How to Bleed a Radiator (and Check the Valves Are Open)

If your heating is on but one radiator stays cold (especially at the top), it usually doesn’t mean anything serious. In many cases, it just needs bleeding or the valves need a quick check.

Here’s how to safely check both.

What you’ll need

• A radiator key (or a flathead screwdriver for some radiators)

• An old cloth or towel

• A small container (optional)

Step 1: Make sure the heating is off

Turn your heating off and let the radiators cool down fully.

This helps prevent hot water spraying out and gives a more accurate bleed.

Step 2: Locate the bleed valve

The bleed valve is usually:

• At the top of the radiator

• On one side

• A small square or slotted screw

Pop a cloth underneath just in case.

Step 3: Bleed the radiator

• Insert the radiator key

• Slowly turn it anti-clockwise

• You’ll hear a hissing sound (that’s trapped air escaping).

Once water starts to come out in a steady stream, turn the key clockwise to close it.

Don’t over-tighten.. snug is enough.

Step 4: Check the radiator valves

Each radiator has two valves at the bottom:

TRV (thermostatic valve): usually has numbers on it

Make sure it’s turned up, not set to zero valve: usually has a plain cap

Make sure it hasn’t been knocked closed (this can happen when cleaning or decorating)

Both should be open for the radiator to heat properly.

Step 5: Check your boiler pressure

Bleeding radiators can reduce boiler pressure slightly.

• Check the pressure gauge on your boiler

• It should usually sit around 1–1.5 bar (check your boiler manual)

• If it’s low, you may need to top it up

Step 6: Turn the heating back on

Once everything’s done:

• Turn the heating back on

• Give the radiator time to warm up fully

If it’s now hot top to bottom, job done 👍

When to stop and get help

It’s time to get advice if:

• The radiator still won’t heat up

• Multiple radiators are cold

• The valve won’t turn

• The boiler pressure keeps dropping

At that point, it’s best not to force anything.

A quick reassurance

Radiators staying cold is one of the most common December issues, especially when heating gets used more than usual. Often, it’s a simple fix and knowing what to check first can save time, stress and unnecessary call outs.

And if you’re ever unsure, it’s always better to ask than guess.