Home Q&A: Why do we bleed radiators, and how do I do it?

If your radiators are not hot over their entire surface, they cannot radiate or convect heat well. File picture

Question

Why do we bleed radiators, and how do I go about doing it?

Answer

If your radiators are not hot over their entire surface, they cannot radiate or convect heat well. 

Oddly, though called radiators, they do most of their work through convection. If the top edge is cool, you likely have a pocket of air trapped in the system. Here’s a quick fix. 

Turn off the central heating and ensure the radiators are completely cool. Look for a little metal nub set into the top edge of the radiator. This is your bleed valve. 

Spread some old towels across the floor and tuck them under the area of the radiator, paying attention to the point where we’re going to bleed the radiator at that valve. Using a radiator key (available at any DIY outlet), make a one-quarter turn to the left (lefty loosey, rightly tighty). You may hear a hissing rush of air. 

Stop and just let the air out, and when you start to detect a trickle of water, smartly turn the key to the right again. Don’t over-tighten as this can damage the thread. 

If your radiators are not heating up enough generally, it’s a good idea to check the pressure in your boiler when it’s cold and before you bleed radiators (we’re looking for 1bar-1.5 bar in the green area of the gauge). 

If the pressure reads too low, the system may need topping up at the filling valve. Your plumber can do this for you if you prefer, and this is a little more challenging, as you may have to attach a filling-loop and then deal with bleeding the radiators.

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