“The boiler seems on, but the radiators are stone cold” usually means you can hear the boiler doing something, the display looks normal, or the hot water works, but the radiators never warm up. In most UK homes that points to a central heating demand or circulation issue rather than a simple temperature setting problem.
It helps to separate hot water and heating. Many boilers can run hot water independently of the radiator circuit, so the boiler can be “on” because it is responding to hot water demand while central heating is not actually being called for properly.
If you want the cleanest path from symptom to the right set of checks, use the house cold diagnostic rather than trying three fixes at once and not knowing which one changed anything.
When no heat reaches any radiators, it’s worth noticing whether every radiator is cold or whether one gets slightly warm and the rest do nothing. If everything stays cold, the problem is often that heat is not being sent through the heating circuit at all. In zoned systems that can be a stuck motorised valve. In other cases it can be a pump or flow issue where the boiler is active but the system isn’t circulating through radiators.
People sometimes get misled by one warm pipe near the boiler. Pipes can feel warm locally even when the wider circuit isn’t moving properly. The real test is whether radiators begin to take heat after a reasonable run, not whether the boiler casing feels warm.
If one radiator works but most stay cold, it can still be a flow distribution issue rather than “the boiler isn’t heating”. That’s why the symptom described in radiator only heats when others are off is useful, because it shows what happens when heat is being made but not shared evenly around the system.
If the problem began after bleeding radiators or topping up pressure, trapped air and circulation changes are still realistic. In that situation it’s worth reading radiator not heating after bleeding, because the sequence of events is often the same even if the symptom feels like a boiler issue at first.
Controls still matter here. If the thermostat is already satisfied, the programmer is off, or smart controls have decided there’s no heating demand, the boiler can appear active for other reasons while the radiator circuit stays cold. That’s why the best “first” move is confirming that heating demand is genuinely being called for, not just that the boiler has power.
During colder spells, people often turn TRVs down hard to save money. That can be sensible, but if the heating circuit becomes too restricted, some systems behave poorly and you can end up with heat not reaching the places you care about. The system needs a sensible path for flow, even if you’re not trying to heat every room equally.
If you want the bigger picture on how UK homes actually stay warm without wasting money, the complete warm home guide keeps you focused on what changes comfort rather than what looks “active” on a boiler screen.
If you ever smell gas, see signs of active leaks, or the boiler is showing fault codes you don’t recognise, that’s the stop line. But if the boiler seems on and radiators stay cold, treating it as a demand-and-circulation problem first is usually the fastest way to avoid the expensive wrong diagnosis.