Radiator Cold at Top and Hot at Bottom: Airlock or Sludge?
A radiator that is hot at the bottom and cold at the top is a distinctive pattern that points to one of two causes: trapped air in the top of the radiator, or a build-up of iron oxide sludge in the lower section that is blocking heat distribution. Both are common central heating faults in London homes, and both are resolvable, but the approach differs depending on which cause is responsible. This guide explains how to distinguish between the two and what action to take.
Distinguishing an Airlock from Sludge
When a radiator is hot at the bottom but cold at the top, the first step is to run your hand slowly up from the bottom to the top of the radiator surface to understand the temperature distribution. An airlock produces a sharp transition from hot metal to cold metal roughly halfway up or in the upper third of the radiator, with the cold upper section feeling uniformly cold. The cold portion corresponds precisely to the volume of trapped air, which sits above the water level inside the radiator. Iron oxide sludge produces a different pattern: the bottom of the radiator feels warm or only mildly hot because the sludge deposits at the lowest point of the radiator partially block the flow and reduce heat transfer across the whole panel.
A useful test to help distinguish the two is to bleed the radiator at the bleed valve, which is located at the top of one of the radiator ends. Use a radiator bleed key to turn the valve a quarter turn anti-clockwise. If air hisses out and then water follows, the cause was a trapped airlock and bleeding the radiator will resolve the problem. If no air emerges when the bleed valve is opened, or only a small puff of air followed by very little water despite the radiator still feeling cold at the top, sludge is the more likely cause. A radiator fully charged with sludge can exhibit the same cold-top symptoms as an airlock but will not release air at the bleed valve because the air has been displaced by the sludge layer.
How to Bleed a Radiator to Clear an Airlock in London
To bleed a radiator, first ensure the central heating is switched on and the system is at operating temperature, which gives the trapped air maximum pressure and makes it easier to release. Place a small container or cloth beneath the bleed valve to catch any water. Insert the bleed key and turn it slowly anti-clockwise by no more than half a turn. Air will escape with a hiss. Hold the key in position until the hiss stops and a steady stream of water begins to emerge, then close the valve by turning it clockwise until snug. Do not overtighten, as the brass valve seat is easily damaged. After bleeding one or more radiators, check the boiler pressure gauge, as releasing air reduces the volume in the system and may cause the pressure to drop below the minimum operating level. Repressurise the system using the filling loop if the pressure has dropped below 0.8 bar.
Air in a central heating system is introduced when water is added to the system during repairs, when the system pressure drops and air is drawn in through micro-leaks, or when the system inhibitor has depleted and corrosion gases are being generated. If a radiator needs bleeding repeatedly, the root cause of air introduction should be investigated. A system that consistently loses pressure and requires frequent bleeding may have a small leak somewhere in the pipework or at a radiator valve that is allowing water out and air in.
Clearing Sludge From a London Central Heating System
If sludge is the cause of poor heat distribution in one or more radiators, the sludge must be removed from the system rather than simply bled. A single affected radiator can be isolated at its thermostatic radiator valve and lockshield valve, removed from the wall, flushed through externally with a garden hose to remove the sludge, and refitted. This is effective for localised sludge deposits. Where sludge is present throughout the system and multiple radiators are affected, a power flush is the appropriate treatment. A power flush involves connecting a specialist machine to the central heating circuit that circulates water and descaling chemicals at high velocity to dislodge and remove sludge from the pipework, radiators, and heat exchanger. Prestige Engineers carry out power flushes and system cleans across all London boroughs, with same-day radiator bleeding and sludge diagnosis also available.