Cold at the top and warm at the bottom is air. Cold at the bottom and warm at the top is sludge. That single distinction tells you whether this is a ten minute job with a bleed key or a conversation about flushing, and it is worth getting right before you spend money.
Bleeding radiators is a normal homeowner task and there is no gas work involved. What matters is doing it in the right order, protecting your floor, and knowing what a repeat air problem is telling you about the system.
How to bleed a radiator safely
- Turn the heating off and let the radiators cool so you are not releasing hot water.
- Start with the radiator on the lowest floor closest to the boiler, then work outward and upward.
- Hold a cloth and a small container under the bleed valve at the top corner, turn the key a quarter turn anticlockwise and wait for hissing to become a steady dribble of water.
- Close the valve firmly as soon as water runs cleanly. Do not over tighten.
- Repeat around the house, then check and correct the boiler pressure once you are finished.
Check the pressure afterwards, every time
Bleeding removes volume from a sealed system, so pressure will read lower afterwards. If you leave it low, the boiler may lock out on a low pressure code the next time it fires. Top back to around 1.2 bar cold if your manual sets out the procedure for your model, and close the filling loop fully.
On an open vented system with a tank in the loft, the tank refills automatically, but a header tank that keeps demanding water is worth investigating rather than ignoring.
If air keeps coming back
Air that returns within weeks is a symptom, not a maintenance chore. The usual causes are a small leak drawing air in, an expansion vessel that has lost its charge, a pump set too fast and pulling air past a poorly positioned automatic air vent, or corrosion inside the radiators producing hydrogen gas because the inhibitor has been diluted by repeated top ups.
That last one is the most common in older Borders systems that have been topped up for years. The fix is a proper clean, a magnetic filter and correct inhibitor dosing rather than another round of bleeding.
When a cold radiator is not about air at all
- Cold at the bottom, warm at the top means sludge and needs flushing, not bleeding.
- One radiator cold throughout often means a seized thermostatic valve pin, which is a straightforward mechanical fix.
- Whole zones cold points at a motorised valve or wiring centre rather than the radiators.
- Upstairs cold and downstairs fine can mean a failing pump or an incorrectly balanced system.
Frequently asked questions
- Which radiator should I bleed first?
- Start downstairs nearest the boiler and work away from it, finishing with the highest radiators furthest away. Air collects at high points, so working in that order pushes it in the right direction.
- Do I need to turn the heating off to bleed a radiator?
- Yes. Let the system cool first. Bleeding a hot pressurised radiator risks scalding, and a cool system releases air more predictably.
- How often should bleeding be necessary?
- A healthy sealed system might need it once a year at the start of the heating season, if at all. Monthly bleeding means something is drawing air in or corroding, and that is worth diagnosing.
- Can you sort out unbalanced radiators as well?
- Yes. Balancing the flow across radiators is part of the central heating work we do, and it often makes more difference to comfort than turning the boiler temperature up. It also improves efficiency, which matters on modern condensing boilers.
Bled them and they are still cold?
Tell us which radiators, whether they are cold top or bottom, and whether it is one room or a whole floor. That usually identifies the fault before we arrive.