🥶 Why Are My Radiators Cold Even When the Boiler’s On?
You’ve turned the heating on, your boiler is working — but your radiators are still cold. Frustrating, right?
If your radiators aren’t warming up as they should, it usually means there’s an issue somewhere in your heating system. The good news? Many of these problems are common, fixable, and preventable.
In this guide, we’ll break down the most likely causes — and what you can do to get your home warm again.
🔧 Common Reasons Radiators Stay Cold
Let’s take a look at the most frequent culprits when your radiators aren’t heating up properly:
🌬️ 1. Air Trapped in the Radiator
Symptoms:
- Radiator is cold at the top, warm at the bottom.
Cause:
- Air gets trapped inside the radiator, blocking the flow of hot water.
Fix:
- Bleed the radiator using a radiator key. Open the valve slightly until air hisses out, followed by a trickle of water. Then close it.
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🔄 Tip: Always check your boiler pressure after bleeding — it can drop slightly and may need topping up.
🧫 2. Sludge or Blockages
- Symptoms:
- Radiator is cold at the bottom, warm at the top.
- Some radiators work, others don’t.
- Cause:
- Over time, sludge (rust, dirt, limescale) builds up in the system, especially in older properties.
- Fix:
- You may need a chemical flush or a powerflush to clean the system.
- In severe cases, individual radiators may need replacing.
🔁 3. Radiator Valves Are Closed or Faulty
- Symptoms:
- One radiator is cold, others are hot.
- Cause:
- The thermostatic radiator valve (TRV) or manual valve may be closed or stuck.
- Fix:
- Make sure the valves are fully open.
- If a TRV is stuck in the closed position, it may need loosening or replacing.
🕹️ 4. Boiler Pressure Is Too Low
- Symptoms:
- Multiple radiators not working properly.
- Boiler may display a low pressure warning.
- Cause:
- Most boilers operate best at 1.0–1.5 bar when cold. Low pressure means water isn't circulating properly.
- Fix:
- Top up your system using the filling loop beneath the boiler (see manufacturer instructions).
- If pressure drops again quickly, there may be a leak.
💨 5. Faulty Pump or Circulation Issues
- Symptoms:
- Boiler is on, but no radiators are heating.
- Rattling or humming noises from the boiler or pipes.
- Cause:
- The system’s pump may be stuck, broken, or not circulating water properly.
- Fix:
- You’ll need a Gas Safe registered engineer to inspect and possibly replace the pump.
⚙️ 6. Diverter Valve Fault (Combi Boilers Only)
- Symptoms:
- You get hot water, but no heating.
- Radiators don’t heat even though boiler fires up for hot water.
- Cause:
- The diverter valve controls whether your boiler sends heat to your taps or radiators. It can get stuck or fail over time.
- Fix:
- A qualified heating engineer will need to test and repair or replace the valve.
✅ Quick Troubleshooting Checklist
| ✅ Check This | Why |
|---|---|
| Bleed the radiators | To release trapped air |
| Check boiler pressure | Should be 1.0–1.5 bar |
| Make sure all radiator valves are open | TRVs can stick |
| Isolate cold radiators | Could indicate sludge |
| Call an engineer if unsure | For safety and proper diagnosis |
🧠 When to Call a Heating Engineer
While bleeding radiators and checking pressure are safe DIY jobs, anything beyond that — like replacing valves, flushing the system, or fixing the pump — should be handled by a Gas Safe registered engineer.
Call a professional if:
- The problem keeps coming back
- You suspect sludge or major blockages
- There are error codes or pressure drops
- Only hot water is working (on a combi boiler)
🏠 Final Thoughts
Radiators staying cold while your boiler is on isn’t just annoying — it’s a sign that your heating system needs some attention. From trapped air to faulty components, the key is identifying the cause early and addressing it before it becomes a bigger (and more expensive) issue.
📞 Need Help Getting Warm Again?
At Gas-Go Heating we offer radiator repairs, system flushing, and full heating system diagnostics.
✅ Fast response. ✅ Friendly service. ✅ Gas Safe registered.
