Combi Boiler Not Heating Radiators – Common Causes & How to Fix Them

A combi boiler is the primary source of central heating and hot water in most homes throughout the UK. When something malfunctions within your boiler, you will usually be met with an error code or warning light that tells you there is a problem. But if your combi boiler is not heating the radiators, there could be an issue within the system somewhere.

It may not be the end of the world if your combi boiler is not heating radiators, especially if your boiler is still providing hot water when you turn on your taps. But cold radiators aren’t very inviting and may make your home uncomfortable.

In this comprehensive guide, we will cover everything you need to know about why your combi boiler may not be heating radiators and what you can do to try to solve the problem. We will also explain what you can do to get your boiler working again and when to call out a local engineer for expert assistance. Finally, we will let you know how Local Boiler can help with the troubleshooting process and ensure your heating system is running as it should, safely and efficiently.

Understanding How a Combi Boiler Works

Before we start looking into the potential causes of cold radiators, let’s first have a quick recap of how a combi boiler works.

A combi (combination) boiler supplies both heating and hot water to your home directly from the mains supply. There is no need for a separate water cylinder or storage tank, as with a conventional boiler. The boiler heats water to taps and showers on demand, and also circulates hot water around the radiators for space heating.

Key Components of a Combi Boiler:

• Heat Exchanger: Transfers heat from the burner to the water.
  
• Diverter Valve: Directs hot water to either the taps or radiators.
  
• Pump: Circulates water throughout the heating system.
  
• Pressure Gauge: Maintains system pressure (usually between 1–2 bar).
  
• Thermostat & Timer: Controls when and how the heating operates.

When any of these components develop faults, the radiators may fail to heat properly.

Common Reasons Why Your Combi Boiler Isn’t Heating the Radiators

1. Diverter Valve Malfunction

The diverter valve is a critical component that switches the boiler’s focus between heating and hot water. If you notice that you have hot water but no heating, the diverter valve may be stuck or faulty.

Signs of a faulty diverter valve:

• Hot water works, but radiators stay cold.
  
• Some radiators get warm while others don’t.
  
• The heating indicator doesn’t activate on the boiler.

Solution:
The valve can be inspected by a competent engineer. In some cases, cleaning or replacing the diverter valve may solve the issue. Local Boiler engineers have come across this issue numerous times and can carry out a quick repair.

2. Low Boiler Pressure

Boiler pressure is vital for circulating hot water around the heating system. If the pressure drops below the recommended level (usually below 1 bar), the radiators may not heat properly.

How to check boiler pressure:

• Look at the pressure gauge on the front of your boiler.
  
• The ideal range is between 1.0 and 1.5 bar when the system is cold.

If pressure is low:

• You can re-pressurise the system by using the filling loop.
  
• Follow your boiler’s manual carefully, or ask for guidance from a professional.
  

Tip: If your boiler pressure keeps dropping frequently, there could be a leak in the system. Contact a Gas Safe engineer for inspection, Local Boiler can help with diagnosing such pressure-related issues safely.

3. Air Trapped in the Radiators

Air pockets inside the radiators can block the flow of hot water, leading to uneven heating or cold spots.

How to identify trapped air:

• The radiator feels warm at the bottom but cold at the top.
  
• You hear gurgling or bubbling sounds.

Fix:

• Bleed the radiator using a radiator key.
  
• Start from the radiator farthest from the boiler and work your way closer.
  
• Make sure to top up the boiler pressure afterwards.

4. Faulty Thermostat or Settings Issue

A malfunctioning thermostat or incorrect settings could prevent your boiler from turning the heating on.

Checklist to ensure your thermostat is working properly:

• Make sure the temperature setting is above the current room temperature.
  
• Check if the timer is set correctly for your heating schedule.
  
• Replace batteries in wireless thermostats if applicable.

If the thermostat is outdated, upgrading to a smart thermostat could improve heating efficiency.

5. Circulation Pump Failure

The pump is responsible for circulating hot water throughout your radiators. When it fails, the boiler may heat water, but it won’t reach your radiators effectively.

Common signs:

• Radiators stay cold even though the boiler is running.
  
• You hear strange humming or vibrating noises from the pump.
  
• The pump feels unusually hot or completely cold to the touch.

Fix:
An engineer can assess whether the pump needs cleaning, unblocking, or replacing.

6. Sludge or Debris Build-Up

Over time, sludge (a mixture of rust, dirt, and debris) can accumulate in your radiators and pipes. This build-up restricts hot water flow and makes radiators inefficient.

Symptoms of sludge build-up:

• Radiators heat unevenly.
  
