Almost always air in the radiator. Hot water comes in and heats the lower part, but air has collected at the top where it cannot escape on its own, so the top stays cold. Bleeding the radiator is the first step and usually fixes it.
If the same thing comes back after bleeding, or several radiators behave the same way, it is rarely the radiator. That points at system pressure, or at air getting in somewhere — and you cannot bleed that away.
In detail
Open the bleed valve at the top of the radiator carefully with a bleed key or screwdriver, with a cloth and a bowl underneath. It hisses while the air escapes. As soon as steady water comes out instead of air, close it. Never open further than necessary, and do not do it on a system you do not know how to refill.
Because the radiator is filling with air again, and that air is coming from somewhere. Common causes are system pressure that is too low, topping up in a way that draws air in, or a leak that sucks air in as the system cools. That is the question to answer — not the radiator.
Most systems have a pressure gauge at the boiler or the heat interface unit. In a house the normal cold reading is often around 1–1.5 bar, but the correct value is in the system’s own documentation and differs between installations. If the needle is near zero, pressure is what gets sorted first.
If the radiator is cold all over rather than just at the top, it is more often the valve or the thermostatic head than air. If the whole house is cold, it is the heat source. And if a radiator is warm at the top but cold at the bottom, that is close to the opposite — usually sludge or a blocked return valve.
The whole radiator so it is visible where it is warm and where it is cold, the valve and thermostatic head close up, and the pressure gauge at the boiler or heat interface unit. Say as well whether it is one radiator or several, and whether this is new or has happened before.
What this page is not
This page describes what the symptom usually comes down to. It is not an assessment of your particular installation and it does not replace a professional who can see it. If water is running, you can smell gas, or anything involving electricity is in play: shut it off and call a professional now.
Other symptoms
Partly, and the part that is settled is the application key. An ECL Comfort 210 or 310 is an empty shell without one: the key carries the software layout, such as A266 for heating plus domestic hot water. Keys are not locked to a serial number and move freely between controllers of the same type.
Read the answer →There are three common sources, and they can be told apart without tools. The trap and its connections only leak when you run water. The supplies to the tap leak continuously, including overnight, because they are under pressure. The dishwasher hose or connection leaks during and just after a cycle.
Read the answer →Condensation spreads evenly along a cold pipe, often as many small droplets on the underside, and gets worse when it is warm and humid indoors. A leak has a point: the damp is heaviest at a joint, a valve or a weld, and runs from there.
Read the answer →Not without knowing what it sits on. A valve can be the isolation for a single tap, for a whole flat, for a heating circuit, or for a system that cannot be left standing — and from the outside they look much alike. A closed heating or circulation circuit can cause frost damage or stagnant hot water long after you have forgotten you touched it.
Read the answer →The heating working while the hot water does not already says a good deal: the fault is usually in the part that only concerns hot water, not in the heat source. The most common are the cylinder or the heat exchanger, a broken or misadjusted mixing valve, or a stopped hot water circulation pump.
Read the answer →One thing to do next
Write a few lines and send a photo or a short video. You get a written first assessment of your own case — what it most likely is and what the next step is — for €9.99. A professional reads it, not just a machine.