If your radiators are heating unevenly, the fix usually isn't just "bleed the cold one". It's bleeding all of them, in a specific order, so trapped air is pushed out of the system rather than shuffled from one radiator to another.
This guide explains exactly which radiator to bleed first, why the sequence matters on multi-floor properties, and how to tell when a radiator is cold for a reason bleeding won't solve.
Bleed the radiator on the lowest floor that sits furthest from the boiler first. Work through that floor towards the boiler, then move up to the next floor and repeat the same furthest-to-nearest pattern, finishing at the radiator closest to the boiler on the top floor.
The Correct Bleeding Order, Floor by Floor
Central heating water is pushed out from the boiler and circulates through the pipework to each radiator in turn. Air that becomes trapped in the system tends to collect furthest from the boiler, where flow is weakest. That's why the sequence starts there, not at the nearest or most obviously cold radiator.
On a bungalow or single-storey flat, simply start with the radiator furthest from the boiler and work back towards it. On a three-storey house, repeat the pattern on each additional floor before moving higher.
Why the Sequence Actually Matters
Bleeding radiators in a random order isn't dangerous, and it won't damage your heating system. But it does make the job less effective and more likely that you'll be back doing it again within weeks.
What happens if you bleed out of sequence
When you open a bleed valve, water and air both move to fill the space you've created. If you bleed a radiator close to the boiler first, you can draw air from further down the pipe run into that section of the system, effectively relocating the air pocket rather than removing it. Working furthest-to-nearest avoids this, because each radiator you clear is drawing air forward, out of the system, rather than back into a radiator you've already dealt with.
Why floor order matters too
Air is lighter than water and naturally rises. On a multi-floor property, clearing ground-floor radiators before moving upstairs lets residual air continue rising and clearing through the upper radiators as you go, rather than getting trapped in sections you've already finished.
The "Bleed the Highest Radiator First" Myth
You may come across advice suggesting you should start at the highest point in the house and work downwards, on the logic that air rises and collects at the top of the system. This is a common misunderstanding, and it isn't the sequence recommended by radiator manufacturers or heating engineers for a standard UK domestic system.
Why "highest first" causes problems
Most sealed central heating systems circulate water outwards from the boiler through the pipe run, not simply bottom-to-top by floor. Starting at the highest, most distant radiator before the ground floor has been cleared can draw air from the unbled ground-floor radiators upward into sections you've just finished, meaning you end up repeating parts of the job. The furthest-from-boiler, lowest-floor-first sequence is consistent with air removal and with the direction water is actually flowing.
If you have a combi boiler feeding a two-storey house, this means you start in a ground-floor room like a living room or dining room, not in a loft conversion or top-floor bedroom, even though that upper radiator may be furthest from the floor in a literal sense.
Do You Bleed Radiators from the Top or the Bottom?
This question usually means two different things, so it's worth separating them clearly.
The valve itself
- The bleed valve is almost always at the top of the radiator, because that's where trapped air collects
- It's usually at one end, opposite the thermostatic radiator valve (TRV)
- Some double-panel radiators have a second valve recessed on the inner panel, sometimes under a small plastic cap
The house sequence
- You work bottom-up through the property, not top-down
- Start on the ground floor, finish on the highest floor
- Within each floor, start furthest from the boiler and finish nearest to it
How to Tell Which Radiators Need Bleeding
With the heating on and radiators up to temperature, run your hand from top to bottom of each one.
Cold at the top, warm at the bottom
This is the classic sign of trapped air. Bleeding will typically resolve it.
Gurgling or banging pipework
Air moving through the system as the heating starts up often causes noise before a radiator feels obviously cold.
Slow to heat up
A radiator taking noticeably longer than others to reach temperature can indicate a partial air lock.
Uneven room temperatures
Rooms that never quite reach the thermostat setting despite a hot boiler are worth checking radiator by radiator.
How to Bleed Each Radiator, Once You've Reached It
Once you're following the correct sequence, the process at each individual radiator is straightforward.
Switch off the heating and let it cool
Allow at least 20 minutes for radiators to cool before starting, to reduce the risk of scalding from escaping water.
Fit the key and prepare a cloth
Slot a radiator bleed key, or a flathead screwdriver if the valve accepts one, onto the square fitting. Hold a cloth beneath it to catch drips.
Turn a quarter turn anti-clockwise
You should hear a hissing sound as trapped air escapes. Avoid turning more than one and a half turns, as some valve mechanisms can come out fully.
Close as soon as water flows steadily
Once the hissing stops and a smooth stream of water appears, close the valve clockwise firmly but without overtightening.
Move to the next radiator in sequence
Repeat at the next radiator along the furthest-to-nearest, floor-by-floor order, until every radiator has been bled.
Checking Boiler Pressure After Bleeding
Bleeding releases a small amount of water along with the trapped air, which typically causes the pressure in a sealed central heating system to drop.
- Check the pressure gauge on the boiler once all radiators have been bled
- Most domestic sealed systems run best at around 1.0 to 1.5 bar when cold, though this varies by boiler, so check the manufacturer's guidance
- Use the filling loop or internal filling key to top up slowly while watching the gauge
- Stop as soon as the correct pressure is reached and close the filling valves securely
When Bleeding Won't Fix the Problem
Bleeding only addresses trapped air. Several common radiator faults look similar but have different causes and need a different fix.
| Symptom | Likely Cause | Typical Fix |
|---|---|---|
| Cold at the top, warm at the bottom | Trapped air | Bleeding, in the correct sequence |
| Warm at the top, cold at the bottom | Sludge or debris settled in the base of the radiator | Radiator flush or full system power flush |
| Same radiator needs bleeding every few weeks | Air being continuously drawn in, often a leak or faulty auto air vent | System diagnosis to find the source |
| Whole radiator stays cold after bleeding | Closed or faulty valve, or a blockage | Valve inspection and repair or replacement |
| Upstairs radiators cold, downstairs fine | Circulation or pump issue, or system needs balancing | Engineer diagnosis of pump and balancing valves |
If you've bled every radiator in the correct order and one or more still won't heat properly, that's a reliable sign the issue isn't air, and continuing to bleed it won't help.
