Guides ยท Heating
Bleeding and balancing radiators
How to bleed a radiator with a key, what to watch on a sealed system's pressure gauge, how lockshield balancing evens out the heat, and when cold spots mean sludge rather than air.
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Radiators that are cold at the top, gurgle, or heat up unevenly from room to room can often be improved with two simple jobs: bleeding out trapped air, and balancing the flow so every radiator gets its share. Neither needs you to touch the boiler.
Cold at the top, or cold at the bottom?
Feel the radiator carefully once the heating has been on a while. It can be hot.
- Cold at the top, warm at the bottom: usually trapped air. Bleeding should help.
- Cold at the bottom or in the middle, warm at the top: often sludge (magnetite and debris) settled in the radiator. Bleeding will not fix this. Cleaning the system, for example a chemical clean or a powerflush, is a job for a heating engineer or plumber.
- Cold all over while others are hot: check the valves are open, then look at balancing below. A stuck thermostatic valve pin is another common cause.
How to bleed a radiator
- Turn the heating off and let the radiators cool. Hot water can spray out and scald.
- Find the bleed valve at the top corner of the radiator. It has a small square spindle or a slot.
- Hold a cloth around it and a small container underneath.
- Fit the radiator key and turn it slowly anticlockwise, about a quarter turn. You should hear air hiss out.
- When water starts to come out steadily, close the valve clockwise. Do not overtighten.
- Repeat on other radiators, usually starting with those on the lowest floor.
- Turn the heating back on and check the radiators heat evenly.
Sealed systems and the pressure gauge
Many homes, particularly those with a combi or a system boiler, have a sealed (pressurised) heating system with a pressure gauge on or near the boiler. Bleeding lets out a little water, so the pressure may drop.
Your boiler's user instructions say what pressure it should show and whether you can top it up yourself. On many systems the householder control for this is a filling loop, used only as the boiler maker's user instructions describe. Follow the maker's instructions for your boiler. If you are unsure, the pressure keeps dropping, or anything involves the boiler's gas components or casing, call a Gas Safe registered engineer. See when to call a Gas Safe engineer.
Older open vented systems are topped up automatically from a small feed and expansion cistern, usually in the loft, so there is no gauge to watch.
Balancing radiators
Balancing sets how much water each radiator gets, so the ones nearest the boiler do not take it all.
- One valve on each radiator is the control you use day to day: a thermostatic radiator valve (TRV) or a manual wheel head.
- The other is the lockshield valve, under a plastic cap. It is set once and left.
The general approach:
- With the heating off and cold, open all TRVs or manual valves fully.
- Turn the heating on and note the order in which radiators warm up.
- The radiators that heat fastest, usually nearest the boiler, have their lockshields closed most of the way and then opened a little.
- Work outwards, opening the lockshields more on radiators further from the boiler.
- The aim is a similar temperature drop across each radiator. Heating engineers measure this with thermometers; at home you can get closer by checking each radiator warms at a similar rate.
It takes patience. If it does not help, or the system is noisy or slow, ask a heating engineer to look at it.
Radiator keys
We haven't tested these. Bleed valves come in a few sizes, so a multi-end key or one matched to your radiators avoids rounding the spindle.
We haven't tested this. Figures are as the maker states them (checked 13 November 2026).
Wickes 3 way radiator key. A carbon steel key with three different-sized ends, which Wickes says are for a variety of radiator valves and bleed screws.
Primaflow radiator key, bleed plug and cap plug. A bleed key with a replacement bleed valve plug and blanking cap, which Wickes lists as 1/2in BSP. The plug and cap are for replacing radiator parts, which means draining or isolating that radiator first.
Solid brass radiator bleed key. The Amazon UK listing describes a solid brass bleed key with an easy-grip design. The listing text mentions both a pack of 2 and a pack of 1, so check the quantity before buying.
How often should I bleed my radiators?
When they show signs of air: cold at the top, gurgling, or after work on the system. There is no need to do it on a schedule if they heat evenly.
Why has my boiler pressure dropped after bleeding?
On a sealed system, bleeding lets out water. Check the gauge against the boiler maker's user instructions. If it needs topping up and you are unsure how, or it keeps dropping, call a Gas Safe registered engineer.
Will bleeding fix a radiator that is cold at the bottom?
Usually not. That pattern often means sludge, which needs a system clean by a heating engineer or plumber.