Which Radiator to Bleed First, and Should It Be Hot or Cold?

Bleed radiators with the heating switched off and the radiators cool, or at most lukewarm. Start downstairs with the radiator furthest from the boiler, work back towards it, then do the same upstairs, and check the boiler pressure when you have finished. That order and that temperature get the most air out with the least risk of scalding. If you find yourself doing it every week or two, though, air is a symptom and something else needs fixing.

Hot or Cold: Why the Heating Should Be Off

There are two reasons to switch the heating off and let the radiators cool before you start.

• Safety. System water runs hot enough to scald, and when the bleed screw is opened on a hot radiator the first thing out after the air can be a spurt of very hot water.
• The air settles. While the pump is running, air bubbles are carried round the circuit. Once it stops, they rise and collect at the top of each radiator, which is exactly where the bleed screw is. On some older open-vented systems, opening a bleed screw with the pump running can even pull air in rather than let it out.

Running the heating briefly beforehand is useful, because it shows which radiators are cold at the top. Then switch it off and give them time to cool before bleeding.

Which Radiator to Bleed First

The usual order is:

1. Downstairs, the radiator furthest from the boiler.
2. Work through the downstairs radiators towards the boiler.
3. Upstairs, again starting furthest from the boiler and working back.

Starting low and far away pushes air towards the high points, where the upstairs radiators then catch it. Upstairs radiators usually hold the most air, because air rises through the system, so it makes sense to do them last and pick up what the downstairs bleeding moved along. If one radiator is obviously worst, you can deal with it first, but go round the whole house rather than bleeding one and stopping. Heated towel rails in upstairs bathrooms trap air the same way; see our guide to a heated towel rail not getting hot.

How to Bleed a Radiator Properly

• Have a radiator key, or a flat screwdriver for the slotted type, plus a cloth and a small container.
• Hold the cloth under the bleed screw at the top corner and open it a quarter to half a turn. You do not need to open it further. Our step-by-step guide on how to bleed a radiator covers which key fits and what to do when the screw will not turn.
• You will hear a hiss as air escapes. When it turns to a steady spit of water, close the screw firmly but without forcing it.
• Wipe the valve dry and check a minute later that it is not weeping.

If no air and no water come out at all, the radiator may be isolated at its valves, or the system pressure may be too low to push water up to it.

Check the Pressure Afterwards

This is the step most people forget. On a sealed system, which covers most combi and system boilers, every bit of air released is replaced by water from within the system, so the pressure on the boiler gauge falls. After bleeding several radiators it can drop low enough for the boiler to lock out. Check the gauge against the range in your boiler manual and top it up through the filling loop if needed. Our article on repressurising a system yourself explains how, and when not to.

Older open-vented systems with a feed and expansion tank in the loft top themselves up automatically, so there is no gauge to watch.

When to Bleed Radiators

Bleed when there is a reason to, not on a calendar. The signs are:

• A radiator that is warm at the bottom and cold across the top.
• Gurgling or trickling sounds in radiators or pipes when the heating starts.
• After the system has been drained and refilled, for example following a radiator replacement.
• At the start of the heating season, as part of a first switch-on check. Our guide to when to turn the heating on covers the rest of that list.

When Bleeding Does Not Fix It

Bleeding only solves one fault, trapped air. These are the patterns that point somewhere else:

• Cold at the bottom, warm at the top. That is sludge settled in the panel, not air. Bleeding will not touch it. See our guide to a radiator cold at the top or bottom.
• Cold all over, even after bleeding. Check the TRV pin has not stuck down after the summer, and that the lockshield is open. Our article on TRV faults shows how.
• Air back within days. Frequent air, or gas, usually means one of two things. Corrosion inside steel radiators produces hydrogen gas, which is a sign the system water has lost its protection and sludge is forming. Or air is being drawn in, through a leaking joint, an automatic air vent or, on open-vented systems, a pump that is pulling air in. Either way, it needs diagnosing rather than more bleeding.
• A whole floor or circuit stays cold. After a drain-down, an airlock in a section of pipe can stop water reaching several radiators at once. Clearing it can mean bleeding the pump and the high points of the pipework, or pushing water through that circuit under pressure.
• The pressure keeps falling. If you have to top up the pressure regularly after bleeding, the system is losing water somewhere. Our guide to a system that won't hold pressure covers the causes.

When to Call an Engineer

Bleeding is a sensible homeowner job. Constant bleeding is not. If radiators keep filling with air, heat unevenly after bleeding, or the water from the bleed screw comes out black, the system is telling you something about the water inside it. That often leads to a power flush, a magnetic filter and a fresh dose of inhibitor rather than more trips round the house with the radiator key.

If your radiators still are not heating properly after bleeding, book a free survey. We will find out whether it is air, sludge, valves or balance, and give you a fixed quote for putting it right.