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Quick answer
When only one radiator stays cold, the problem is almost always at that radiator rather than the boiler. The most common causes are a stuck thermostatic valve (TRV) pin, air trapped at the top, a lockshield valve that has been closed, or a system that is not balanced so water takes the easier route to other radiators. Sludge can also block a single radiator. Most of these can be checked in a few minutes with nothing more than a radiator key and a small spanner.
Key takeaways
- A single cold radiator is usually a local fault: TRV pin, air, valve position or balancing.
- A stuck TRV pin is the most common cause after a summer with the heating off.
- Cold at the top means air; cold at the bottom or in the middle usually means sludge.
- Lockshield valves are easy to knock shut during decorating or flooring work.
- If the radiator is hot at one end only and warm at the other, balancing is often the fix.
Step 1: Check the TRV
Turn the TRV head to maximum. If the radiator stays cold, unscrew the head (most unscrew by hand or with a collar) and look at the small metal pin underneath. It should move up and down freely when pressed. After months of the heating being off, it often sticks down, holding the valve closed. Tap it gently or work it up and down with a pair of grips, being careful not to force it. Refit the head and test. The same thing happens most autumns, and we see it constantly in October.
Step 2: Check for air
If the radiator is warm at the bottom but cold at the top, it has air in it. Bleed it with a radiator key, holding a cloth under the bleed screw until water appears. If you have a sealed system, check the boiler pressure afterwards and top up if it has dropped. Our guide to repressurising a combi covers this.
Step 3: Check the lockshield valve
At the other end of the radiator is the lockshield valve, usually under a plain plastic cap. Remove the cap and turn the spindle anticlockwise with a spanner. If it has been fully closed, the radiator will get no flow at all. Count the turns as you open it in case you need to rebalance later.
Step 4: Look for sludge
A radiator that is cold in the middle or along the bottom, with hot pipes feeding it, is often partly blocked with magnetite sludge. This is a sign the whole system needs attention, not just that radiator. We explain it in radiators cold at the bottom. A magnetic system filter and correct inhibitor help stop it coming back.
Step 5: Consider balancing
If the radiator warms slowly and only at one end, especially if it is the furthest from the boiler, the system may be unbalanced. Water takes the path of least resistance, so radiators near the boiler can grab most of the flow. Balancing adjusts the lockshield valves to share flow fairly. It takes patience but no special tools beyond a thermometer.
When it is not the radiator
If the pipes to the radiator are cold too, the problem is further back: a closed or faulty zone valve, a failing pump on a large system or a blocked section of pipe. And if several radiators on one floor are cold, see upstairs radiators not heating. These are jobs for a heating engineer.
Book with Gasmaster
Still got a radiator that will not warm up? Gasmaster Plumbing & Heating, Gas Safe registered (220932), fixes central heating problems across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. Get in touch or book through our boiler service booking page.
Frequently asked questions
Why is only one radiator cold after the summer?
A stuck TRV pin is by far the most likely reason. The pin sits in one position for months and seizes.
Can I replace a TRV myself?
Replacing a TRV head is simple. Replacing the valve body means draining at least part of the system and refilling a sealed system correctly, which many people prefer to leave to an engineer.
Should the pipes to a cold radiator be hot?
If the flow pipe is hot and the radiator is cold, the issue is the valve or the radiator. If the flow pipe is cold, look further back in the system.
Will a power flush fix one cold radiator?
If the cause is sludge across the system, cleaning the system helps. If it is just a stuck pin or a closed valve, a flush is unnecessary.
Sources
- Energy Saving Trust, heating controls guidance (energysavingtrust.org.uk)
- BEAMA, thermostatic radiator valve guidance (beama.org.uk)
- Gas Safe Register (gassaferegister.co.uk)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
