Radiator Not Getting Hot? Here's What to Check

Why Would a Radiator Stop Working?

A radiator that's cold or only partially warm is a common heating problem. Before assuming the worst, work through these checks — many can be resolved without calling an engineer.

Is It Just One Radiator or Several?

Just one radiator cold: The problem is likely local to that radiator — a closed valve, stuck TRV, or trapped air.

Multiple radiators cold: The problem may be system-wide — low pressure, pump failure, sludge buildup, or a boiler fault.

All radiators cold but hot water works: Points to the heating circuit specifically — could be a zone valve fault or programmer issue.

Check 1: Are the Valves Open?

Every radiator has two valves:

**TRV (thermostatic radiator valve):** The numbered valve usually on one side. Make sure it isn't set to 0 (fully closed).

**Lockshield valve:** The capped valve on the other side. This is set at installation and should be left as-is, but check it hasn't been accidentally closed.

Check 2: Bleed the Radiator

If the radiator is warm at the bottom but cold at the top, trapped air is the likely cause. Bleed the radiator using a bleed key to release the air. Check our separate bleeding guide for the full process.

Check 3: Check Boiler Pressure

Low boiler pressure (below 1 bar) can prevent water reaching all radiators. Check the pressure gauge and repressurise if needed.

Check 4: Is the TRV Stuck?

A TRV that's stuck in the closed position will prevent hot water reaching the radiator. Remove the TRV head and check if the pin is stuck down (closed). Try pressing it gently — if it won't move or springs back immediately, the valve may need replacing.

Check 5: Sludge and Blockage

If the radiator is cold at the bottom but warm at the top, sludge (iron oxide deposits) may be blocking the bottom of the radiator. This requires a power flush or radiator replacement.

Check 6: Balancing

If radiators further from the boiler are consistently cooler than those nearby, the system may need balancing — a process of adjusting lockshield valves to ensure even heat distribution.

If you've checked all of the above and still have a cold radiator, Halo Heating can diagnose and fix the problem. Call 01483 676108.