One Radiator Not Working When the Rest Are Hot

Why only one radiator?

Every radiator on the system gets water from the same boiler and the same pump. If all the others are warm, the water is hot and moving. What is different about the cold one is what happens at its two valves and inside its panel. That narrows the job to a handful of causes, and you can rule most of them in or out in twenty minutes.

Before you start, note two things. Is the pipe going into the radiator hot, or cold as well? And was anything done recently: decorating, a new radiator, a drain-down, a new boiler? A radiator that stopped after the walls were painted almost always has a valve left shut. One that stopped at the start of the heating season usually has a stuck pin.

Pipe hot, radiator cold: the valve is not letting water in

Follow both pipes down from the radiator to where they leave the floor or wall. If the one feeding it is hot right up to the valve and the radiator is cold after that, water is stopping at the valve.

• Turn the numbered TRV head to maximum. If nothing changes, unscrew the head (most unscrew by hand or have a locking ring) and look at the small metal pin underneath. Press it gently: it should spring back up. If it stays down, it has stuck shut. See freeing a stuck thermostatic radiator valve
• With the head off and the pin up, the valve is fully open. If the radiator now heats, the pin was the problem, or the head itself has failed
• Pull the cap off the valve at the other end. That is the lockshield. If it turns anticlockwise more than a turn or two, it was nearly shut
• If there is no TRV, just a manual wheel head, check that it opens. Old wheel valves can shear internally and spin without moving the gate

If a valve starts weeping when you move it, stop, put a cloth under it and do not force it further. A leaking radiator valve is a repair, not a DIY tweak.

Warm in places: air or sludge in the panel

If the radiator is getting some heat, the valves are open and the problem is inside it. Cold across the top is air. With the heating off and the radiator cool, open the bleed valve at the top corner a quarter turn with a radiator key until air stops and water spits, then close it. On a sealed system, check the boiler pressure afterwards and top up to about 1 to 1.5 bar if it has dropped. The guide to bleeding radiators with the heating on or off has more on this.

Cold along the bottom while the top is warm is sludge. Black magnetite builds up in steel panels and settles where water moves slowly. One radiator full of sludge can be taken off, flushed out with a hose outdoors and put back. If one is bad, others are usually getting there, so it is also worth asking about a system clean and a magnetic filter.

Less common causes worth knowing

If the valves open, bleeding does nothing and both pipes stay cold, the cause is further back in the pipework.

• A blocked or kinked microbore pipe. Houses from the 1970s and 80s often feed radiators through 8mm or 10mm copper that blocks with sludge far more easily than standard 15mm pipe
• An unbalanced system. A radiator at the far end of the run, or added later onto a long branch, can be starved while near ones take all the flow. Closing the lockshields a little on the hottest radiators pushes more water to it
• A separate heating zone. Larger homes and some flats have zone valves controlling upstairs and downstairs separately. If one radiator is on the other zone, its thermostat may simply not be calling for heat
• A new radiator fitted with the flow and return swapped onto a one-way TRV body, which can make it hum, bang or refuse to open

What an engineer checks and what it costs

An engineer will measure the temperature of the flow and return pipes, try the valve with a known-good TRV head, and check whether water reaches the radiator at all. A faulty TRV head is swapped in minutes. A faulty valve body can often be changed without draining the whole system, using a pipe-freezing kit or a partial drain-down, because shutting the radiator’s own valves does not isolate the valve body from the pipework. A radiator full of sludge comes off the wall for flushing. A blocked microbore run is traced and cleared or replaced.

As a rough guide to typical UK market prices, replacing one radiator valve is often somewhere around £80 to £200 fitted, and removing, flushing and refitting a single radiator is often in a similar range. Our radiator repair visit covers all of these. The £60 call-out applies at any hour, and the repair price is agreed with you on site before any work starts.

Is one cold radiator urgent?

Rarely. It is uncomfortable, not dangerous. It becomes urgent when it is the only heat in a room used by someone elderly, a baby or someone unwell in cold weather, or when a valve you have touched starts leaking. If more than one radiator is cold, or the cold spreads, read radiator not heating up instead, because the cause is probably not at the radiator.

Nothing here needs you to go inside the boiler. If you smell gas, open windows, turn the gas off at the meter if you safely can, do not touch switches, and call the National Gas Emergency Service on 0800 111 999 once you are outside.