When the boiler is running but the radiators are not getting properly hot, it usually means heat is being made but it is not being moved around the system efficiently. In UK homes that tends to show up as lukewarm radiators, cold patches, upstairs lagging behind, or one or two radiators barely doing anything while others feel fine.
It’s tempting to assume the boiler is failing, but a lot of the time the boiler is doing what it can with what it’s being given. The real issue is often flow, balance, trapped air, sludge, a sticky valve, or a thermostat and programmer setup that is limiting central heating demand.
If you want the quickest route to the most likely cause based on what you can actually feel in the house, start with the house cold diagnostic and let it point you to the right set of checks.
A useful first step is to compare radiators rather than focusing on the boiler. If the whole house is lukewarm, that points to the system not delivering heat well overall. If only certain radiators are weak, that points more towards balancing, air, or a restriction on one branch of pipework.
Uneven radiator temperature is one of the clearest clues. If parts of a radiator stay cold while the boiler is running, it often comes down to flow patterns through the radiator and the wider system, and it helps to think in terms of why radiators heat unevenly rather than blaming the boiler straight away.
Another common pattern is where one radiator only performs when others are turned down. That can look like the boiler is “not powerful enough”, but it is more often the system distributing heat badly, which is why the symptom in radiator only heats when others are off matters so much.
Controls can also create this “boiler running, radiators weak” feeling. If the thermostat is satisfied quickly, if smart controls are limiting demand, or if the programmer is not calling for a steady heating run, radiators never get the chance to build proper temperature. That is especially noticeable in draughty homes where the house loses heat quickly and the system needs longer runs to make a difference.
If you’ve recently bled radiators, topped up pressure, or had any work done, trapped air and circulation changes are still realistic even if nothing sounds obviously “airy”. Air reduces effective surface area and can reduce circulation, particularly upstairs and on towel rails, which can make radiators feel like they’re underperforming even with the boiler running.
Where the symptoms are widespread and have slowly worsened over time, sludge becomes more likely. The clue is usually that radiators take longer each winter or certain rooms repeatedly underperform. The mistake people make is jumping straight to expensive work before confirming the simpler flow-and-control issues, because those are common and cheaper to fix.
If you want the whole-house approach that ties these patterns together for typical UK properties, anchor your thinking to the complete warm home guide so you’re not treating one symptom while missing the bigger cause.
If anything smells of gas, you can see a leak, or the boiler shows fault codes you don’t recognise, stop and get a qualified engineer. But for the common “boiler runs, radiators stay weak” scenario, the useful mindset is simple: the boiler may be producing heat, and the system may be failing to deliver it evenly. Solve it in that order and you avoid the expensive wrong diagnosis.