Why Trapped Air Stops Your Radiators Working

If a radiator is hot at the bottom and stubbornly cold along the top, air is almost certainly trapped inside it. Water should fill the whole panel, but pockets of air rise to the highest point and push the hot water aside, so that section never warms up. The same air can travel around the system, causing gurgling noises, banging pipes and a general loss of heat output in every room.

Air creeps in for all sorts of ordinary reasons: draining the system to fit a new valve or radiator, work on the pump, and even the natural corrosion that occurs inside older steel panels. That corrosion gives off small amounts of gas over time, which collects at the top of radiators. Bleeding is simply the act of releasing that trapped air through a small valve so water can fill the space again.

Before You Begin: Safety and Preparation

Bleeding is a straightforward DIY job, but a few precautions keep it safe.

• Let the heating cool first. Turn the heating off at the programmer or thermostat and wait at least 30 minutes. Water in a hot system is under pressure and can scald; cold water is far more forgiving.
• Gather the right kit: a radiator key (widely available for a couple of pounds), a cloth or old towel, a small container or mug, and a pair of gloves.
• Never open the boiler casing or touch gas pipework. That work is for a Gas Safe registered engineer only. If you ever smell gas, do not use light switches or your phone indoors — open windows, leave the property and call the National Gas Emergency Service on 0123456789.
• Work one radiator at a time, starting on the lowest floor and moving upwards through the house. This lets air escape rather than simply shifting it around.

How to Bleed a Radiator, Step by Step

• Find the bleed valve — a small square spindle inside a round nut, usually at one top corner.
• Hold the cloth under the valve and place the container beneath it to catch drips.
• Fit the radiator key onto the spindle and turn it anticlockwise about a quarter to half a turn; you should hear a steady hiss of escaping air.
• Keep the valve open until water dribbles out in a continuous stream rather than spitting and hissing.
• Close the valve firmly — clockwise — as soon as the water runs steadily. Do not overtighten; the spindle is brass and easy to damage.
• Wipe the area dry and check for weeping around the valve over the next few minutes.

Repeat on every radiator that feels cold at the top, then turn the heating back on and let the system warm through. Feel each radiator from top to bottom. If one still refuses to heat evenly, it may need balancing or further attention.

Checking the Boiler Pressure Afterwards

Releasing air lowers the pressure in a sealed system, which most modern boilers use. Once the heating has run and cooled, check the pressure gauge on the boiler. The reading should typically sit between 1.0 and 1.5 bar when cold, although your manufacturer's instructions are the final word.

If it has dropped below 1 bar, top it up using the filling loop — a braided hose or built-in lever connecting the mains supply to the heating circuit. Open it slowly, watch the gauge, and stop as soon as you reach the correct figure. Overfilling above roughly 2 bar can force water out through the pressure relief valve.

If the pressure keeps falling over days or weeks, there is likely a leak or a faulty component somewhere in the system. That is a job for a heating engineer.

When to Call a Professional

• Cold at the bottom, warm at the top: this points to sludge and corrosion debris rather than air. A chemical clean or power flush is usually needed.
• Air returns within days: repeated bleeding suggests ongoing corrosion. A corrosion inhibitor and possibly a new radiator may be required.
• The valve leaks and will not seal: a replacement is inexpensive, but the radiator may need draining — fiddly work best left to a plumber.
• Anything involving the boiler, gas supply or flue: always use a Gas Safe registered engineer, and keep up an annual boiler service.

Keeping the Whole House Warm

Bleeding once a year, ideally in early autumn before the heating goes on in earnest, keeps radiators performing at their best. Check the boiler pressure monthly during the cold months, and make sure thermostatic radiator valves are not stuck shut.

If some rooms warm faster than others, the system may need balancing: adjusting the lockshield valves so hot water is shared evenly rather than rushing to the nearest radiator. Small habits help too — avoid draping towels over radiators, and leave a gap between furniture and the panels so warm air can circulate.

Trapped air is the most common cause of a lukewarm radiator, and usually the easiest to fix. Ten minutes with a radiator key, a cloth and a little patience will restore even warmth across every room — and if anything feels beyond a simple bleed, a qualified heating engineer is only a phone call away.