Bleeding a radiator without a key is something that can actually be done in a lot of cases. While using a key is obviously recommended, it is not something that you will be hopeless without. Whenever you are in a pinch there are a few other methods that you can still use to get the job done.
Step 1 - Turn Off Heating Switch
Make sure that the heat has been shut off. You will need to have your radiator as cool as possible for this job so you can avoid injuring yourself. It is important that you use special caution with the water that is inside of the radiator. Just because the radiator is cooled off does not necessarily mean that the water itself is. Be prepared to work with scalding hot water. Cover bleeder screw and tools with heavy cloth or towel to help prevent any hot water from scalding you.
Step 2 - Try a Screwdriver
You may be able to use a slotted screwdriver to get the job done. You will first need to check to see if your radiator has a slotted bleed screw. If this is the case, then your screwdriver should work as a perfect substitute for a key. Insert the head of the screwdriver into the slot and turn it counterclockwise to bleed the radiator properly.
Step 3 - Allen Key
If the screw is not slotted then an Allen key, or hex key, should be able to do the trick. You will simply repeat the same steps as you did with the slotted screwdriver. It will work the same way.
Step 4 - Try the Last Resort
If your bleed screw happens to be too badly damaged for either of these substitute tools to work, then you may need to move on to more extreme measures. As a last resort, you can loosen up the radiator bleed key assembly instead. It is not something that you should do without first trying other methods, as it can be very dangerous and harmful to your radiator if not done properly.
Understand What Not to Use
A lot of people think that a pair of pliers will do the trick, but typically, all pliers do is cause damage to your screw. Pliers often cannot get a firm enough grasp on the screws to turn them, but they can get enough hold to completely shred the screw instead. This could be what has caused the damage to the screws that are past the point of using Allen keys and screwdrivers. You will want to keep your screws maintained and in good condition for the next time that you need to bleed out your radiator without a key, so avoid using pliers at all.