• The boiler makes kettling noises.
  
• It takes longer for rooms to warm up.

Solution:
A power flush can remove sludge and restore full heating performance. Regular system cleaning is recommended every 5–6 years to prevent this issue.

7. Frozen Condensate Pipe

During winter, the condensate pipe (which carries waste water from your boiler) can freeze, blocking the system and shutting down the boiler.

How to spot a frozen condensate pipe:

• The boiler displays a fault code.
  
• You hear gurgling noises.
  
• The boiler repeatedly shuts off.

What you can do:

• Gently pour warm (not boiling) water over the pipe.
  
• Once thawed, restart the boiler.

If the issue persists, call an engineer, Local Boiler provides fast callouts for such winter emergencies.

Step-by-Step: How to Troubleshoot When Your Radiators Aren’t Heating

Before calling a professional, there are a few checks you can safely perform:

1. Check Your Boiler Pressure

• Ensure the gauge reads between 1–1.5 bar.
  
• Repressurise if needed.

2. Inspect the Thermostat

• Make sure it’s turned up higher than room temperature.
  
• Check the timer or programmer settings.

3. Bleed the Radiators

• Release trapped air to restore circulation.

4. Check for Power or Error Codes

• Reset your boiler if an error code appears.
  
• Refer to the manufacturer’s manual.

5. Test the Diverter Valve (if confident)

• Run hot water and check if radiators heat slightly — if not, the valve may be stuck.

If none of these resolve the problem, contact a qualified heating specialist like Local Boiler for professional help.

Preventative Maintenance: Keep Your Combi Boiler Running Efficiently

Preventive care can go a long way in avoiding heating problems. Regular maintenance ensures consistent performance, improves energy efficiency, and extends your boiler’s lifespan.

Top maintenance tips:

• Book an annual boiler service with a Gas Safe engineer.
  
• Bleed radiators twice a year.
  
• Check for leaks and corrosion around the boiler and pipes.
  
• Keep the condensate pipe insulated in winter.
  
• Schedule periodic system flushing to remove sludge.

Routine inspections by trusted engineers, such as those from Local Boiler, help identify potential issues early before they turn into costly repairs.

When to Call a Professional Engineer

While minor issues like bleeding radiators or adjusting pressure can be done safely, many boiler problems require expert attention.

You should call a professional if:

• The boiler displays fault codes you can’t reset.
  
• The system pressure constantly drops.
  
• Radiators remain cold after troubleshooting.
  
• You suspect internal faults like a broken diverter valve or faulty pump.

Never attempt to open or repair gas components yourself. Always contact a Gas Safe registered engineer.

How Local Boiler Helps You

If your combi boiler is not heating radiators, you can contact Local Boiler for help. Our expert engineers can quickly diagnose the problem and carry out the necessary repairs to restore your home’s heating system.

Here’s how Local Boiler can assist:

• Expert diagnosis: Experienced engineers quickly pinpoint faults like stuck diverter valves or pump failures.
  
• Reliable repairs: They handle everything from component replacements to system flushing.
  
• Guidance and support: Local Boiler provides clear advice to help you maintain your boiler safely.
  
• Emergency service: Quick response times during heating breakdowns, especially in colder months.

With a strong reputation for quality workmanship, Local Boiler ensures your home’s heating system runs smoothly, safely, and efficiently.

For more information, visit Local Boiler to learn about boiler servicing, repairs, and maintenance tips.

Conclusion

If a combi boiler isn’t heating radiators, there are a number of potential culprits – anything from air in the system to a faulty component. Routine maintenance, regular servicing, and safe troubleshooting can avoid most problems.

If your system is still misbehaving, or you’re not sure where to start, call Local Boiler. Our experts are here to make sure that every home is warm, safe and energy efficient throughout the year.

FAQs

1. Why does my combi boiler give hot water but not heat the radiators?

This is often due to a stuck diverter valve that directs heat to hot water only. A qualified engineer can repair or replace it.

2. Can low pressure stop my radiators from heating?

Yes, if boiler pressure drops below 1 bar, it can prevent hot water from circulating through radiators. Repressurising usually resolves the issue.

3. Should I bleed radiators if they’re not heating up?

Absolutely. Air trapped inside radiators prevents them from heating fully. Bleeding helps restore even heat distribution.

4. How often should I service my combi boiler?

You should service your boiler annually to maintain efficiency and detect early signs of faults.

5. Can I fix a frozen condensate pipe myself?

Yes, by gently thawing the pipe using warm water. However, if it repeatedly freezes, ask a professional like Local Boiler to inspect the insulation or pipe route.