Cold at the top: air, and the easiest fix of all
If a radiator is hot at the bottom but stays cold across the top, that is trapped air, and it is the one problem on this page you can almost always sort yourself in a couple of minutes. Air rises to the top of the radiator and stops the hot water filling it, so the bottom heats and the top does not.
The fix is bleeding. With the heating off and the radiator cool, fit a radiator key to the small square valve at the top corner and turn it slowly anticlockwise a quarter turn. You will hear air hissing out, then a dribble of water, at which point close it again straight away and have a cloth ready. Work round the house doing the same to any radiator with a cold top.
One thing people miss: bleeding lets water out, so your boiler pressure will dip afterwards. Check the gauge and top it back up to around 1.2 bar if it has dropped below about 1 bar, which is walked through step by step in the pressure guide. If you find yourself bleeding the same radiators every few weeks, that is not normal, and it points at something the next sections cover.
Cold at the bottom, hot at the top: that is sludge
The opposite pattern tells a very different story. A radiator that is warm across the top but stubbornly cold along the bottom edge is the classic signature of sludge, the rust and debris that settles where the water moves slowest, which is the bottom of the radiator. Bleeding will not touch it, because the problem is not air.
In a single radiator it can sometimes be cleared on its own. When it shows up across several radiators, along with dirty brown or black water when you bleed them, rooms that never quite warm up, or kettling and banging from the boiler, the whole system is silted up and needs flushing rather than fiddling. That is the power flush job: a standalone MagnaCleanse flush is £499, or £350 done alongside a new boiler, and a lightly affected system can sometimes be sorted with a chemical inhibitor and clean at £120 instead. Lloyd will tell you which your system actually needs rather than defaulting to the dearest option. The full breakdown is in the power flush cost guide.
Some radiators stone cold while others are fine
When a few radiators heat up nicely and others stay completely cold, especially the ones furthest from the boiler or all the ones upstairs or all the ones downstairs, the usual culprit is balancing. Over time, or after work on the system, the hot water takes the path of least resistance and floods the nearest radiators while the far ones get starved. Balancing adjusts the small lockshield valves so every radiator gets its fair share, and it is a fiddly, methodical job rather than a dramatic one.
Before assuming the worst, check the simple things on the cold radiators themselves. Is the valve actually open? A thermostatic radiator valve can stick shut, and a quick way to test is to take the head off and see if the pin underneath springs freely. Is a whole zone cold? Then a stuck motorised valve or a tired pump may be failing to push water to that part of the house, and those are engineer jobs.
Every radiator cold while the boiler seems fine
If the boiler fires and sounds like it is working but not a single radiator gets warm, the heat is being made and not delivered, and that points at the parts that move and direct the water rather than the radiators. The common causes are a seized circulating pump, a stuck diverter or motorised valve, or a controls problem where the boiler is heating water for the taps but never being told to send it to the radiators.
There are two quick homeowner checks worth doing first. Confirm the room thermostat is calling for heat and set above the current room temperature, because flat batteries in a wireless stat are a surprisingly common cause of a whole cold house. And check the programmer is actually set to have the heating on at that time of day. If both are right and the radiators are still cold while the boiler runs, stop there. Pumps and valves sit behind the boiler casing or in the system, and opening the case is Gas Safe registered work, not a DIY job.
When it needs Lloyd
Call a Gas Safe registered engineer when the safe checks above have not moved it: radiators cold at the bottom across the house, dirty water when you bleed, a whole zone or every radiator cold while the boiler runs, a pump or valve you suspect has failed, or balancing that is beyond a comfortable afternoon with a key and a thermometer. None of those are jobs to force, and a sludged system left alone quietly eats pumps and heat exchangers, which turns a flush into a much bigger bill.
If you are in Bridgend, Pencoed, Porthcawl, Cardiff or the surrounding county, the quickest way to get a straight answer is to message Lloyd on WhatsApp describing which radiators are cold and where, or call 07712 646488. He answers his own phone during working hours, tells you honestly whether it sounds like a boiler repair, a power flush or something smaller, and prices the visit before he sets off.
