Why Is Radiator Not Heating Up? (7 Causes & Fixes)

A radiator that stays cold while the rest of your heating system is running is a common problem — and usually one you can fix yourself. Here are the most common causes and how to diagnose and fix each one.

1. Trapped Air in the Radiator

Most common cause. Air trapped inside the radiator prevents hot water from filling it completely. The radiator will be cold at the top but warm at the bottom — a classic sign of trapped air.

Fix: Bleed the radiator. You'll need a radiator bleed key — the Mudder 11-Piece Utility Key Kit includes a 4-way bleed key along with other useful home maintenance keys. See our full guide on how to bleed a radiator.

2. Thermostatic Radiator Valve (TRV) Stuck or Turned Off

The TRV on the side of the radiator controls how much hot water flows through it. If it's turned to 0 or the minimum setting, the radiator won't heat up. TRVs can also get stuck in the closed position, especially after summer when they haven't been used.

Fix: Check the TRV setting — turn it to 3 or higher. If it feels stuck, try gently turning it back and forth to free the pin inside. If the TRV is faulty, replace it. The Honeywell Home Thera-5 TRV is a reliable replacement that fits the standard M30 x 1.5 connection.

3. Lockshield Valve Closed

The lockshield valve on the other side of the radiator (the capped one) controls flow rate. If it was accidentally closed during maintenance or balancing, the radiator won't receive hot water.

Fix: Remove the cap from the lockshield valve and open it by turning counterclockwise. Start with 1–2 full turns and check if the radiator warms up.

4. Radiator Needs Balancing

If radiators closer to the boiler are very hot while distant ones stay cold, the system is out of balance. Hot water takes the path of least resistance and doesn't reach the furthest radiators.

Fix: Balance the radiators by adjusting the lockshield valves. See our guide on how to balance radiators.

5. Sludge Buildup Inside the Radiator

Over time, rust and debris accumulate inside radiators as a black sludge (magnetite). This blocks water flow and creates cold spots — often cold at the bottom rather than the top. The radiator may feel warm at the top but cold in the middle or bottom.

Fix: A power flush by a heating engineer clears sludge from the whole system. For a single radiator, you can remove it and flush it with a hose. Adding a magnetic filter to the system prevents future sludge buildup.

6. Boiler Pressure Too Low

Central heating systems need to maintain adequate water pressure (typically 1–2 bar) to circulate water through all radiators. If pressure drops too low, distant radiators may not receive enough flow.

Fix: Check the pressure gauge on your boiler. If it's below 1 bar, top up the system using the filling loop (consult your boiler manual). If pressure keeps dropping, there may be a leak in the system.

7. Pump Failure

The circulation pump moves hot water from the boiler through the radiators. If it fails, water doesn't circulate and radiators stay cold even though the boiler is running. Signs include the boiler firing but no radiators heating up, or the boiler overheating.

Fix: This requires a heating engineer. Pump replacement is a common repair and usually straightforward for a professional.

Quick Diagnostic Guide

Start with the simplest checks — TRV setting and bleeding — before moving to more complex causes. Most single-radiator problems are solved by bleeding or freeing a stuck TRV.