Understanding Why Your Radiators Stay Cold
It's a familiar and frustrating scenario: you turn on the heating, the hot water flows freely from your taps and shower, yet your radiators stubbornly refuse to warm up. This common issue, where the hot water is working but not the radiators, often points to a problem within the central heating circuit, distinct from the domestic hot water supply. While your boiler is clearly functioning to heat water, a blockage, pressure drop, or a specific component failure is preventing that heat from reaching your radiators.
The good news is that many of these problems can be diagnosed and even resolved with some straightforward checks around your home, saving you the cost of an immediate engineer call-out. Understanding the distinction between your hot water system and your central heating circuit is key. The hot water working suggests the boiler's main heat exchanger and primary water flow are functional. The issue lies with the secondary loop that circulates hot water specifically to your radiators.
- Differentiate between domestic hot water and central heating circuits.
- Boiler functioning for hot water is a good sign for primary heating.
- Blockages or pressure issues often affect radiators specifically.
- DIY checks can resolve many 'radiators not working' faults.
Diagnosing this fault effectively means looking at components and processes exclusive to the heating side of your system. We'll explore the most probable culprits, starting with the simplest potential fixes.
Common Causes and Affordable Fixes for Cold Radiators
When your radiators are not working but the hot water is, the problem often lies with the central heating circulation system. The most frequent offenders are air build-up, low system pressure, or a faulty diverter valve. Air trapped in the radiators prevents hot water from circulating effectively, leading to cold spots or entirely cold radiators. Low system pressure can mean the pump isn't strong enough to push water through the entire system. A stuck or faulty diverter valve (in combi boilers) might be directing all hot water to the taps and not the heating circuit.
Check your system pressure gauge. This is usually found on the boiler itself and should typically be between 1 and 1.5 bar when the system is cold. If it's below this, you may need to repressurise your boiler. Refer to your boiler's manual for specific instructions on how to do this safely, usually involving a small filling loop tap. If the pressure is fine, the next step is bleeding your radiators. Use a radiator bleed key to carefully open the valve at the top of each cold radiator; you'll hear a hiss as air escapes. Once water starts to drip out, close the valve immediately. Repeat for all cold radiators. Be prepared for a small amount of water to escape, so have a cloth and container ready.
If bleeding radiators causes a significant drop in system pressure, you'll need to repressurise again afterwards. If after repressurising and bleeding, the radiators still remain cold, and hot water continues to function normally, the issue might be more complex, potentially involving a sludge build-up in the system that requires a power flush, or a problem with the pump or diverter valve itself, which would likely require a qualified heating engineer.
If you notice several radiators are only cold at the top and warm at the bottom, they definitely need bleeding. If they are cold all over, it points to a circulation issue rather than just air.
Investigating the Pump and Valves
The central heating pump is responsible for circulating hot water from the boiler to your radiators. If the pump has failed or is jammed, heat won't reach the radiators. Similarly, the diverter valve (common in combi boilers) switches flow between the heating and hot water circuits. If it's stuck in the hot water position, your radiators will stay cold. These components are best inspected by a professional, but their failure is a primary reason for the 'hot water working, radiators not' scenario.
When to Call a Professional
What should you do if you've repressurised the system, bled all the radiators, and the problem persists? When your radiators aren't working but your hot water is still flowing, and your DIY efforts haven't resolved the cold radiators, it's time to seek expert advice. This usually indicates a more significant issue that requires specialised tools and knowledge.
Potential problems include a blocked pump, a faulty diverter valve that isn't switching correctly between heating and hot water, significant sludge build-up within the pipework and radiators (requiring a power flush), or a control system issue. An experienced heating engineer can accurately diagnose the exact fault. They can check if the pump is receiving power and is operational, if the diverter valve is functioning as it should, and assess the overall health of your central heating system. Attempting repairs on these components without expertise could lead to further damage or safety risks.
When hot water works but radiators don't, focus your initial checks on system pressure and air in the radiators, as these are the most common and affordable fixes.
For those with a Worcester Bosch or Vaillant boiler, or any other brand, specific internal components might be involved. Don't hesitate to contact a Gas Safe registered engineer. They possess the diagnostic equipment to pinpoint issues like low flow rates, solenoid valve faults, or control board errors that prevent the radiators from heating up, even when the boiler is producing hot water.