No hot water but heating works? Having working radiators but no hot water can be confusing.
If your home is warm but the taps are running cold, it doesn’t necessarily mean the entire boiler has failed. The cause can be something as simple as a control setting, low pressure, or a temporary fault — but it can also point to a faulty diverter valve, hot-water cylinder, thermostat, or another component.
The first thing to understand is that the likely cause depends heavily on the type of heating system in your London home.
A combi boiler heats domestic hot water on demand and does not normally use a separate hot-water cylinder. System and conventional boilers, on the other hand, can work with a hot-water cylinder that stores heated water for use around the home.
In this guide, we’ll explain why your heating may work while your hot water doesn’t, how to narrow down the problem and when you should call a qualified heating engineer.
Why Do I Have Heating but No Hot Water?
When the radiators are heating normally but the taps are cold, the problem is often related specifically to the domestic hot-water side of the system.
Possible causes include:
- A faulty diverter valve
- A problem with the hot-water cylinder
- A faulty cylinder thermostat
- An immersion-heater problem
- Incorrect timer or programmer settings
- Low boiler pressure
- A boiler fault
- A problem with the hot-water controls
- An airlock in the hot-water pipework
- An electrical supply problem
The most important question is:
Do you have a combi boiler or a boiler connected to a hot-water cylinder?
That distinction can completely change the diagnosis.
First: Do You Have a Combi Boiler?
A combi boiler heats domestic hot water when you turn on a hot tap.
It doesn’t normally store hot water in a separate cylinder.
When you turn on a hot tap, the boiler detects the demand and heats the water as it passes through the appliance.
Baxi’s guide to how a combi boiler works
This means that if your radiators work but your hot taps stay cold, a combi-boiler problem may involve the boiler itself, its controls, the hot-water demand sensor or the diverter valve.
If You Have a Combi Boiler, Check the Diverter Valve
The diverter valve is one of the most important components to consider when a combi boiler provides heating but struggles to provide hot water.
Its job is to direct heat either towards the central heating system or towards your domestic hot water.
Vaillant’s guide to boiler diverter valves
A faulty or stuck diverter valve can prevent the boiler from directing heat where it is needed.
What Are the Signs of a Faulty Diverter Valve?
Possible symptoms include:
- Heating works but hot water is cold
- Hot water is only lukewarm
- Hot water works only when the heating is running
- Water temperature changes unexpectedly
- The boiler appears to fire when a hot tap is opened, but the water doesn’t become properly hot
Vaillant identifies lukewarm hot water and hot water that only works when the central heating is on as possible symptoms of a faulty diverter valve.
Can You Fix a Diverter Valve Yourself?
It’s not a good DIY repair.
The diverter valve is part of the boiler system, and accessing or repairing internal boiler components can be dangerous.
Vaillant’s advice on faulty diverter valves recommends using a qualified heating engineer rather than attempting the repair yourself.
A qualified engineer can determine whether the valve needs cleaning, freeing or replacing — or whether another component is actually responsible.
What If You Have a Hot-Water Cylinder?
If your home has a separate hot-water cylinder, the diagnosis is different.
A cylinder stores hot water so that it can be supplied to taps and showers when required.
There are two broad types:
- Direct cylinders, which heat the water using an immersion heater
- Indirect cylinders, which use an external heat source such as a boiler or heat pump
Vaillant’s guide to hot-water cylinders
If your radiators work but the cylinder isn’t producing hot water, several different components may need checking.
Check the Hot-Water Timer or Programmer
One of the simplest possibilities is that the hot-water schedule has been changed.
If your heating works but your hot water doesn’t, check whether the programmer or timer is actually requesting hot water.
This is particularly worth checking if:
- You’ve recently had a power cut
- The clocks changed
- Someone changed the heating controls
- The programmer was replaced
- The system has been switched to a different operating mode
Vaillant notes that timer settings can be one reason for hot-water problems on systems where hot water is stored separately.
If the controls look normal, move on to the next checks.
