Radiators Not Working, but Hot Water Is? Here’s How We Can Help

If your radiators are not working but your hot water is, it can be both confusing and frustrating. After all, if your boiler is producing hot water for your taps and shower, why isn’t it heating your home?

The good news is that this is one of the most common heating problems homeowners experience, particularly during the colder months. In many cases, the boiler itself is functioning correctly, but something is preventing heat from circulating properly through the central heating system.

You may find that all the radiators remain cold, only one radiator isn’t heating up, or that certain radiators have cold spots. Issues such as trapped air, low boiler pressure, faulty radiator valves, sludge buildup, or a faulty diverter valve can all prevent warm water from reaching your radiators.

In this guide, our heating engineers explain the most common causes of cold radiators, how to identify the problem, and when it’s time to call a professional heating engineer.

Why Are My Radiators Not Working, but My Hot Water Is?

If your hot water is working normally, your boiler is still producing heat. This usually means the problem lies somewhere within the heating system rather than the boiler’s ability to generate hot water.

Several factors can stop the flow of hot water from circulating properly through your radiators, including:

• Trapped air
• Low boiler pressure
• Faulty radiator valves
• Sludge buildup
• A faulty pump
• Diverter valve issues
• Motorised valve faults
• An unbalanced heating system

The exact cause often depends on the type of boiler and central heating system installed in your property.

Check for Trapped Air in Your Radiators

One of the most common reasons for cold radiators is trapped air.

Air trapped inside a radiator prevents hot water from circulating properly through the entire radiator. As a result, you may notice that the radiator is cold at the top while remaining warmer at the bottom.

Common signs of trapped air include:

• Cold at the top of the radiator
• Gurgling or funny noise sounds
• Uneven heat distribution
• One radiator not heating while others work normally

Fortunately, this is often one of the easiest heating issues to fix.

How to Bleed Your Radiators

To release trapped air, you can bleed your radiators using a radiator key.

1. Turn the heating off.
2. Locate the bleed valve at the top of the affected radiator.
3. Insert the radiator key into the bleed screw.
4. Slowly turn the key a quarter turn.
5. Listen for a hissing sound as the air escape begins.
6. Once water appears, close the bleed valve immediately.

As the radiator releases trapped air, hot water should begin circulating properly again.

If air trapped inside the system repeatedly returns, it may indicate a larger issue that requires professional assistance.

Check Your Boiler Pressure

Low boiler pressure is another frequent cause of cold radiators.

Most manufacturers recommend that boiler pressure sits between 1 and 1.5 bar, although some systems operate correctly between 1 and 2 bar. The correct pressure for your system can usually be found in your boiler manual.

You can quickly check the pressure gauge on the front of your boiler.

If the pressure gauge reads below 1 bar, low water pressure may be preventing hot water from circulating properly around your entire central heating system.

In many cases, repressurising the system using the filling loop will restore normal operation.

However, if boiler pressure continues to drop after topping it up, there could be a leak or another fault within the heating system that requires investigation.

Are Your Radiator Valves Working Properly?

Faulty radiator valves are another common cause of radiators not heating correctly.

Modern radiators usually have two valves:

• A thermostatic radiator valve (TRV)
• A lockshield valve

Both need to be functioning correctly to allow proper heat distribution throughout the property.

Thermostatic Radiator Valve Problems

The thermostatic radiator valve controls the temperature of the radiator.

Over time, a thermostatic valve can become stuck in the closed position, particularly after long periods without use during warmer weather.

If the valve head feels stiff or doesn’t move properly, the thermostatic radiator valve may be preventing hot water from entering the radiator.

A stuck thermostatic radiator valve can often cause:

• One radiator remaining cold
• Uneven heating
• Poor heat distribution

Lockshield Valve Problems

The lockshield valve controls water flow through the radiator and helps keep the entire heating system properly balanced.

It is usually located beneath a plastic cover on the opposite side of the radiator.

If a lockshield valve becomes stuck or is accidentally turned too far clockwise, it can restrict the flow of hot water into the radiator.

In some cases, simply adjusting the valve can restore proper circulation.

Could Sludge Be Blocking Your Radiators?

If your radiator feels cold at the bottom but warm at the top, sludge buildup is often the culprit.

Sludge can accumulate in radiators and pipes over time. It is made up of rust, dirt, and debris that forms naturally within older heating systems.

Because sludge is heavier than water, it settles at the bottom of radiators and pipework, gradually restricting the flow of hot water.

