How to Bleed a Radiator (Step-by-Step)

Cold at the top is the one radiator fault you can usually fix yourself. Here are the seven steps, the right order for a whole house, and the faults that look like air but are not.

How to tell air from something else

Air rises. It collects at the top of a radiator and stops hot water filling that space. Three signs point to it:

• The radiator is hot at the bottom but cold at the top.
• It gurgles or bubbles when the heating runs.
• It heats up more slowly than the others.

There is no fixed bleeding schedule. Bleed a radiator when it shows these signs, and check them when you first turn the heating on each autumn.

Two patterns are not air. A radiator cold at the bottom has a flow problem. A radiator cold all over while its neighbours are hot often has a stuck valve pin. Bleeding fixes neither, and the cards further down explain both.

What you will need

• A radiator key from any hardware or DIY shop. Many modern valves take a flat-head screwdriver instead.
• A dry cloth to catch drips.
• A small bowl to hold under the valve. Heating water is dirty and stains carpets.

The seven steps, from cool-down to test

1. Cool the system. Turn the heating off at the programmer and wait until the radiators are cool to the touch. A bleed valve on a hot system can spray scalding water.
2. Prepare your tools. Find the bleed valve. It is a small square pin inside a round nut at one top corner of the radiator. Put the cloth under it and hold the bowl beneath.
3. Open the bleed screw. Fit the key and turn it a quarter-turn anticlockwise. No more. The screw does not need to come out, and unscrewing it further lets water jet out.
4. Vent the air. You will hear a hiss. That is the air escaping. Wait until it stops. When water begins to dribble, the air is out.
5. Close it securely. Turn the screw clockwise until it is snug. Do not force it. Over-tightening can shear the screw.
6. Check the boiler pressure and top up. Read the gauge with the boiler cold. If it has fallen below 1 bar, top it up to 1 to 1.5 bar using the filling loop, then close the filling loop fully. Worcester Bosch’s filling guide shows the keyless lever and keyed loop methods. Follow your own boiler’s manual for how its filling control works.
7. Test the heating. Switch the heating back on. Let the radiators warm. Feel the top and the bottom. Warm all over, a steady gauge and no more gurgling means the job is done. Still cold somewhere? Read on.

Which order to bleed radiators in

Start on the ground floor with the radiator closest to the boiler. Work outwards from there, then move upstairs and finish with the highest radiators in the house. Air gathers at the high points, so they go last.

Doing several? Glance at the gauge between radiators. Top up before it falls below 1 bar, and check it again once you finish.

After bleeding: the autumn pressure trap

Every autumn, our engineers get the same call. A customer bleeds several cold radiators ahead of winter, switches the heating on, and the boiler refuses to fire. The gauge reads close to zero.

Air does not leave a sealed system alone. Water and pressure go with it. Bleed enough radiators and the pressure falls far enough to trigger a low-pressure lockout, such as F.22 on Vaillant boilers. It is not a fault. You caused it. A top-up cures it.

Always finish with the gauge. Top up cold to 1 to 1.5 bar, close the filling loop fully, and reset the boiler as its manual describes. Leaving it open pushes the pressure too high. Shut it. If the gauge keeps dropping after a top-up, water is escaping. Our guide to why a boiler keeps losing pressure explains where to look.

Bled it, and the radiator is still cold?

Match what you feel to one of these four patterns. Each card shows what it means, what is safe for you to do, and when to stop.

How to free a stuck TRV pin

A stuck pin is the easiest of these faults to fix. It happens most in autumn, when valves that stood still all summer are asked to open again.

1. Turn the heating off and let the radiator cool.
2. Remove the plastic thermostatic head from the valve.
3. Find the small metal pin in the centre of the exposed valve.
4. Gently press or tap the pin with a flat tool until it moves freely.
5. Refit the head and turn the heating back on.

Never yank the pin out with pliers. You can bend it or snap the head, and a damaged valve is a replacement job. If it will not move, stop.

What a visit costs if bleeding does not fix it

Our diagnostic rates are fixed and published. The rate depends on when we attend, not on how stubborn the radiator is.

The £120 rate covers weekday daytime visits only. It does not cover weekends, evenings or emergencies, which use the rates above.