Check the Cylinder Thermostat
A hot-water cylinder normally has a thermostat that controls the temperature of the stored water.
If the thermostat develops a fault, the cylinder may not receive the correct heating signal.
You might notice:
- Hot water remains cold
- Hot water is only lukewarm
- The boiler doesn’t appear to heat the cylinder
- The problem occurs even though central heating works normally
Don’t remove electrical covers or attempt to rewire the thermostat yourself.
A qualified heating engineer or suitably competent professional can test the controls safely.
What About the Immersion Heater?
If you have a hot-water cylinder, you may also have an immersion heater.
An immersion heater uses electricity to heat water directly inside the cylinder.
If the immersion heater fails, you may lose hot water even though the central heating continues to work.
Possible causes include:
- Failed heating element
- Electrical supply problem
- Faulty thermostat
- Tripped electrical protection
- Wiring or control fault
If your cylinder has an immersion heater, check whether its switch is on and whether there are any obvious electrical issues.
Don’t remove electrical covers or attempt electrical repairs yourself.
Could Low Boiler Pressure Cause No Hot Water?
Yes.
Low boiler pressure can affect boiler operation and may result in a lack of hot water as well as heating problems.
Vaillant’s guide to low boiler pressure
Check the pressure gauge or digital pressure display if your boiler has one.
The correct pressure depends on the appliance, so use the manufacturer’s recommended range rather than assuming that every boiler should show exactly the same reading.
If the pressure is repeatedly falling, don’t simply keep topping it up without finding out why.
Could the Boiler Have a Fault Code?
Yes.
If your boiler display is showing an error or fault code, it may be preventing normal hot-water operation.
Check the exact code and look it up in the manual for your particular boiler.
Don’t assume that the same code means the same thing on every manufacturer or model.
If the fault keeps returning after a reset, professional diagnosis may be needed.
What If the Boiler Fires but the Water Stays Cold?
This can be an important clue.
If you open a hot tap and the boiler appears to respond but the water remains cold or only becomes lukewarm, the problem may involve the hot-water side of the system.
On a combi boiler, a diverter valve or another hot-water component could be involved.
On a cylinder-based system, the problem could instead be related to:
- Cylinder controls
- Thermostat
- Pump
- Motorised valve
- Immersion heater
- Boiler-to-cylinder circulation
The system type therefore matters before diagnosing the fault.
What If Hot Water Works Only When the Heating Is On?
This symptom can be particularly useful when diagnosing a combi boiler.
If your hot water only becomes hot when the central heating is switched on, a faulty diverter valve may be responsible.
Vaillant’s explanation of diverter-valve problems
The valve may not be correctly switching the boiler between heating and hot-water demand.
This is not something you should attempt to repair by opening the boiler yourself.
Could an Airlock Cause No Hot Water?
In some systems, trapped air can interfere with water flow.
An airlock can occur when air becomes trapped in part of the heating or hot-water system, potentially restricting water flow.
Vaillant’s guide to airlocks in hot-water systems
Possible signs can include:
- No hot water
- Reduced hot-water flow
- Banging or tapping sounds
- Poor heating performance
- Unusual behaviour from the hot-water system
If you suspect an airlock and aren’t familiar with your heating system, it’s better to have it investigated rather than randomly opening valves or draining components.
What Can You Safely Check Yourself?
Before calling an engineer, there are several basic checks you can make.
Check the Boiler Display
Look for:
- Fault codes
- Warning symbols
- Pressure readings
- Error messages
Write down the exact information shown.
Check the Heating Controls
Make sure:
- Hot water is switched on
- The programmer is set correctly
- The thermostat is operating normally
- The system hasn’t accidentally been switched to heating-only mode
Check the Boiler Pressure
If your boiler has a pressure gauge, check whether the reading is within the manufacturer’s recommended range.
Check the Power Supply
If the boiler or cylinder controls have no power, check whether there has been a power cut or whether an obvious electrical switch has been turned off.
Don’t remove electrical covers.
Check Whether You Have a Cylinder
If you’re unsure what type of heating system you have, look around the property.
