No Hot Water Coming From Your Boiler? Causes & How To Fix It

Last Updated on February 18, 2025

If you’ve turned on your taps only to find no hot water coming from your boiler, you’re not alone. This is a common issue for UK homeowners and landlords, but the good news is that there are several possible causes, some of which you can troubleshoot yourself before calling in a heating engineer.

In this guide, we’ll walk you through the most common reasons why your boiler isn’t producing hot water and what you can do about it.

Common Reasons Why Your Boiler Isn’t Producing Hot Water

Below, we break down the most common reasons your boiler might not be heating water and what you can do about it.

1. No Power or Gas Supply to the Boiler

Your boiler needs a reliable power source and, for gas boilers, an uninterrupted gas supply to function correctly. If either is disrupted, your boiler won’t be able to ignite and produce hot water.

What to Check:

• Power Cut: If your home has experienced a recent power cut, the boiler may have switched off and failed to restart. Check if other appliances are working.
• Tripped Fuse: Boilers are connected to a fused spur switch or a dedicated circuit on your consumer unit (fuse box). If a fuse has tripped, reset it and try turning the boiler back on.
• Boiler Power Switch: Make sure the boiler’s power switch is turned on. It might have been accidentally switched off during routine cleaning or maintenance.

Gas Supply Issues (For Gas Boilers) – If your boiler runs on mains gas, check the following:

• Other gas appliances (e.g., gas hob) – If they aren’t working either, your gas supply may be off.
• Emergency Gas Shut-Off Valve – Make sure it hasn’t been accidentally turned off.
• Supplier Issues – Contact your gas supplier to check if there’s a network issue or planned maintenance in your area.

What to Do:

If your boiler is still unresponsive after these checks:

• Electrical faults should be checked by an electrician.
• Gas supply issues should be assessed by your gas supplier or a Gas Safe registered engineer.
• If your boiler is repeatedly losing power or struggling to ignite, there may be an internal component failure requiring professional repair.

2. Low Boiler Pressure

Most modern combi boilers rely on adequate pressure to circulate hot water effectively. If the boiler pressure drops too low, it may struggle to function.

Why Does Boiler Pressure Drop?

• Leaks in the System: Even small leaks in pipework or radiators can cause pressure loss over time.
• Bleeding Radiators: Releasing trapped air from your radiators can lower pressure in the system.
• Faulty Pressure Relief Valve: If the valve is damaged, it may be causing water to escape, reducing pressure.

How to Fix It:

1. Locate the pressure gauge on your boiler—most models display this on the front panel.
2. If the pressure is below 1 bar, it needs to be increased.
3. Use the filling loop (a small flexible hose underneath the boiler) to slowly add water to the system until the pressure reaches 1.5 bar.
4. Close the filling loop and restart your boiler.

Warning: If pressure keeps dropping even after topping it up, there may be a hidden leak. Contact a Gas Safe engineer for further inspection.

3. Faulty Diverter Valve

The diverter valve is a critical component in combi boilers, directing hot water either to your radiators or taps. If the diverter valve is stuck or malfunctioning, your central heating may work fine, but you won’t get hot water from your taps.

Signs of a Faulty Diverter Valve:

• Hot radiators but no hot water at taps.
• Intermittent hot water that turns cold quickly.
• Boiler heating up but no hot water being delivered.

Solution:

This isn’t a DIY fix. Replacing or repairing a diverter valve requires a professional. A Gas Safe registered engineer will assess whether the valve needs adjustment, cleaning, or a complete replacement.

4. Frozen Condensate Pipe (Common in Winter)

In colder months, your boiler’s condensate pipe (which removes acidic water waste from the boiler) can freeze, blocking the flow and causing the boiler to shut down as a safety precaution.

How to Identify a Frozen Condensate Pipe:

• Your boiler may display a fault code (check the user manual).
• You might hear gurgling noises from the boiler.
• The pipe (usually running outside to a drain) may be visibly frozen or blocked with ice.

How to Fix It:

1. Locate the condensate pipe (a plastic pipe running from the boiler to the outside).
2. Carefully pour warm water (not boiling) over the pipe to thaw the ice.
3. Once the ice has melted, reset your boiler and check if hot water is restored.

Prevention Tip: Insulating the condensate pipe with foam lagging can help prevent freezing during winter.

5. Thermostat or Timer Issues

If your thermostat or timer settings are incorrect, your boiler may not be activating when needed.

What to Check:

• Thermostat Settings: Ensure the temperature is set high enough (above 50°C) to trigger the boiler.
• Timer/Programmer: If you’ve had a recent power cut, the timer may have reset, meaning your boiler isn’t turning on when expected.
• Battery-Powered Thermostats: If your thermostat uses batteries, check if they need replacing.

How to Fix It:

• Adjust the thermostat to a higher setting and see if the boiler responds.
• Check the timer and reset it if necessary.
• If the thermostat isn’t working correctly, it may need to be replaced.

6. Pilot Light or Ignition Failure

Older boilers have a pilot light (a small flame that stays lit to ignite the boiler), while modern models use an electronic ignition system. If either fails, your boiler won’t fire up.

Common Causes:

• A strong draft may have blown out the pilot light.
• A blocked burner can prevent ignition.
• A faulty gas supply could stop the boiler from lighting.

What to Do:

• For older boilers: Check the manufacturer’s guide on how to relight the pilot light safely. If it keeps going out, there may be a bigger issue.
• For modern boilers: Check for an error code related to ignition failure. A Gas Safe engineer can diagnose and fix the issue.

Safety Warning: If you smell gas, do not attempt to relight the boiler. Call the National Gas Emergency Service on 0800 111 999 immediately.

7. Faulty Heat Exchanger

The heat exchanger is responsible for transferring heat from the boiler’s burner to the water supply. If it’s damaged, the boiler will struggle to heat water efficiently.

Symptoms of a Faulty Heat Exchanger:

• Water temperature fluctuates (hot then cold).
• Unusual noises from the boiler, such as whistling or banging (this may indicate limescale buildup).
• Heating works, but no hot water.

Solution:

A blocked or damaged heat exchanger is a serious issue that typically requires professional repair or boiler replacement. A Gas Safe engineer can assess whether a power flush (to remove sludge and limescale) or a full heat exchanger replacement is necessary.

When to Call a Professional

While some boiler issues can be fixed with basic troubleshooting, others require a trained heating engineer. You should call a professional if:
✅ Your boiler keeps losing pressure.
✅ The pilot light won’t stay on.
✅ The boiler is making unusual noises.
✅ There’s a suspected gas leak.
✅ The problem persists after troubleshooting.

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Is your boiler beyond repair, or are you tired of constant breakdowns? Upgrading to a new energy-efficient boiler can save you money on energy bills and ensure you have reliable hot water when you need it.