Common symptoms include:

• Cold at the bottom of radiators
• Multiple radiators not heating properly
• Black water when bleeding radiators
• Slow warm-up times
• Noisy pipework
• Poor heating performance

Accumulation of debris or sludge can prevent proper heating throughout the whole system.

A professional power flush may be required for severe sludge issues.

Power flushing removes sludge from heating systems effectively and restores circulation throughout the entire central heating system.

Installing a magnetic filter can also help prevent future sludge buildup and protect your boiler.

Could It Be a Diverter Valve Problem?

If you have a combi boiler, a faulty diverter valve may be responsible.

The diverter valve directs heat either towards the hot water supply or the radiators. If the valve becomes stuck, your boiler may continue producing hot water while preventing heat from reaching the heating system.

This is a particularly common fault when:

• All the radiators remain cold
• Hot water is working normally
• The boiler appears to be functioning correctly

A diverter valve fault generally requires professional diagnosis and repair by a heating engineer.

Could a Motorised Valve Be Causing the Problem?

If your home has a system boiler and hot water cylinder, a faulty motorised valve may be responsible.

Motorised valves act as traffic controllers within the heating system, directing warm water between the radiators and the hot water cylinder.

When a motorised valve fails, your hot water may continue working while the radiators receive little or no heat.

This is a common issue in larger homes with more complex heating systems and often requires professional assistance.

What If Only One Radiator Isn’t Working?

If only one radiator is cold while the rest of the heating system is working properly, the issue is usually isolated to that radiator.

The most common causes include:

• Trapped air
• A stuck thermostatic radiator valve
• A closed lockshield valve
• Sludge buildup
• Restricted water flow

The affected radiator can often be repaired without carrying out work on the entire central heating system.

Could the Heating System Need Balancing?

If the first radiator heats up quickly but others further away remain cold, the heating system may not be properly balanced.

Balancing involves adjusting lockshield valves on each radiator to control flow rates and ensure equal heat distribution throughout the property.

An unbalanced system can result in:

• Some radiators becoming very hot
• Other radiators remaining cold
• Reduced efficiency
• Higher energy consumption

A properly balanced heating system ensures all radiators receive an appropriate flow of hot water.

Heating engineers often work towards achieving approximately a 12°C difference between the flow and return pipes when balancing a system.

Could It Be a Faulty Pump?

The circulation pump is responsible for moving warm water around your heating system.

If the pump becomes faulty, hot water may still be available at your taps while your radiators remain cold.

Common signs of a faulty pump include:

• Cold radiators throughout the property
• Strange noises from the airing cupboard or boiler
• Poor circulation
• Inconsistent heating

A faulty pump can affect the performance of the entire heating system and will usually require professional repair or replacement.

Can I Fix Radiators That Are Not Heating?

Some radiator issues can be resolved without calling an engineer.

Homeowners can often:

• Bleed radiators to release trapped air
• Check boiler pressure
• Repressurise the system
• Ensure radiator valves are open
• Check thermostat settings
• Review fault codes in the boiler manual

However, issues involving a faulty boiler, diverter valve, motorised valve, or pump should always be assessed by a professional heating engineer.

Attempting repairs on heating components without the necessary experience can sometimes make the problem worse.

When Should You Call a Professional Heating Engineer?

You should call a professional if:

• All the radiators remain cold
• Boiler pressure repeatedly drops
• The heating system won’t circulate properly
• You suspect a faulty pump
• A power flush may be required
• Your boiler displays fault codes
• You hear unusual noises from the system
• Your radiators still don’t work after bleeding

These symptoms often indicate deeper problems within the whole system that require professional diagnosis.

How WPJ Heating Can Help

At WPJ Heating, we regularly diagnose and repair heating problems across South and West London.

Our experienced heating engineers can quickly identify whether the issue is caused by trapped air, low boiler pressure, faulty radiator valves, sludge buildup, motorised valve faults, diverter valve problems, or circulation pump issues.

We also provide:

• Boiler repairs
• Power flushing
• Radiator balancing
• Heating system diagnostics
• Central heating repairs
• Boiler servicing

Our Gas Safe-registered engineers work on all major boiler brands and heating systems, providing honest advice and long-term solutions rather than temporary fixes.

Need Help With Cold Radiators?

If your radiators are not working but your hot water is, don’t ignore the problem.

What starts as a minor issue can often lead to reduced efficiency, higher heating bills, and larger repairs if left unresolved.

Whether you’re dealing with one radiator that won’t heat up or a problem affecting your entire central heating system, WPJ Heating’s experienced engineers can help.

Contact WPJ Heating today to arrange a visit and get your heating system working properly again.