Bled your radiator and it’s still cold?
If a radiator is still cold after bleeding, the problem isn’t air. A radiator that stays cold at the bottom has sludge in it: black, rusty magnetite that settles and blocks the hot water. Bleeding only lets out air from the top, so it can’t shift sludge. A powerflush can.
What bleeding a radiator actually does
Bleeding releases air or gas that has collected at the top of a radiator. Air rises, so it pushes the hot water down and leaves a cold band across the top. Open the bleed valve, the air hisses out, water follows, and the top of the radiator heats up again.
So bleeding fixes a radiator that’s cold at the top. It does nothing for a radiator that’s cold at the bottom, because nothing at the bottom is air.
Still cold at the bottom? That’s sludge
Inside every heating system, water slowly reacts with the steel of the radiators and pipes. That corrosion makes magnetite: a fine, black, magnetic sludge. It’s heavy, so it sinks and settles along the bottom of each radiator, where the hot water can’t push through it.
At first you might notice a slightly cooler strip along the bottom. Over time the sludge builds up and the cold patch grows, the radiator gives out less heat, and the boiler has to run longer to warm the room.
Was the water black when you bled it?
The colour of the water that comes out of the bleed valve tells you a lot about the inside of your system:
Black water is magnetite circulating round the system, the raw material of sludge. Brown or orange water is rust. Clear water is a good sign that the system is clean.
If you saw black or brown water, that’s worth getting looked at, even if the radiators still feel warm.
Other reasons a radiator stays cold
Sludge is the most common cause, but not the only one. These are worth checking first, and they’re safe for a homeowner to look at:
A valve that’s shut or stuck. Check both valves are open. The pin in a thermostatic radiator valve (TRV) can stick after a summer of not moving.
Low system pressure. On a combi or system boiler, look at the pressure gauge. Your boiler manual gives the right figure, often around 1 to 1.5 bar when the system is cold. Bleeding radiators lowers the pressure, so check it afterwards.
An unbalanced system. If the radiators nearest the boiler are hot and the furthest ones are lukewarm, the system may need balancing.
A pump problem. If several radiators are cold and the boiler is running, the pump may not be moving the water properly. That’s one for an engineer.
How a powerflush fixes it for good
A powerflush machine is connected to your heating and pushes water and a cleaning chemical round the whole system at high flow but low pressure. Each radiator is flushed in turn, with the flow reversed, until the water runs clear. See how a powerflush works.
Afterwards we add inhibitor to slow future corrosion, and fit a magnetic filter if your system doesn’t already have one, so new debris is caught before it can settle.
In Kent?
Kent Powerflush, Gas Safe 660156, covers Faversham, Canterbury, Whitstable, Herne Bay, Thanet, Sandwich, Deal, Dover, Folkestone, Ashford and Sittingbourne. Call 07540 335 087 or get a free quote.
Local pages: powerflush in Faversham, powerflush in Herne Bay, powerflush in Dover, or see all the areas we cover.
Questions people ask
Why is my radiator still cold after bleeding?
Because the problem isn’t air. If the radiator is cold at the bottom, sludge (magnetite) has settled inside it and is blocking the hot water. Bleeding only releases air from the top, so it can’t shift sludge. A powerflush clears it.
Why is the water black when I bleed my radiator?
Black water is magnetite: rust from the inside of your radiators and pipes, circulating in the water. It’s the same material that settles and turns into sludge.
How often should I need to bleed radiators?
Bleeding a radiator once in a while is normal. If you need to bleed the same radiators every few weeks, corrosion inside the system is likely producing gas, and the system needs cleaning and protecting.
Can I flush a radiator myself?
One radiator can be taken off the wall and hosed out outside. Sludge sits in the pipes and the other radiators too, though, so the whole system needs a powerflush machine to clean it properly.
Related guides
Hot radiators across East Kent
Faversham, Canterbury, Whitstable, Herne Bay, Thanet, Sandwich, Deal, Dover, Folkestone, Ashford and Sittingbourne. Tell us about your system and Pete will come back with a price.