Bleeding fixes exactly one problem: air trapped at the top of a radiator. If you have bled a radiator properly, water came out cleanly, and it is still cold, then air was never the fault. That is useful information rather than a dead end, because it rules out the easiest cause and points you at a short list of real ones.
The next question is which part of the radiator is cold, and how many radiators are affected. Those two answers separate a stuck valve from sludge, and sludge from a system that has simply never been balanced. Work through the sections below in order.
Check whether you bled it properly in the first place
Bleeding only works with the heating switched off and the system cooled, so that air has risen and settled at the high points. Bleeding a hot, running system pushes water out while the pump keeps air circulating, which is why people repeat the job several times and get nowhere.
You should hear a hiss, then get a small spurt of clean water, then close the valve. If nothing at all came out, the radiator is not full of water, which means flow is being blocked further back rather than air being trapped at the top.
- Turn the heating off and let the system cool before bleeding.
- Start with the lowest radiator on the ground floor and work up.
- Recheck system pressure afterwards, since bleeding lowers it.
- Top back up to about 1.2 bar cold if your boiler is a sealed system.
Cold at the bottom, hot at the top: that is sludge
A radiator that is warm across the top but cold along the bottom edge is holding sludge, which is iron oxide debris that has settled out of the system water. It sits in the bottom of the panel where flow is slowest and it displaces the water that should be carrying heat.
Bleeding will never fix this because the trapped material is not air and it is not at the top. The options are removing and flushing that radiator individually, or power flushing the system and fitting a magnetic filter and inhibitor so it does not simply return within a couple of years.
Cold all over: look at the valves
A radiator that is uniformly cold while others are hot usually has a valve problem. Thermostatic radiator valves have a pin under the head that seizes when it has not moved for months, most often after a summer with the heating off. Take the head off and look: the pin should stand slightly proud and move when pressed.
A gently freed pin often restores the radiator immediately. At the other end of the radiator sits the lockshield valve, which controls balancing. If that has been closed off by a previous occupant or a decorator, no amount of bleeding will help until it is opened again.
Only the far radiators cold: balancing
If your cold radiators are consistently the ones furthest from the boiler, or always the upstairs back bedroom, the system is out of balance. Water takes the path of least resistance, so radiators near the pump take more than their share of flow and the distant ones get what is left.
Balancing is the fix. It means adjusting lockshield valves so each radiator gets a fair share, checked by temperature difference across each one. It takes patience, it is done with the system running, and it improves both comfort and efficiency across the whole house rather than one room.
| Pattern | Cause | Fix |
|---|---|---|
| Cold at the bottom only | Sludge in the panel | Flush the radiator or power flush the system |
| Cold all over, others hot | Seized TRV pin or closed lockshield | Free the pin or reopen the valve |
| Furthest radiators always cold | System out of balance | Balance the lockshield valves |
| Nothing came out when bled | No flow reaching the radiator | Investigate valves and circulation |
| All radiators cold, hot water fine | Diverter, three way valve or pump | Engineer diagnosis |
When the whole house is cold
If every radiator is cold but your hot water is fine, this is not a radiator problem at all. The heat is not reaching the heating circuit, which puts the diverter or three way valve, the pump and the controls in the frame. Our guide on hot water but no heating covers that path properly.
If neither heating nor hot water works, start with power, controls, pressure and lockout codes instead. Different symptom, different checklist.
Where we come in
- You have freed the valve pin and the radiator is still cold.
- Bleeding produces dirty black water, which confirms sludge.
- Radiators need bleeding repeatedly, which means air is getting in from somewhere.
- You want the system balanced properly rather than room by room guesswork.
- Pressure will not hold after topping up, which points at a leak.
Frequently asked questions
- Why is my radiator still cold after bleeding it?
- Because air was not the cause. Cold at the bottom means sludge, cold all over means a seized thermostatic valve pin or a closed lockshield, and cold only at the far end of the house means the system needs balancing. None of those are fixed by bleeding.
- Should radiators be hot or cold when you bleed them?
- Cold. Switch the heating off and let the system settle so air can rise to the top of each radiator. Bleeding while the pump is running keeps air moving around the system, which is why it often seems not to work.
- Does black water when bleeding mean sludge?
- Yes. Dirty or black water is iron oxide from inside the system. A small amount is normal in an older system, but heavily discoloured water alongside cold spots means a flush and a magnetic filter are worth doing.
- How do I free a stuck radiator valve pin?
- Remove the thermostatic head to expose the pin and press it gently a few times. Many free up immediately. If it will not move, do not force it, because a snapped pin turns a five minute job into a valve replacement with the system drained down.
- Is balancing something I can do myself?
- A patient homeowner can, using a thermometer and adjusting lockshield valves. It takes a couple of hours across a full house. If you would rather it was set correctly first time, we do it as part of central heating work across the Borders and East Lothian.
Bled it and it is still cold?
We flush, balance and repair heating systems across Kelso, the Borders and East Lothian.