How to fix a radiator that's gargling or cold at the top

February 13, 2026

Have you ever noticed your radiators only ever seem to be warm at the bottom? Maybe you hear a strange gargling sound when the heating is on? These are tell-tale signs that your central heating system has unwanted air circulating. Bleeding your radiators is the simple way of purging this air, and ensuring your system is running as efficiently as possible.

Don't worry though; bleeding a radiator is a simple DIY job, just follow these simple steps:

Step 1: Turn off the heating

This might seem obvious, but you'd be surprised. You can either:

Turn the thermostat down very low
Switch the heating off at the timer
Switch off the boiler

Make sure you give the system time to cool down. The radiator should be lukewarm at most; 30 minutes is usually enough.

Step 2: Find the bleed valve

The bleed valve is a small slotted square usually encased in a lug on the top left or right corner of the radiator. Insert your bleed key over the small square and hold your cloth directly underneath it.

Step 3: Slowly open the valve

Turn the bleed key anti-clockwise, very slowly. You should hear a gentle hiss. That's the sound of the air escaping. Just hold the key steady. Opening the valve further may allow the air out slightly quicker but you'll run the risk of the bleed screw coming loose. Trying to rethread it while the water escapes is not fun.

Step 4: Wait for water to appear

Once the hissing stops, water will start to dribble out. As soon as the spluttering has stopped, close the valve by turning the key clockwise. If the hissing stops but the water hasn't come through, check the next step and repeat this step until it does.

Don't forget to wipe away any drips from the radiator and wall, before they stain. The water inside your central heating system will often have been circulating for many years and it's usually a dark brown/black colour.

Step 5: Check the boiler pressure

Bleeding radiators can lower system pressure, especially if you’ve done more than one. If you have a sealed system make sure you check the pressure gauge on your boiler. (An older/vented system should refill itself). The ideal pressure (when cold) is usually 1 - 1.5 bar. If it’s too low, check the manufacturer's instructions on how to top it up.

Step 6: Turn the heating back on

Switch the heating back on and wait for the system to warm up. Make sure the radiator is now getting hot at the top and the bottom, and ensure the bleed valve is fully tightened.

Bleeding a radiator is a quick job that can make a big difference. Anyone can do it, so why go through the time and expense of calling out a handyman. It can even end up saving you money on your gas bill, as when a radiator isn't heating the room properly you'll likely turn the thermostat up, and/or keep the heating on for longer.