How to Bleed a Radiator (and Get Even Heat Back)

Quick answer

A radiator that is warm at the bottom and cold at the top has trapped air blocking the hot water. To fix it: turn the heating off, hold a cloth under the bleed valve at the top corner, turn a radiator key slowly anti-clockwise until air hisses out, and close the valve as soon as water runs steadily. Then check the boiler pressure and top it up if it has dropped.

October is the month radiators get their first real workout after summer, and it is also the month homeowners notice the classic symptom: a radiator that heats at the bottom and stays stubbornly cold at the top. The cause is almost always trapped air, and the fix takes about ten minutes per radiator with a key that costs next to nothing.

Why air gets trapped in radiators

Heating systems are filled with water, but small amounts of air enter over time — through tiny leaks, during maintenance, or as dissolved gases come out of the water when it heats. Air rises, so it collects at the highest point it can reach: the top of a radiator. Once enough air gathers, it forms a pocket that hot water cannot pass, so the top of the radiator stays cold while the bottom heats normally.

This is a maintenance task, not a fault. But if the same radiator needs bleeding every few weeks, air is entering the system faster than normal, which points to a failing expansion vessel, a small leak, or internal corrosion — all worth a professional look.

Tools needed

• Radiator key (or flat-head screwdriver for screw-type valves)
• Old towel or cloth
• Small container or jug to catch drips
• Cloth for wiping up
• Boiler manual (for pressure top-up instructions)

Step-by-step: bleed a radiator

Step-by-step instructions

1. 1

   Identify the radiators that need bleeding

   Turn the heating on and wait 10–15 minutes. Walk around and feel the top and bottom of each radiator. Any radiator that is noticeably colder at the top than at the bottom has trapped air. Note them all — bleeding one radiator can shift air to another, so plan to check the whole house.

2. 2

   Turn the heating off and let things cool

   Switch the heating off at the thermostat or boiler and wait a few minutes. The water inside stays hot for a while, and opening a valve on a fully hot radiator risks scalding. Lukewarm is fine — you do not need the system stone cold.

3. 3

   Find the bleed valve

   The bleed valve sits at the top corner of the radiator, usually on the opposite end from the thermostatic valve. It is a small square peg (for a radiator key) or a slotted screw. Older radiators sometimes hide it behind a cap — unscrew the cap first.

4. 4

   Open the valve slowly and listen

   Hold a cloth and a small container under the valve, insert the radiator key, and turn it slowly anti-clockwise — about a quarter to half a turn is usually enough. You should hear air hissing out. If water sprays immediately with no hiss, that radiator did not have trapped air; close the valve and move on.

5. 5

   Close the valve when water runs steadily

   When the hiss fades and a steady, bubble-free stream of water appears, turn the key clockwise to close the valve firmly. Do not over-tighten — bleed valves are soft brass and a snapped valve is a much bigger job than a bleed. Wipe up any drips.

6. 6

   Check boiler pressure and top up if needed

   Bleeding releases water, so system pressure drops. Check the boiler pressure gauge — most sealed systems should read 1–1.5 bar when cold. If it has fallen below the range in your boiler manual, top it up using the filling loop, following the manual. Then restart the heating and re-check the radiators after 15 minutes.

Bleeding order: start at the bottom, work up

If several radiators need bleeding, work from the ground floor upward, starting with the radiator furthest from the boiler on each floor. Air migrates upward through the system, so bleeding in this order pushes it out efficiently instead of chasing it from room to room. After bleeding all of them, re-check the first one — air shifted from a later radiator sometimes re-collects there.

What if the radiator still does not heat?

Bleeding fixes trapped air, and trapped air is the most common cause of a cold top. If the radiator stays cold after bleeding, work through these checks in order:

Repeated topping-up deserves emphasis: every top-up adds fresh, oxygen-rich water that accelerates internal corrosion. A system that needs topping up more than once or twice a heating season has a fault worth fixing, not a maintenance routine worth repeating.

A 10-minute autumn habit

Add radiator bleeding to your start-of-heating-season routine, alongside the furnace checks in our fall maintenance guide. Ten minutes with a radiator key each October keeps heat even, rooms comfortable, and the boiler working efficiently — and it catches the early signs of system trouble while they are still cheap to fix.