Turn the heating off and allow the system to cool before bleeding or adjusting radiators
Radiators contain hot pressurised water. Stop if a fitting is seized, leaking heavily or boiler pressure repeatedly drops.
MOST LIKELY CAUSES
Start with these
Sludge buildup
Magnetite settles at the bottom and blocks water flow.
Restricted valve
A partly closed or blocked valve limits circulation.
Poor system balance
Other radiators may take too much flow.
Weak circulation
Pump or pipe restrictions may reduce movement.
Symptom variations Match the exact heat pattern, sound or leak position. ⌄
- Bottom strip stays cold
- Radiator warms slowly
- Dark water when drained
- Several older radiators affected
- Heat improves when others are closed
Quick checks Simple checks before removing fittings. ⌄
- Confirm the heating is on.
- Check whether one radiator or the whole system is affected.
- Inspect TRV and lockshield positions.
- Look for leaks, air and corrosion.
- Let the system cool before bleeding.
Step 1 — Check both valves Turn the heating off and allow the radiator to cool first. ⌄
Confirm the TRV and lockshield are open and not seized.
Step 2 — Compare other radiators Turn the heating off and allow the radiator to cool first. ⌄
Work out whether the problem is local or system-wide.
Step 3 — Arrange flushing Turn the heating off and allow the radiator to cool first. ⌄
A heating engineer can flush the radiator or system if sludge is suspected.
Common causes explained Why radiator faults develop. ⌄
A radiator that is warm at the top but cold at the bottom often has sludge or debris restricting flow through the lower waterways.
Advanced diagnostics Heating-engineer checks and repair work. ⌄
- Measure flow and return temperatures.
- Inspect valve internals.
- Assess system water quality.
- Check pump performance.
Parts that may need replacing Common components linked to this fault. ⌄
- Radiator valve
- Lockshield valve
- Magnetic filter
- Inhibitor
- Radiator
When to call a heating engineer Conditions that should end basic troubleshooting. ⌄
- Boiler pressure keeps dropping.
- A radiator or valve is leaking heavily.
- Valves are seized or corroded.
- Several radiators remain cold.
- The pump or boiler may be faulty.
Frequently asked questions Common radiator questions. ⌄
Should I bleed radiators while heating is on?
No. Turn the heating off and allow the system to cool first.
Why did boiler pressure drop?
Releasing air can reduce pressure in a sealed system.