How to bleed a radiator

What you are actually letting out

A sealed heating system holds a fixed volume of water going round and round, and gas has no business being in there. When some appears, it does what gas does: it rises, and it collects at the highest point it can reach inside each radiator. The water carries on flowing underneath it. That is why the signature of trapped air is so specific — cold across the top of the panel, warm along the bottom — and why the fix is simply to open a hole at the top and let the gas out until water arrives.

It is worth knowing where the gas came from, because it changes whether you will be doing this again. Some of it is ordinary air, dissolved in every litre of fresh water added to the system and released as the water heats. The rest may not be air at all. A system corroding inside generates hydrogen, steadily, as steel gives itself up to oxygenated water. Both bleed out the same way and feel identical at the valve. The difference is that one is a consequence of a top-up you did last spring, and the other is a system quietly eating itself.

First, make sure it is air

Bleeding is quick and free, so people do it to radiators that do not need it, and then conclude the radiator is broken when nothing improves.

Cold at the top, warm at the bottom, is air. Bleed it.

Hot at the top and stubbornly cold along the bottom edge is the opposite substance. Iron oxide is far denser than water, so it settles out and lies along the floor of the panel while water flows over the top of it. There is nothing gaseous in there and opening the bleed valve will produce nothing but a squirt of black water. That one needs the system cleaning or the radiator removing and flushing.

A radiator that is cold all over while others in the house are hot is usually neither. Look at the valves at each end — a thermostatic head that has sat closed all summer commonly seizes with its pin held down and never opens again in October.

What you need

A radiator key, which costs about a pound and fits the square spindle in the bleed valve. Many valves made in the last fifteen years take a flat-blade screwdriver instead, so look at the end of the radiator before you buy anything. Beyond that, a cloth and a small container — a mug is enough, an old towel underneath is better, because what comes out of an old system is black and it stains.

The procedure

1. Turn the heating off and let the radiators go cold. This is the step people skip and it is the one that matters. Water in a running system is at seventy degrees or more and under pressure, so a valve opened too far sprays it across the room. Cold also stops the pump from pulling more air in behind you while you work. Half an hour is usually enough; if the panels are cool to the palm, you are ready.
2. Start with the ground floor, and start furthest from the boiler. You are going to push the remaining gas upward and outward as you go, and working from the low, far end means you are always moving it towards the radiators you have not done yet rather than back into the ones you have.
3. Fit the key onto the square spindle at the top corner of the radiator, hold your cloth under it, and turn it slowly anticlockwise. A quarter to half a turn is plenty. You are opening a hole, not removing a component, and a valve wound all the way out will come free in your hand along with everything behind it.
4. Listen. You will hear a steady hiss as gas escapes. Let it run until the hiss stops and the first dribble of water appears, which is the system telling you the gas has gone and water has taken its place.
5. Close it as soon as water runs, turning clockwise until it is firm. Firm, not forced — these are small brass fittings and they strip.
6. Work up through the house, finishing at the highest radiator, and do not forget a towel rail in a bathroom or a radiator on a landing. They are usually the highest points in the system and therefore where the most gas ends up.

The part nobody tells you about

Everything you just let out was occupying volume in a sealed system, and the water that replaced it came from the pressure you had in reserve. So the gauge will have dropped, and after a whole houseful it can drop a long way.

This is why a great many people bleed their radiators on a Saturday morning and find the boiler dead by Saturday afternoon. Below roughly 0.5 bar the appliance stops the burner rather than risk running its heat exchanger without enough water moving through it, and it announces that with a code — F1 on an Ideal, F.22 on a Vaillant, CE 207 on a Worcester, E119 or E118 on a Baxi. Nothing has broken. You took the water out yourself an hour earlier.

So check the gauge when you have finished, cold, and bring it back to between 1 and 1.5 bar. How to repressurise a boiler covers the job itself and, more usefully, how to tell the difference between a top-up you caused and a leak you have not found. If your boiler has no gauge at all and there is a small tank in the loft, you have an open-vented system: it refills itself from that tank and there is nothing for you to do afterwards.

Once the pressure is right, run the heating and feel the radiators again after twenty minutes. Any that are still cold at the top have more gas that has migrated up while the pump ran, and a second pass will usually finish them.

When bleeding is the wrong answer

Doing this once a year, in autumn, is maintenance. Doing it every few weeks is a symptom, and it is worth treating as one, because the system is getting gas from somewhere and each of the explanations leads somewhere different.

Air is being drawn in — commonly through an automatic air vent that is passing the wrong way, or on the suction side of the pump. Or the system is corroding and making hydrogen, which points at water that has never been inhibited or has been diluted by years of topping up. Or there is a leak: a system losing water is also a system letting air in, and the pressure drop and the air are the same fault seen twice.

The quick test for the second of those costs nothing. Bleed into a glass and look at what comes out. Dark grey or black water is magnetite, which is corrosion in suspension, and a system producing it is a system that would benefit from a chemical clean, an inhibitor dose and — if it does not have one — a magnetic filter on the return.

Where to stop

Bleeding a radiator is genuinely owner's work, and one of very few heating jobs that is. The boundary is the boiler casing. Everything behind that front panel sits within inches of a gas valve, a sealed combustion chamber and 230 volts, and in Great Britain work on a gas appliance is restricted by law to Gas Safe registered engineers.

Three things should stop you rather than slow you down. A bleed valve that will not close, because you now have a hole in a pressurised system and it will not stop by itself — shut both valves at the ends of that radiator to isolate it and call somebody. Water appearing from the body of the radiator rather than the valve, which means the panel has corroded through and bleeding has found it. And any smell of gas at any point, which is not a heating problem at all: leave the property, do not touch a switch on the way out, and ring the national gas emergency line on 0800 111 999 from outside.