Cold at the bottom, cold at the top: what your radiator is telling you
Put your hand on the radiator and work out which part is cold. That one observation separates a two-minute job with a bleed key from a day's work and a bill to match — and it stops you being sold the wrong one.
Radiators fail in patterns, and the pattern is the diagnosis. Before you ring anyone, walk round the house with the heating on for twenty minutes and feel each radiator top and bottom, at both ends. What you find tells you most of what an engineer would charge to find out.
Cold at the top, warm at the bottom → air
Air rises, so trapped air collects along the top of the panel. This is the easy one.
- Turn the heating off and let the system cool.
- Put a bleed key on the small square valve at the top corner and turn it slowly anti-clockwise, with a cloth underneath.
- Let the hiss run until water appears, then close it.
- Check the boiler pressure afterwards and top it back up if it has dropped.
The important bit: if you are bleeding the same radiator every few weeks, the air is being made, not trapped. That usually means a small leak drawing air in, a pump running too fast, or corrosion producing hydrogen inside the system. Repeated bleeding is a symptom worth investigating.
Cold at the bottom, warm at the top → sludge
This is magnetite: black iron oxide, the product of steel radiators slowly corroding from the inside. It is heavier than water, so it drops out where the flow is slowest — the bottom of the panel — and builds into a sediment that heat cannot get through.
Bleeding will not touch it. You cannot flush it out with a bleed key or by turning the pump up. The system needs cleaning: either a power flush, or in bad cases the affected radiators removed and flushed out individually.
Two signs that confirm it: the water is black when you bleed or drain, and more than one radiator has the same cold band at the bottom. Once the system is cleaned it should be dosed with inhibitor and fitted with a magnetic filter — otherwise you will be doing it again in a few years.
Sludge is not only a radiator problem. The same debris circulates through the boiler's heat exchanger and the pump. It is a common cause of kettling noises and of pumps failing early, and most boiler manufacturers require a clean system for the warranty to stand.
Cold all over, while the rest of the house is hot → a valve or balancing
If one radiator stays stone cold top and bottom while everything else works, start at the valves.
- A stuck thermostatic valve. The pin underneath the head seizes, usually over summer. Take the head off and press the pin. If it does not spring back, that is your fault, and it is a cheap part.
- The lockshield is closed. The plain valve at the other end, usually under a plastic cap. Someone may have closed it and forgotten.
- The system was never balanced. Water takes the easiest route, so radiators nearest the boiler get the flow and the far ones get what is left. Balancing is adjusting each lockshield so every radiator gets its share. It costs an engineer's time and no parts, and it transforms houses where the back bedroom has always been cold.
Warm at one end, cold at the other
Usually a partial blockage in that radiator, or the valves at each end being throttled very differently. On a long radiator it can also be a flow rate too low to carry heat all the way across — which is a balancing problem again.
Whole house lukewarm
If nothing gets properly hot, stop looking at radiators. That points at the boiler, the pump, a failed motorised valve or the controls — and it is worth checking the boiler's flow temperature has not simply been turned down.
Rough guide to what each one costs you: bleeding is free and takes minutes. A seized TRV head is a small part and half an hour. Balancing is an hour or two of labour and no parts. A power flush is most of a day. Anyone who quotes you a power flush for one radiator that is cold at the top has not done the diagnosis.
If you are in Manchester and want it looked at properly, call 07454 118822, or read more about central heating and power flushing.
Cold radiator questions
Why is my radiator cold at the top but hot at the bottom?
Trapped air. Turn the heating off, let it cool, and bleed the radiator with a bleed key until water appears, then check the boiler pressure and top it up if needed. If the same radiator needs bleeding every few weeks, something is generating that air and it needs looking at.
Why is my radiator cold at the bottom but hot at the top?
Sludge — black iron oxide settling where the flow is slowest. Bleeding will not help. The system needs cleaning, usually a power flush, followed by inhibitor and ideally a magnetic filter to stop it coming back.
Do I need a power flush?
Only if several radiators are cold along the bottom, the water runs black when you bleed or drain, or the boiler is kettling. If one radiator is cold at the top and the rest are fine, you almost certainly do not — and a good engineer will tell you so.
What is balancing a heating system?
Adjusting the lockshield valve on each radiator so every one gets its fair share of flow instead of the nearest ones taking it all. It costs labour and no parts, and it is often the fix for a house where one room has always been cold.
How long does a power flush take?
Most houses take the best part of a day, depending on the number of radiators and how bad the sludge is. You will have heating again the same evening.