Cold at the top means trapped air, so bleed it. Cold along the bottom means sludge, and bleeding will not shift it. A radiator cold all over is usually a stuck valve. If you are bleeding the same radiator every few weeks and the boiler gauge keeps falling, the heating circuit is probably losing water.
You put a hand on the radiator and part of it is stone cold while the rest is warm. Maybe it is the top few inches. Maybe it is a band along the bottom. Maybe the whole thing is lukewarm while the one next door is roasting. Each pattern has a different cause, and the one that keeps coming back however often you bleed it can be the first hint of a hidden heating leak. Here is what each cold spot on a radiator means, what you can do yourself, and the point where it stops being maintenance.
Read the radiator before you reach for the radiator key
Anything that stops water filling part of a radiator leaves that part cold. Air is lighter than water, so it collects at the top. Sludge, the black iron oxide that forms as a steel radiator corrodes from the inside, is heavier, so it settles at the bottom. That is why the position of the cold spot tells you so much.
| What you notice | Most likely cause | What to do |
|---|---|---|
| Hot at the bottom, cold across the top | Trapped air | Bleed it, then check the boiler pressure |
| Hot at the top, cold along the bottom | Sludge inside the panel | Flush the radiator, or the system if several are affected |
| Cold patch in the middle, or one panel cooler than the other | Poor flow or a partial blockage | Have an engineer isolate and flush it |
| Whole radiator cold, others fine | Stuck valve or poor balancing | Check the thermostatic valve pin and the lockshield |
| Cold spots return and the boiler needs topping up | Water escaping from the circuit | Log the gauge for a week, then have the leak traced |
Cold at the top: bleed it, then look at the gauge
A radiator that is hot low down and cold across the top has air sitting in it. Bleeding takes five minutes.
- Turn the heating off first. Bleeding with the pump running can draw more air in, and cooler water will not scald you.
- Fit the radiator key to the bleed valve. It is the small square fitting at one top corner. Hold a cloth under it.
- Open it a quarter turn. You will hear a hiss. The moment water appears, close it. Do not unscrew the valve fully, because on some radiators it comes out in your hand.
- Check the boiler pressure afterwards. Air leaving is replaced by water, so the gauge drops. On a sealed system it should read about 1 to 1.5 bar when cold. Top it up through the filling loop if it has fallen below that.
Bleeding once a year is normal. Bleeding the same radiator every fortnight is not, and we come back to that below.
Cold at the bottom: sludge, and why bleeding will not help
A radiator that is warm at the top and cold along the bottom has nothing to do with air, so opening the bleed valve only drops the pressure a little more. The cold band is sludge lying in the bottom of the panel, blocking circulation through that section. It builds up over years as steel radiators corrode, and faster in a system with no inhibitor, no magnetic filter, or a history of being drained and refilled.
If one radiator has a cold bottom and the rest are fine, an engineer can take it off the wall and flush it through outside. If three or four show the same band, the sludge is through the whole circuit and a system flush is the usual answer, followed by fresh inhibitor and a magnetic filter next to the boiler.
Cornwall’s water is soft, so limescale is rarely the culprit in a cold radiator here. What we do see a great deal of is corrosion sludge in older systems. The granite cottages and farmhouses around Camborne, Redruth and the mid-Cornwall villages often have heating fitted decades ago and extended a room at a time, with old steel radiators and pipe runs added by three different plumbers. Those systems make sludge readily, and they tend to have a weeping joint or two tucked away as well.
The whole radiator is cold
When one radiator stays cold from top to bottom while the others heat up, water is not getting into it at all. The usual reasons are mechanical.
- The thermostatic valve pin is stuck. After a summer idle, the small pin under the thermostatic head can seize shut. Take the head off and press the pin gently. If it will not spring back, the valve needs replacing.
- The lockshield is shut. The capped valve at the other end may have been closed during decorating and never reopened.
- The system needs balancing. If radiators near the boiler are hot and the far ones cool, water is taking the easy route. Balancing is fiddly and worth handing to an engineer.
