No Hot Water but Heating Works? Common Causes and What to Check
If your radiators are working but your hot taps run cold, it can be frustrating and confusing. The good news is that this fault is usually quite specific, and a few simple, safe checks can help you work out what is going on before you call in an engineer.
First safety notes before you touch anything
Always put safety first with any boiler or hot water issue. You should never remove boiler covers, touch internal components, or attempt gas work of any kind. Those jobs are strictly for a Gas Safe registered engineer.
As a homeowner, it is generally safe to check your controls, look at the boiler display, read the pressure gauge, and follow the reset instructions in your manual. If at any point you smell gas, see scorch marks, or feel unsure, turn the boiler off and call a professional straight away.
Quick checks you can safely do yourself
Before assuming a major fault, it is worth ruling out a few simple issues that are within your control. These checks only involve controls and basic readings, not tools or covers.
Work through the following list in order, and stop if you find a clear issue or anything that concerns you.
Check hot water settings on the programmer: Make sure the hot water is actually turned on and the timer is calling for hot water.
Check the room and cylinder thermostats: A hot water or cylinder thermostat that is turned down too low may prevent the boiler from firing for hot water.
Look at the boiler display: Note any fault codes or flashing lights, and whether the hot water symbol appears when you run a tap.
Check the system pressure (sealed systems): On combi and system boilers, the pressure gauge should usually be around 1 to 1.5 bar when cold.
Try a safe reset: If your manual explains a simple reset button or sequence, you can try this once and see if the boiler recovers.
If the boiler locks out again after a reset, or you get the same fault code repeatedly, do not keep resetting it. Make a note of the code and contact a Gas Safe engineer.
Do you have a combi boiler or a cylinder?
How your hot water is made affects the likely cause of the problem. A couple of quick checks can help you narrow it down and give clearer information to an engineer.
If you have a combi boiler
A combi boiler heats water directly from the mains, with no hot water cylinder. When you turn on a hot tap, the boiler should fire up, and you may see a tap or hot water symbol on the display.
Run a hot tap fully open and watch the boiler. If the heating works but the boiler does not react to the tap, or fires briefly then goes off, it points to an internal combi fault such as a diverter valve, flow switch, or sensor problem that needs an engineer.
If you have a cylinder
A system or regular boiler usually works with a hot water cylinder in an airing cupboard. Most have a cylinder thermostat on the side, and sometimes a separate programmer channel for hot water.
Check that the programmer is set to provide hot water, and that the cylinder thermostat is turned up above the current water temperature. If the boiler still does not fire for hot water, or the cylinder feels cold after being on for an hour or more, you will need an engineer to investigate controls, valves, or sensors.
Common causes when heating works but hot water does not
Once you have ruled out simple settings and programming issues, the fault is usually within the boiler or associated controls. Below are the most common technical causes your engineer may look for.
Diverter valve stuck or faulty
In many combi boilers, a diverter valve sends heat either to the radiators or to your hot water. If it sticks in the heating position, your radiators may work but your taps stay cold.
Symptoms can include radiators heating up when you run a hot tap, or hot water that runs warm then quickly goes cold. Fixing or replacing a diverter valve is a job for a Gas Safe engineer, as it involves opening the boiler and working with pressurised components.
Hot water sensor / NTC problems
Modern boilers use temperature sensors, often called NTCs, to monitor the hot water flow. If a sensor fails or gives the wrong reading, the boiler may not recognise that hot water is needed.
You might see a fault code related to domestic hot water temperature, or notice that the boiler fires for heating but ignores the hot taps. Sensor testing and replacement must be carried out by a qualified engineer.
Plate heat exchanger scaling or blockage
Combi boilers use a plate heat exchanger to transfer heat from the boiler water to your tap water. Over time, especially in hard water areas, limescale or sludge can build up and restrict this component.
This can lead to lukewarm water, water that gets hot then cools, or no usable hot water at all even though the boiler appears to be running. Cleaning, descaling, or replacing a plate heat exchanger is specialist work and should not be attempted as a DIY job.
Low pressure and airlocks in system boilers
For sealed systems, low pressure can prevent hot water from circulating properly, even if the boiler is firing. If your gauge is below the level stated in your manual, the system may need repressurising.
Some homeowners are comfortable topping up pressure following the exact instructions in the manual. If you are unsure, or if the pressure keeps dropping after you top up, stop and call an engineer as there may be a leak or airlock that needs professional attention.
Flow switch and flow rate issues
A flow switch tells the boiler that a hot tap has been opened. If it sticks or fails, the boiler may never receive the signal, so it stays in heating mode only.
Occasionally, running multiple taps at once or very low flow from a tap can also confuse the boiler. Try one hot tap fully open. If there is still no response on the boiler display, a faulty flow switch or related sensor is likely and needs expert diagnosis.
When to stop troubleshooting and call a professional
There is a clear point where safe homeowner checks end and professional work begins. Anything that involves taking covers off, undoing screws, or touching internal components is not suitable for DIY.
Call a Gas Safe engineer promptly if you notice repeated fault codes, pressure that keeps dropping, banging or kettling noises from the boiler, or hot water that has suddenly failed without any settings being changed.
Information to have ready when you phone an engineer
Having a few key details to hand can make your repair visit quicker and more efficient. It helps the engineer bring likely parts and understand the fault before they arrive.
Before you ring, note down:
Boiler make and model: Usually found on the front panel or inside the flap where the controls are.
Any fault codes or lights: Write them down exactly as they appear on the display.
System pressure reading: If you have a pressure gauge, note the bar reading when the system is cold.
Recent noises or leaks: Mention any dripping, staining, gurgling, or banging you have noticed.
When the fault started: Say if it was sudden, after other work, or has been getting worse.
FAQ: common questions about heating but no hot water
Is it safe to use the heating if there is no hot water?
In many cases the heating can still be used, but it depends on the fault. If the boiler is not showing fault codes, pressure is stable, and there are no unusual noises or smells, it is often safe to keep the heating on until an engineer visits.
If you are in any doubt, turn the system off and ask a Gas Safe engineer for specific advice for your boiler and setup.
Can I fix a diverter valve or flow switch myself?
No, these components sit inside the boiler and involve working on gas or pressurised water parts. Only a qualified engineer should repair or replace them.
Attempting this work yourself can be dangerous and may also invalidate your boiler warranty or home insurance.
Will a boiler service help prevent this problem?
Regular servicing helps keep sensors, valves, and heat exchangers clean and working correctly, which can reduce the risk of losing hot water or heating. It also gives an engineer the chance to spot early signs of wear.
If your boiler has not been serviced in the last year, it is sensible to book a service once the immediate fault has been dealt with.
Next steps and how 7 Day Plumbing Services Ltd can help
If you have worked through the safe checks and still have no hot water but your heating works, the next step is a professional diagnosis and repair. Do not ignore the issue, as continued use can sometimes make faults worse.
For fast, friendly help with boiler repairs, contact 7 Day Plumbing Services Ltd on 07896487603. To learn more about how we look after local homes, you can also visit our boiler repair and boiler servicing pages and arrange a convenient appointment.
