No Hot Water From Boiler? 7 Causes & What to Check

No hot water from boiler? Start by checking whether the problem affects every hot tap, whether your central heating still works and whether the boiler is displaying a fault code. These details can quickly help narrow down what is happening.

Some causes are simple, such as a loss of power or low boiler pressure. Others, including a faulty diverter valve, sensor or internal boiler component, need professional diagnosis.

No Hot Water From Boiler: What Should I Check First?

Before assuming a component has failed, check the basics that are safely accessible to you:

• Does cold water still come from the taps?
• Is the boiler display switched on?
• Is there a fault code showing?
• What does the boiler pressure display?
• Does your central heating still work?
• Is every hot tap affected or only one?

If only one tap or shower has a problem while the other hot-water outlets work normally, the fault may be with that outlet rather than the boiler.

If you have no heating and no hot water, check the boiler display, power, pressure and any fault code before arranging a repair.

7 Common Causes of No Hot Water

1. The Boiler Has Lost Power

Even a gas boiler needs electricity for its controls, pump and other components.

If the display is completely blank, check whether there has been a power cut and whether the normal boiler electrical switch has been turned off.

If other electrical equipment is also not working, there may be a wider electrical-supply issue rather than a boiler fault.

2. Boiler Pressure Is Too Low

Low pressure can stop some sealed heating systems from operating correctly and may result in no heating or hot water.

Check the pressure gauge or digital pressure reading. The correct range depends on the boiler, so follow the manufacturer’s instructions rather than assuming one pressure figure applies to every appliance.

If you keep restoring the pressure and it falls again, see our guide to why boiler pressure keeps dropping.

If you simply want to understand the normal pressure range, read our boiler pressure guide.

3. The Boiler Has Locked Out

Modern boilers can stop operating when they detect a fault. When this happens, the display will often show an error or fault code.

Write the code down before resetting the appliance.

If your boiler manual allows a normal user reset, you can follow the manufacturer’s instructions. If the fault returns, repeatedly resetting the boiler is not fixing the underlying problem.

For recurring shutdowns, see why your boiler keeps turning off.

4. A Diverter Valve May Be Faulty

This is particularly relevant to combi boilers.

A combi boiler needs to direct heat either towards your radiators or towards domestic hot water. The diverter valve helps control this.

Possible clues include:

• the radiators get hot but the taps stay cold;
• the tap water only becomes lukewarm;
• hot water seems to work only while the central heating is on.

These symptoms do not prove the diverter valve has failed because other faults can create similar problems.

Engineer note: this is why we would not recommend buying a replacement component purely from the symptom. The boiler should be diagnosed first.

Read more in our guide to a faulty boiler diverter valve.

5. The Boiler Is Not Detecting Hot-Water Demand

When you open a hot tap on a combi boiler, the appliance needs to recognise that hot water is being requested.

A useful check is simply to watch the boiler display while opening a hot tap. Does the display change? Does the boiler attempt to fire?

You do not need to open the appliance or test components yourself. Just tell the engineer what you observed.

6. A Condensate Problem Has Stopped the Boiler

Condensing boilers produce condensate water that is normally carried away through a condensate pipe.

In very cold weather, exposed condensate pipework can sometimes freeze and cause the appliance to shut down.

A fault code may appear when this happens. Check the boiler manufacturer’s instructions before taking any action.

For more information, see our guide explaining what a boiler condensate pipe is.

7. A System or Regular Boiler Has a Hot-Water Control Problem

If you have a system or regular boiler with a separate hot-water cylinder, the diagnosis is different from a combi boiler.

The boiler may still be producing heat while a control, valve, cylinder thermostat or another part of the hot-water system prevents the cylinder from heating correctly.

If you’re unsure which type of boiler you have, see our guide: What Type of Boiler Do I Have?

Heating Works but There Is No Hot Water

This is useful information to give an engineer.

If the radiators heat normally but the taps remain cold, we know the boiler can still produce heat and the fault may be more specific to domestic hot-water operation.

On a combi boiler, a diverter-valve problem is one possibility, but sensors, controls and other components may also need checking.

The important point is to diagnose the boiler rather than replacing parts based on one symptom.

No Heating and No Hot Water

If both the heating and hot water have stopped, start with:

• boiler electrical power;
• normal boiler controls;
• boiler pressure;
• any fault code displayed;
• water supply;
• whether other gas appliances are operating normally, where applicable.

Manufacturer homeowner guidance similarly recommends checking basic supplies, pressure and the boiler display before arranging professional diagnosis. Vaillant also notes that many boiler faults should be handled by a Gas Safe registered engineer.

Why Does My Hot Water Keep Going Hot and Cold?

Intermittent hot water is slightly different from having no hot water at all.

Before booking, note whether:

• the water begins hot and then turns cold;
• the temperature repeatedly rises and falls;
• the problem happens at every outlet;
• it happens when another tap or shower is opened;
• the boiler repeatedly fires and stops.

These details can help distinguish between a boiler fault, water-flow problem and an issue affecting one particular tap or shower.

Can I Fix No Hot Water Myself?

You can carry out normal homeowner checks such as looking at the boiler display, checking the pressure and following manufacturer-approved controls or reset instructions.

Do not remove the boiler casing or attempt internal repairs.

If you still have no hot water from boiler after checking the normal controls, pressure and fault display, professional fault diagnosis may be needed.

See our boiler repair service if the problem continues.

When Should I Call a Boiler Engineer?

Arrange professional diagnosis if:

• the boiler repeatedly locks out;
• the same fault code keeps returning;
• the heating works but there is consistently no hot water;
• the boiler does not respond when a hot tap is opened;
• the hot water repeatedly changes between hot and cold;
• the boiler pressure keeps falling;
• water is leaking from the boiler;
• the normal homeowner checks do not solve the problem.

If the boiler is also leaking, see our guide to a boiler leaking water.

No Hot Water in Cardiff?

If you’re in Cardiff and your boiler has stopped producing hot water, visit our dedicated Boiler Repair in Cardiff page.

When booking, tell us whether the heating still works, the boiler make and model, any fault code displayed and whether every hot tap is affected.

Quick Questions

Why do I have heating but no hot water?

If the radiators work but the hot taps remain cold, the fault may be specific to domestic hot-water operation. On a combi boiler, the diverter valve is one possible cause, but the appliance should be diagnosed before replacing parts.

Can low boiler pressure cause no hot water?

Yes. If system pressure is too low, some boilers may stop operating correctly. Check the pressure display and the instructions for your particular boiler.

Why do I have no hot water but no fault code?

Not every boiler problem necessarily creates an obvious fault code. The way the appliance responds when a hot tap is opened can still provide useful diagnostic information.

Why does my hot water only work when the heating is on?

On a combi boiler, this can be associated with a diverter-valve problem, although other faults can produce similar symptoms. Proper diagnosis is needed before replacing components.

Should I keep resetting the boiler?

No. If your manufacturer allows a normal user reset, trying it once may be appropriate. If the same fault returns, repeatedly resetting the appliance is not a repair.

Need Help With a Hot-Water Boiler Fault?

Let’s Heat can investigate no-hot-water faults, boiler lockouts, pressure problems, leaks and other boiler issues.

You don’t need to know which component has failed before booking.

Book Boiler Repair →

Sources Checked

This guide has been checked against current homeowner boiler guidance from
Vaillant.