Radiators cold at the top? Here's the real fix
Cold at the top usually means trapped air — but if bleeding doesn't solve it, there's a bigger problem. Here's the ladder of causes from cheapest to biggest.
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The likely causes, in order
- Trapped air — Bleed the radiator with a key at the top — you'll hear air hiss out before water comes.
- System needs balancing — Radiators furthest from the boiler often get less flow. Adjusting the lockshield valves distributes heat properly.
- Sludge (black iron oxide) — If the radiator is cold at the BOTTOM instead, it's magnetite sludge sitting in the base. A power flush restores full circulation.
- Failed TRV — Thermostatic valves can seize in the closed position over summer — often solved by loosening the top and freeing the pin.
What you can safely try first
- Turn off heating, let it cool, bleed radiator with a key at the top
- Check pressure afterwards — top up if it drops below 1 bar
- If it comes back cold within days, the underlying issue isn't air
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Frequently asked questions
How often should I bleed radiators?▾
Once a year is normal. More often and there's likely air being drawn in somewhere — often via a leaking automatic air vent.
What's a power flush?▾
A high-flow cleaning process that removes sludge and debris from your entire heating system. Typically £450-750 depending on system size.
Related pages you might want
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