If your radiators are warming up nicely but your taps and shower run cold, the problem is usually in the hot water side of the boiler or cylinder, not the whole heating system. With a few safe checks you can often narrow down the cause and explain it clearly to an engineer.
Safe checks first before calling an engineer
Before assuming a major fault, it is worth ruling out a few simple issues. These checks do not involve opening the boiler or touching gas components, so they are safe for most homeowners.
Check the timer/programmer: Make sure the hot water is actually set to be on and that the clock time is correct.
Look at the boiler pressure gauge: Most systems should sit around 1.0 to 1.5 bar when cold. If it is below 1.0, that may be the problem.
Try one reset: Follow the manual and press the reset button once. Do not keep resetting a boiler that keeps locking out.
Note any error code: If the display shows a fault code, write it down or take a photo.
Check other gas appliances: If your gas hob or other gas appliances will not light, you may have a supply issue.
Look for leaks: Check around the boiler, radiators and cylinder for any obvious dripping or damp patches.
If basic checks do not help, avoid taking the casing off the boiler or touching any gas or combustion parts. At that point you need a Gas Safe engineer to investigate safely.
Understanding your setup: combi vs system boiler
The fault and fix often depend on whether you have a combi boiler or a system/regular boiler with a separate hot water cylinder. Knowing which you have will help your engineer diagnose faster.
A combi boiler heats water on demand, with no cylinder. If you turn on a hot tap, the boiler should fire up immediately. A system or regular boiler works with a cylinder that stores hot water, often in an airing cupboard.
Common combi boiler causes when heating works
With a combi, working radiators but no hot water normally points to a fault inside the boiler affecting the domestic hot water side.
Diverter valve issues: The diverter valve changes the flow of hot water between your radiators and the taps. If it sticks in the heating position, the central heating works but hot water does not.
What you can do: Listen when you open a hot tap. The boiler should fire and you may hear valves moving. If nothing changes or the heating gets hotter instead, mention this to the engineer.
Engineer job: Testing, freeing or replacing a diverter valve involves opening the boiler and working on pressurised and gas‑related parts. This must only be done by a qualified engineer.
DHW plate heat exchanger blockage: In hard‑water areas, limescale and sludge can block the plate heat exchanger that transfers heat to your hot water. You may get lukewarm water that quickly runs cold.
What you can do: Note whether hot water starts warm then cools rapidly, and whether the boiler display shows any temperature spikes or fault codes.
Engineer job: Cleaning or replacing the plate heat exchanger and flushing the system. This is internal boiler work and not a DIY task.
Faulty flow sensor: The flow sensor tells the boiler that a tap is open. If it fails, the boiler may not fire for hot water, even though the heating works on a timer.
What you can do: Try different taps and note whether the boiler fires for any of them. Tell the engineer which outlets are affected.
Engineer job: Testing the sensor with a meter and fitting a replacement if needed.
Low boiler pressure: Some combis cut hot water or behave unpredictably when pressure is low, even if heating is struggling along.
What you can do: If your manufacturer’s manual explains how to top up the pressure using the filling loop, you can usually do this yourself. Only top up slowly to the recommended level. If pressure keeps dropping, stop topping up and call an engineer.
System boilers and cylinders: typical no‑hot‑water causes
If you have a hot water cylinder and the heating is fine, the issue is often with controls, valves or the cylinder itself rather than the main boiler.
Programmer or thermostat settings: The cylinder thermostat may be set too low or the programmer may have hot water off, even though heating is on.
What you can do: Check the hot water is actually scheduled to come on. Ensure the cylinder thermostat is usually set around 60 °C. Do not remove covers from any wiring centre or junction box.
Motorised valve fault: Many systems use a motorised valve to send hot water either to the cylinder, the radiators, or both. If that valve sticks in the heating position, radiators get hot but the cylinder stays cold.
What you can do: Place your hand carefully on the pipes going into the cylinder (avoiding very hot surfaces). If they stay cold while the boiler runs for “hot water”, mention this to your engineer.
Engineer job: Testing and replacing motorised valves, and checking wiring and controls safely.
Limescale in the cylinder coil: In hard‑water areas, the heating coil inside the cylinder can scale up, reducing heat transfer so the stored water never gets properly hot.
What you can do: Note how long the cylinder has been heating and how hot the water actually gets at the tap. If it only reaches lukewarm even after hours, tell your engineer.
When the problem is urgent
Most no‑hot‑water issues are inconvenient rather than dangerous, but some situations need urgent attention.
No hot water plus unusual smells, soot marks or staining around the boiler or flue
A carbon monoxide alarm sounding or showing a warning
Repeated boiler lockouts or error codes after a reset
Any signs of burning, melting or scorching on or near the boiler
If you notice any of these, turn off the boiler, ventilate the area and contact a Gas Safe engineer urgently. Do not attempt further resets or DIY repairs.
Information to give your engineer to speed things up
Good information helps your engineer arrive prepared with the right parts and saves time during the visit.
Before you call, make a note of:
Boiler make and model: This is usually on the front panel or inside the user manual. A quick photo is useful.
Any fault or error codes: Write down exactly what the display shows, including flashing lights or symbols.
System pressure reading: Note the pressure from the gauge when the system is cold.
Symptoms in your own words: For example, “radiators hot, taps cold”, “shower runs hot then cold” or “hot water only works sometimes in the evening”. Mention any recent changes, such as a new shower or radiator.
Frequently asked questions
Why is my shower hot then cold?
This can point to a partially blocked plate heat exchanger in a combi, a faulty thermostat, or an undersized or faulty shower mixer. The boiler may be overheating then shutting down briefly, which you feel as hot then cold water.
An engineer will check the boiler temperatures, plate heat exchanger, sensors and the type of shower valve you have to pinpoint the cause.
Why does the problem happen intermittently?
Intermittent hot water often involves sticking valves, failing sensors, or borderline low pressure. The system works sometimes, then fails when a component sticks or when demand is higher.
Log when it happens: time of day, number of taps running, and whether heating is on at the same time. This pattern can be very helpful for diagnosis.
Can low pressure affect hot water only?
Yes, low pressure can affect the hot water side more noticeably, especially in combi boilers. If the boiler cannot circulate water properly, it may struggle to produce stable hot water, even if it just about runs the heating.
If topping up the pressure restores hot water but the pressure keeps dropping, you likely have a leak or expansion issue that needs an engineer’s attention.
Next steps: get your hot water running again
If you have checked your controls, noted the pressure and any fault codes, and still have no reliable hot water, it is time for a professional repair. Do not open the boiler casing or work on any gas components yourself.
For clear advice and quick diagnosis, book a boiler repair visit with Welm Facilities Maintenance Ltd. Call 07854896435 to arrange a convenient time and get your hot water and heating back to normal as soon as possible.
