Radiator cold at the bottom but hot at the top: what it means and how to fix it

Put your hand on the radiator once the heating has been running for twenty minutes and feel where the cold is. If the top is cold and the bottom is warm, the radiator has air trapped in it. Air rises, sits in the top of the panel and stops water reaching it. Bleeding with a radiator key lets the air out and the water in, and the problem is solved until the next time air gets in.

If the bottom is cold and the top is hot, bleeding will do nothing, because what is sitting in the bottom is not air. It is sludge: a black, oily mud made of rust particles from the inside of the radiators and steel pipework, bound together with old inhibitor and sometimes limescale. It is heavier than water, so it settles in the lowest part of every radiator and the lowest pipe runs, and the heating water simply flows over the top of it.

A third pattern, cold in the middle with a warm edge, is usually the same sludge in a double-panel radiator where one panel has blocked more than the other. And a radiator that is cold everywhere, top to bottom, is normally a valve problem or a balancing problem rather than a sludge problem at all.

Every wet central heating system corrodes from the inside. Water plus steel plus a little oxygen makes iron oxide, and the inside of a panel radiator is thin pressed steel with a lot of surface area. Inhibitor slows the reaction right down, which is why a system that was dosed properly and left sealed can run for fifteen years and drain almost clean.

The reaction speeds up whenever fresh water gets in. Every time a combi is topped up, every time a radiator is taken off for decorating, every time the system is drained and refilled without new inhibitor, oxygenated water arrives and the corrosion restarts. Open-vented systems with a tank in the loft are worse still, because the tank breathes and a slow leak keeps drawing new water in. Twenty years of that produces kilos of magnetite, and it all ends up in the bottom of the radiators, the boiler heat exchanger and the pump.

You can see it for yourself. Bleed a radiator into a white cup and look at the colour. Clear water is a healthy system. Grey is a system that needs inhibitor. Black, with a metallic sheen and grit at the bottom of the cup, is a system that needs cleaning.

Cold patches are the visible symptom. The invisible ones cost more:

• The boiler works harder. A radiator with the bottom third blocked gives out a third less heat, so the rooms take longer to reach temperature and the boiler runs longer to get them there.
• The pump wears out. Magnetite is abrasive and magnetic. It collects on the pump rotor, makes it noisy and eventually seizes it.
• The heat exchanger blocks. Modern condensing boilers have narrow aluminium or stainless waterways. Sludge that reaches them causes kettling (a rumbling, banging noise as water boils on hot spots), and a blocked exchanger is often the reason a boiler is written off.
• Pinholes. The corrosion that makes the sludge eventually eats through the radiator wall, usually along the bottom seam. The first sign is a rusty tear-mark on the wall below.

Boiler manufacturers know all this, which is why most warranties on a new boiler require the system to be cleaned before it is fitted and a magnetic filter installed on the return.

Take the radiator off and flush it outside. If only one radiator is affected, close both valves, undo the tails, carry it into the garden and run a hose through it until the water runs clear. Refit it, top up, add inhibitor. Messy but effective for a single radiator on an otherwise healthy system.

Chemical clean. A cleaning chemical is dosed into the system and circulated for a week or two while the heating runs, then the system is drained, refilled and dosed with inhibitor. It loosens light sludge and is a good pre-clean before a new boiler on a system that is not too far gone.

Power flush. A pump is connected to the system, usually at the pump position or across a radiator's tails, and pushes water through the pipework and each radiator in turn at high flow with cleaning chemical, reversing the flow direction to shift stubborn deposits, while a magnet collects what comes out. Each radiator is agitated and flushed individually until the water runs clear. This is the right fix when several radiators are cold at the bottom, when the boiler is kettling, or before a new boiler goes onto an old system. Properly done it takes most of a day.

Replace the radiator. If a radiator is badly pitted inside or has already started to weep at the seam, cleaning it is throwing money after it. A new panel of the same size drops onto the existing tails.

The question people ask is whether a power flush is worth it, and the honest answer depends on the system. On a ten-year-old system with two cold radiators it transforms the heating. On a forty-year-old system with rotten steel pipework under a concrete floor it can open up leaks that the sludge was quietly plugging, and a good engineer will say so before starting. That conversation is the whole point of a power flush being quoted after a look at the system rather than over the phone.

Three things stop the sludge coming back:

1. Inhibitor at the right dose. One bottle per hundred litres of system water, roughly one per ten radiators. It needs testing every year at the boiler service and topping up after any work that drains the system.
2. A magnetic filter on the return pipe. It catches magnetite before it reaches the boiler and is emptied at each service. On a clean system it collects a teaspoon a year. On a dirty one it fills in weeks, which tells you something.
3. Stop the fresh water getting in. Fix the slow leak that is making you top up. On an open-vented system, check the loft tank's ball valve is not dribbling.

Do that and the flush you paid for will still be paying you back in fifteen years.