A large insulated cylinder, often located in an airing cupboard, indicates that your home may have a stored hot-water system rather than a combi boiler.
What You Shouldn’t Do
Don’t:
- Remove the boiler casing
- Attempt to repair a diverter valve yourself
- Rewire a cylinder thermostat
- Open electrical covers
- Work on gas components
- Bypass safety controls
- Keep resetting a boiler with a recurring fault
- Drain the system without knowing what you’re doing
Gas-related boiler work should be carried out by an appropriately qualified professional.
When Should You Call a Heating Engineer?
You should arrange professional help if:
- Heating works but hot water remains cold
- Hot water is only lukewarm
- Hot water works only when heating is on
- The boiler repeatedly displays a fault code
- Boiler pressure keeps falling
- The diverter valve is suspected
- The cylinder isn’t heating
- The immersion heater isn’t working
- The hot-water controls aren’t responding
- You cannot identify the cause
Heating Services can be linked here to Homacord’s Heating page for boiler, hot-water and central-heating assistance.
Does No Hot Water Mean I Need a New Boiler?
Not necessarily.
A lack of hot water can be caused by a relatively small component or control problem.
For example, depending on the system, the issue could involve:
- A diverter valve
- Thermostat
- Timer
- Sensor
- Motorised valve
- Immersion heater
- Low pressure
- Another control component
The boiler should be diagnosed before you decide that replacement is necessary.
Replacing an entire boiler because of a fault that could have been repaired isn’t always the most sensible option.
No Hot Water but Heating Works in Your London Home?
If your radiators are working normally but your taps or shower are producing cold water, don’t immediately assume that the whole boiler has failed.
First identify whether you have a combi boiler or a hot-water cylinder system.
With a combi boiler, the diverter valve is one possible cause, particularly if hot water is lukewarm or only works when the heating is running.
With a cylinder-based system, the problem could instead involve the cylinder thermostat, timer, motorised valve, immersion heater or another part of the hot-water system.
If the basic checks don’t identify the problem, professional diagnosis is the safest next step.
Heating Services can link directly to Homacord’s Heating page for heating, boiler and hot-water assistance.
For wider property maintenance requirements, Homacord links naturally to the main Homacord website.
Frequently Asked Questions
Why do I have heating but no hot water?
The cause depends on your heating system. On a combi boiler, a diverter valve or boiler fault could be responsible. On a cylinder-based system, the problem may involve the cylinder thermostat, timer, motorised valve, immersion heater or another component.
Can a diverter valve cause no hot water?
Yes. A faulty diverter valve can prevent a combi boiler from directing heat towards the hot-water supply correctly.
Why is my hot water only lukewarm?
Lukewarm water can have several causes, including a diverter-valve problem, boiler fault, control issue or another hot-water-system problem.
Why does my hot water only work when the heating is on?
On a combi boiler, this can be a sign that the diverter valve isn’t switching correctly between central heating and hot-water demand.
Can low boiler pressure cause no hot water?
Yes. If boiler pressure falls too low, the appliance may stop operating correctly and can lose both heating and hot-water functions.
What if I have a hot-water cylinder?
If you have a cylinder, the problem may be separate from the boiler itself. The cylinder thermostat, programmer, motorised valve, immersion heater or other components may need checking.
Can an immersion heater cause no hot water?
Yes. If your cylinder relies on an immersion heater and the heater or its controls fail, the stored water may not heat correctly.
Can I fix a diverter valve myself?
It’s not recommended. A diverter valve is an internal boiler component, and work on the boiler should be carried out by an appropriately qualified professional.
Should I reset my boiler if I have no hot water?
If the manufacturer’s instructions recommend a reset, you can follow those instructions. However, if the fault returns, don’t keep resetting the boiler without investigating the cause.
When should I call a heating engineer?
If basic checks don’t restore the hot water, or you suspect a diverter valve, cylinder, immersion heater, boiler or control fault, arrange for a qualified heating engineer to diagnose the system.
