🕐 5 min read · 🔧 DIY difficulty: Easy-Medium · 📅 April 2026
A cold or partially heating radiator is a common issue that's usually straightforward to fix yourself. Whether it's cold at the top, cold at the bottom, or completely cold — the cause and fix are different for each.
Cold at the top, warm at the bottom — trapped air. Needs bleeding.
Cold at the bottom, warm at the top — sludge buildup. Needs flushing.
Completely cold — closed valve, faulty pump, or thermostat issue.
One radiator cold, others fine — usually a valve or balancing issue.
Turn off the heating and let it cool. Use a radiator key to open the bleed valve (small square nut at the top corner). Hold a cloth underneath. When water flows steadily instead of air hissing out, close the valve. Check boiler pressure afterwards and repressurise if needed.
The TRV is the valve with numbers on it. Make sure it's not set to zero or frost. If it's stuck, remove the TRV head and try turning the pin underneath with pliers — it can seize up over summer.
The plain valve on the other side of the radiator (lockshield) should be open. Remove the cap and turn the spindle anticlockwise to open it fully.
If some radiators heat quickly and others are always cold, your system needs balancing. Turn all lockshield valves fully open, then gradually close the ones on radiators that heat first to allow flow to reach the colder ones.
If the radiator is cold at the bottom despite being bled and valves open, sludge has settled. Try a chemical flush first (add central heating cleaner, run for an hour, drain). For severe cases, a professional power flush is the most effective solution.
💡 Pro tip: Add a central heating inhibitor after any flush. It prevents sludge and corrosion from building up again and can extend your boiler's life significantly.
⚠️ If your boiler pressure drops significantly after bleeding multiple radiators, repressurise using the filling loop before turning the heating back on.
Describe your radiator problem to Reslvr and get a specific diagnosis and fix for your exact situation.
Get instant diagnosis — free →