Why radiators heat unevenly
Applies to: Assumes a pumped hot-water heating system with panel radiators and manual or thermostatic radiator valves.
Last worked on 2015-04-06
Where the radiator is cold tells you which of three different problems you have. The pattern is more informative than the temperature.
Cold at the top, warm at the bottom
This is air. Hot water enters the radiator and rises; anything lighter than water collects above it, so gas gathers at the highest point and pushes the water level down. The cold band grows over a season as more gas accumulates.
Bleeding releases it. If it comes back within weeks, the gas is being generated or drawn in rather than left over from filling, and the cause is elsewhere in the system rather than in that radiator.
Cold at the bottom, warm at the top
This is sludge. Corrosion products settle in the lowest part of the radiator where flow is slowest, build a bed, and the water stops passing through it. The affected band is at the bottom and it does not respond to bleeding at all.
The distinction matters because the two look similar from across the room and the fixes have nothing in common. Bleeding a sludged radiator achieves nothing and is the most common wasted afternoon in domestic heating.
Cold everywhere, or cold only at the far end of the run
This is distribution. Water takes the easiest path, so radiators near the pump and on short circuits get more flow than the ones at the end of a long run. Left unadjusted, the near radiators run hot and the far ones never come up.
The correction is balancing: restricting the lockshield valve on the easy circuits so that the resistance across the system evens out. It is done with the system hot, one radiator at a time, and it is slow because each adjustment changes every other one slightly.
Valves that are not doing what the head says
A thermostatic head measures the air immediately around itself. If it sits behind a curtain, above a heat source, or in a draught, it is reading a temperature the room does not have, and the radiator will run hot or shut early with no fault anywhere.
Heads also seize. A pin stuck down leaves the radiator permanently open; a pin stuck up leaves it permanently closed and cold from top to bottom, which is easily mistaken for a distribution problem.
| Cold region | Usual cause | First action |
|---|---|---|
| Top band, sharp horizontal edge | Trapped gas | Bleed at the vent |
| Bottom band, gradual edge | Settled sludge | Isolate and flush the radiator |
| Whole radiator, others fine | Valve closed or pin seized | Remove the head and check the pin moves |
| Whole radiator, all far ones the same | System not balanced | Adjust lockshields from the nearest outward |
| Whole radiator, cycles on and off early | Head reading the wrong air | Move the head or fit a remote sensor |
Scroll the table sideways for the remaining columns.
Work from the pattern rather than from the symptom. Almost all of the frustration in heating troubleshooting comes from applying the fix for trapped gas to a radiator that is blocked with sludge.