Our phone line is answered 24/7 by an AI receptionist who takes your details at any hour. Routine visits run in two arrival windows, 08:00 to 12:00 and 12:00 to 18:00. We dispatch genuine emergencies at any time, at the £220 rate. A cold radiator is a routine fault, so it waits for a scheduled window.

If radiators keep going cold, the Homeowner Plan at £9.99 a month adds the standard annual service, priority booking and 10% off labour. We cover Queen’s Park, Kilburn and the surrounding patch.

When to stop and call a Gas Safe engineer

Smell gas? Call the National Gas Emergency Service on 0800 111 999 first and leave the property.

Stop bleeding and call us if you see any of these:

• A sheared bleed screw. The system is under pressure, so a broken screw leaves an open hole you cannot close.
• A weeping valve gland. Water seeping from the spindle means the seal has failed, and tightening it often makes things worse.
• A seized valve spindle. Forcing it can snap the valve.
• Pressure that keeps dropping. Water is leaving the system, and topping up only hides it.

Anything past the radiator is Gas Safe territory. Work on the boiler casing, gas valves, burners and flue must be done by a Gas Safe registered engineer, under HSE’s domestic gas rules. Balancing a complex system, replacing valves and powerflushing sit on our side of the line too. Our central heating services team does all three.

Frequently asked questions

Can I bleed a radiator without a key?

Sometimes. Many modern valves have a slot that takes a flat-head screwdriver, so try that first. If the valve has only a square pin, you need a radiator key. Do not use pliers, because they round off the pin and can shear the screw. Radiator keys are cheap and stocked by any hardware or DIY shop, so buy one rather than improvising. Keep a cloth under the valve either way.

Is it safe to bleed a radiator with the heating on?

No. Turn the heating off and wait until the radiators are cool. A hot system can spray scalding water from the bleed valve, and a running pump keeps pushing water and air around the circuit while you work. With the heating off, you can open the valve a quarter-turn, let the air out and close it without any rush.

Which radiator should I bleed first?

Start on the ground floor with the radiator closest to the boiler. Work outwards from there, then go upstairs and finish with the highest radiator in the house, because air collects at the highest points. Check the boiler pressure gauge as you go, and top up with the filling loop if it falls towards 1 bar. Check it once more when you finish.

Why has my boiler lost pressure since I bled the radiators?

Bleeding lets air and water out of a sealed system, so the pressure falls. Bleed several radiators and the gauge can drop to zero, which triggers a low-pressure lockout such as Vaillant F.22. This is expected, not a fault. Top the system up cold to 1 to 1.5 bar with the filling loop, close it fully, then reset the boiler as your manual describes. If the pressure keeps falling, call us.

Why is my radiator cold all over, even after bleeding?

It may be a stuck TRV pin, especially if the other radiators are hot. Pins seize when the heating stands idle over summer. Remove the plastic TRV head, then gently press or tap the exposed metal pin with a flat tool to free it. Never pull the pin out with pliers, because you can bend it or snap the head. If the pin will not move, call us.

Why is my radiator still cold at the bottom after bleeding?

Because the problem is not air. A radiator that is hot at the top and cold at the bottom often has dense magnetite debris settled in its base, restricting the flow. Bleeding only removes air from the top, so it cannot shift that. A chemical flush alone will not move compacted magnetite either. An engineer needs to assess the system and powerflush it, then fit a magnetic filter.

Can I fix a cold radiator myself, or do I need a Gas Safe engineer?

You can bleed a radiator, adjust a TRV head, free a stuck TRV pin and top up the boiler pressure yourself. You need a Gas Safe registered engineer to remove the boiler casing, work on the gas or flue, balance a complex system, replace valves or powerflush. If a bleed screw shears, a valve gland weeps or a spindle seizes, stop and call us.

How much does it cost if bleeding does not fix my radiator?

A weekday daytime diagnostic costs £120, covering Monday to Friday, 08:00 to 18:00. Saturday daytime is £160, evening visits from 18:00 to 21:00 are £180, and emergency or out of hours dispatch is £220. Any parts or repair work get a fixed quote before we start. A cold radiator is a routine fault, so it waits for a scheduled arrival window.