off at the controls and call for urgent help if you notice any of the following:
Smell of gas anywhere near the boiler, meter or pipework
Carbon monoxide alarm sounding or showing a warning
Boiler casing, flue or surrounding walls becoming excessively hot
Boiler frequently overheating, banging or "kettling" loudly
Visible burning, scorching or melting near the boiler
If the boiler simply does not provide hot water but the heating is stable and there are no safety alarms or leaks, it is usually a routine repair. You should still arrange a prompt visit to prevent the fault from worsening or affecting other components.
FAQ: common questions when the heating works but hot water does not
Can I still use the boiler?
If the boiler is running quietly, showing normal pressure, and there are no leaks, smells or warning lights, many homeowners continue to use just the heating for a short period. However, you should not ignore the fault.
Book an engineer as soon as possible, and if anything changes or you notice new noises, smells or error codes, switch the boiler off and seek professional advice.
Why does it happen intermittently?
Some hot water faults only appear now and again. This can be due to a sticking diverter valve, a partially blocked plate heat exchanger, or a sensor that fails only at certain temperatures.
Intermittent issues are still a sign something is wearing out or becoming dirty, and they rarely fix themselves. Keeping a simple log of when it happens can really help your engineer track down the cause more quickly.
Will it affect boiler pressure?
Many hot water faults do not immediately affect boiler pressure. Pressure relates mainly to the sealed heating circuit, not the fresh water passing to your taps.
However, leaks, sludge or overheating can gradually cause pressure loss or fluctuations. If you are regularly topping up the system pressure or see big swings when the heating comes on, mention this to your engineer.
Next steps and reducing the risk of future breakdowns
If your radiators are warm but the taps are running cold, the best next step is a professional diagnosis rather than guessing or repeatedly resetting the boiler. Most of the likely causes involve internal components, controls or limescale that need specialist tools and training to put right safely.
To reduce the chance of this problem recurring, an annual boiler service and system check is strongly recommended. Regular servicing allows issues such as sticking valves, scaling and failing sensors to be picked up early, before they leave you without hot water.
If you are experiencing no hot water but your heating still works, contact Beales Services to book a boiler repair and arrange an annual service. Call 02034880883 and a qualified engineer can help get your hot water running reliably again.
