Types of radiator bleed valves
Look at the bleed valve at the top corner of the radiator before you pick a tool. What opens it depends on the shape of the small spindle or screw in the middle. These are the common types in UK homes:
| Valve type | What it looks like | What opens it without a key |
|---|---|---|
| Square spindle, standard size | Small square pin in the centre, the most common UK type | Thin-nosed pliers or a small adjustable spanner, with care |
| Square spindle, larger size | Same, but a slightly bigger square, found on some older radiators | Pliers or a small adjustable spanner |
| Square with a slot | Square pin with a line cut across the top | Flat-head screwdriver |
| Slotted only | Flat or domed screw head with a slot, common on newer and designer radiators | Flat-head screwdriver, or a coin on wide slots |
| Recessed square | Square pin sitting inside a hollow collar | Usually nothing; you need a key with a long socket |
| Plastic or proprietary | Plastic cap or unusual fitting on some designer radiators and towel rails | The maker's own key, or check their fitting instructions |
Many manufacturers now fit valves that take both a key and a screwdriver, precisely so homeowners are not stuck without a key. If you cannot see the valve clearly, a phone torch helps, as the valve is often tucked against the wall.
How to bleed a radiator without a key
The method is the same as a normal bleed. Only the tool changes. Our full guide on how to bleed a radiator covers the order to do the rooms in and the full routine. In short:
- Turn the heating off and wait until the radiator is cool to the touch.
- Hold a cloth under the valve, because a screwdriver or pliers give you less control than a key and water can arrive suddenly.
- Open the valve anticlockwise a quarter to half a turn until air hisses out.
- Close it clockwise as soon as a steady trickle of water appears.
- Check the boiler pressure on a combi or system boiler once you have finished all the radiators.
Slotted valves: use a flat-head screwdriver
Choose a screwdriver whose blade fills the slot fully. A blade that is too small twists in the slot and chews up the soft brass. Push in firmly as you turn, and turn slowly. On a wide slot, a coin can work if you have no screwdriver at all.
Square valves: pliers or a small spanner
Grip the square spindle across two flat faces, not across the corners, with thin-nosed pliers or a small adjustable spanner closed down tight. Turn in small movements. This works, but the spindle is small and soft, and it is easy to round the corners, after which even a proper key will not grip. If the valve is at all stiff, stop and buy a key.
Recessed valves: get the right key
If the square sits deep inside a collar, pliers cannot reach it and a screwdriver has nothing to bite on. A key with a long, narrow socket is the only sensible option.
Do not use a knife, scissors or a hot tool to force a bleed valve. They slip and cut hands, and a damaged valve can let hot, dirty water spray out under pressure. Never try to bleed a radiator while the heating is on.
Radiator bleed key: what size do you need?
Most UK radiators take a standard radiator key with a small square socket, widely sold as a 5mm square key. Some radiators from the 1970s and early 1980s have a slightly larger square, usually listed as 6mm. Plumbers' merchants and DIY shops sell cheap keys in both sizes, and multi-keys that combine both squares with a screwdriver blade are a good buy if you are not sure.
If a key turns without gripping, it is either the wrong size or the spindle is already rounded. If it will not fit over the spindle at all, try the larger size or check for paint on the square. Recessed valves need a key with a longer socket, and some designer radiators and towel rails use the maker's own plastic key, which should have come in the box. The radiator's fitting instructions usually say which key it takes.
Buy two keys and keep one taped to the top of the boiler or inside the airing cupboard. Most people only go looking for a key on the first cold evening of winter.
The radiator bleed screw and how the valve works
The bleed valve has two parts. The outer body screws into the radiator's top corner tapping and stays put. The radiator bleed screw is the small spindle in the middle, with a tapered tip that seals against a seat inside the body. Turning the screw anticlockwise lifts the tip off the seat so air can escape through a tiny side hole. It only needs to move a fraction of a turn to do that.
That is why you should never unscrew it far. On most valves the screw will come right out if you keep turning, and then water runs from the hole at the full system pressure. If that happens, push the screw back in and turn it clockwise straight away. Do not turn the larger nut or hexagon of the valve body, as that unscrews the whole valve from the radiator.
What if the bleed valve is stuck?
A stuck bleed valve is common on radiators that have not been bled for years or have been painted over. Do not force it, because the screw is soft brass and can snap, leaving a stub that is hard to remove.
- Make sure you are turning the right way. Anticlockwise opens it, looking at the valve face on.
- Clear any paint from around the spindle with a small blade, working away from your hand.
- Use a proper key rather than pliers, and try a key with a longer handle or a T-bar for better leverage and control.
- Try a small tightening movement first. A tiny turn clockwise, then anticlockwise, can break the grip of old deposits.
- Stop if it will not move with firm hand pressure on the key.
A penetrating spray applied the day before sometimes helps, but use it sparingly and wipe off any excess, as some leave marks on painted radiators. If the spindle is rounded, the valve needs replacing rather than more attempts.
Radiator bleed valve replacement
A radiator bleed valve that is seized, rounded, snapped or keeps dripping after closing can be replaced without replacing the radiator. The valve body unscrews from the radiator tapping with a spanner, and replacements are sold in the common thread sizes for UK radiators. Take the old one with you or check the radiator maker's details so you get the right thread.
To change it, the radiator has to be isolated by closing the valves at both ends, then drained down far enough that the water level is below the tapping. That is a routine job for a plumber and is often done in under an hour. If you are confident with basic plumbing, our guide on how to turn off and isolate a radiator explains the isolating part. Draining and lifting a radiator is covered in our guide on how to remove a radiator.
On a sealed system you will need to top the pressure back up afterwards, following our guide to repressurising a boiler.
What to do next
If you have a slotted valve and a screwdriver that fits, go ahead and bleed as normal. If you have a square or recessed valve, a key costs very little and saves damaging the spindle. After bleeding, if the radiator is still cold at the top, or cold all over, the air may not have been the cause. Our guide to a radiator cold at the top runs through the other causes. Call a plumber if a bleed valve snaps, spins without opening, or will not stop leaking once closed. Put a container under it and close both radiator valves to take the pressure off while you wait.