When cold spots are the first sign of a leak
This is the part most guides skip. A sealed heating system is a closed loop. Once it is filled and bled it should hold its water and its pressure for months. If air keeps appearing in a radiator, it is getting in from somewhere, and the usual route is the top-up you are doing to replace water that has escaped. Every litre of fresh mains water carries dissolved oxygen. It comes out as the water heats, collects at the top of the highest radiator, and you bleed it. The pressure drops, you top up, and the cycle starts again.
That cycle also feeds fresh oxygen into the system, which speeds up corrosion and makes more sludge. So a house where cold tops and cold bottoms both keep returning, and where the boiler needs a top-up more than a couple of times a year, is very often a house with a slow leak on the heating circuit. Our guide to why a boiler keeps losing pressure runs through the other causes to rule out first, such as a failed expansion vessel or a pressure relief valve discharging outside.
The leak itself is rarely visible. Most heating pipework runs under floors or through wall chases, and a weep of a few litres a day can soak into the ground beneath a solid floor without ever showing as a damp patch. If you have looked under every radiator and found nothing, see our article on a heating system losing water with no visible leak. And if a bottle of sealer is tempting, read why leak sealer often holds for a month and no longer before you pour it in.
Finding a heating leak does not mean lifting floors on a hunch. Our central heating leak detection survey uses thermal imaging to pick up the warm plume a heating leak leaves in a floor or wall, and tracer gas to pinpoint the escape on a buried pipe run. You get one small access point in the right place, and a report you can hand to your insurer if the damage becomes a claim.
What to check this week before you ring anyone
A few days of notes will tell you a great deal.
- Write down the cold gauge reading every morning. A steady reading means the circuit is sound. A reading that creeps down day after day means water is leaving.
- Count the bleeds. Once a season is fine. The same radiator every couple of weeks is a symptom.
- Look at every radiator valve and tail. A green or white crust on a brass fitting is dried water, and it often marks a joint that weeps only when the system is hot.
- Find the pressure relief discharge pipe outside. It is a copper pipe near the boiler flue. Wet ground beneath it means the valve is passing water.
- Feel the floors. A warm patch with no radiator nearby is worth mentioning when you call, because heating water leaking under a screed warms the surface above it.
Bring those notes to the phone call. They shorten the survey, because we can go straight to the circuit that is losing water. We carry out leak detection across Cornwall, from Saltash to Penzance, and most heating surveys are booked within the week. Once the circuit is holding, bleed every radiator each autumn, keep inhibitor in the system, and have the magnetic filter emptied at the annual service.
Frequently asked questions
Why is my radiator cold at the top but hot at the bottom?
Air has collected at the top of the radiator, where it sits above the water and stops the panel filling. Bleed it with a radiator key until water appears at the valve, then check the boiler pressure gauge, because releasing air also lets a little water out and the reading may need topping up.
Why is my radiator cold at the bottom but hot at the top?
Sludge has settled along the bottom of the panel and is blocking the flow through that section. Bleeding will not help because there is no air to release. One affected radiator can be removed and flushed on its own. Several affected radiators usually mean a system flush, fresh inhibitor and a magnetic filter.
How often should I need to bleed my radiators?
Once a year is normal, ideally in autumn before the heating season. Needing to bleed the same radiator every few weeks is not normal. It usually means fresh water is entering the system through repeated top-ups, and the water being replaced has escaped from somewhere on the heating circuit.
Can cold spots on a radiator mean there is a leak?
Not on their own. Cold spots that come back quickly after bleeding or flushing, together with a boiler gauge that keeps dropping, are a different matter. A sealed heating system should hold its water, so that pairing points to a slow escape somewhere in the pipework and is worth having traced.
Bleeding the same radiator again?
If the cold spots keep returning and the gauge keeps falling, the water is going somewhere. We trace heating leaks across Cornwall with thermal imaging and tracer gas, open one small access point in the right place, and give you a report your insurer will accept.
