How to Bleed a Radiator (and When Bleeding Won't Fix It)

As the weather turns colder across Hove, Brighton and Worthing, it's the time of year we get the most calls about radiators that just aren't warming up properly. Often the fix is simple and something you can do yourself in ten minutes: bleeding the radiator.

Here's how to do it safely, how to tell whether air is really the problem, and when it's time to get an engineer in.

The three parts of a radiator you'll need to find

Why radiators need bleeding

Over time, air collects inside a central heating system and rises to the top of the radiators. That pocket of air stops hot water filling the whole panel, so you get a radiator that's warm at the bottom but cold at the top.

Other signs include gurgling or knocking noises from the radiators or pipes, and rooms that take much longer to heat up than they used to.

What you'll need

• A radiator key (available from DIY shops and plumbers' merchants for a few pounds; some newer valves take a flat-head screwdriver instead - we stock these in our shops usually!)

• An old towel or cloth

• A small container to catch any drips

How to bleed a radiator, step by step

Bleeding a radiator in four simple steps

1. Turn your heating off and let the radiators cool down. The water inside can be very hot, and bleeding a system while the pump is running can draw in more air.

2. Find the bleed valve. It's a small square fitting at one of the top corners of the radiator.

3. Hold the cloth under the valve, fit the key and turn it slowly anticlockwise, about a quarter to half a turn. You should hear a hiss as the air escapes.

4. As soon as water starts to come out in a steady trickle rather than spitting, turn the key clockwise to close the valve. Don't overtighten it.

5. Wipe away any drips and move on to the next radiator if needed, doing them one at a time.

6. Check your boiler pressure (see below), then turn the heating back on and check the radiators warm up evenly.

Don't forget to check the boiler pressure

Most systems sit around 1 to 1.5 bar when cold. Check your boiler manual for yours

If you have a combi or system boiler, your heating is a sealed, pressurised system. Letting air out lowers the pressure, and if it drops too far the boiler may stop working.

Look at the pressure gauge on the front of the boiler. Your boiler's manual will tell you the correct level, but many sit at around 1 to 1.5 bar when the system is cold. If it's low, you can usually top it up using the filling loop, following the manufacturer's instructions for your model. If you're not sure how, or you can't find the filling loop, give us a call rather than guessing.

If your home has an older open-vented system with a small feed tank in the loft, it will refill itself and you won't need to top up the pressure.

When bleeding won't fix it

Bleeding only solves trapped air. If you're seeing any of these, something else is going on:

Where your radiator is cold tells you what's wrong

• Cold at the bottom, warm at the top. This usually points to sludge (a build-up of rust and debris) settling in the bottom of the radiator. A system clean or powerflush is often the answer.

• Completely cold radiator. Check the valves at each end are open. If you have a thermostatic radiator valve (TRV), the small pin underneath the head can sometimes stick, especially after the summer.

• Radiators furthest from the boiler are cooler than the rest. The system may need balancing, which means adjusting the valves so hot water is shared out evenly.

• Radiators needing bleeding every few weeks. Regular air build-up can be a sign of corrosion inside the system or a small leak somewhere, and it's worth getting it looked at before it causes bigger problems.

In our experience, when bleeding hasn't done the trick it can be a number of things. Quite often it's a thermostatic radiator valve with its pin stuck down, or a balancing issue that needs adjusting using the valve at the other end of the radiator. We've got a short video on our Facebook page showing how to free a stuck pin and how to help with balancing. Sometimes, though, it's sludge and muck in the system, which brings us on to keeping your system protected.

Keeping your system healthy

A little maintenance goes a long way. An annual boiler service by a Gas Safe registered engineer keeps your boiler running safely and efficiently, and gives us a chance to spot problems with the wider system early.

We also fit a magnetic system filter with every new boiler installation we do, and we highly recommend adding one to existing systems too. It catches the sludge and debris circulating around your heating before it can build up in your radiators and boiler. They're relatively low cost, well worth having, and a real benefit to just about any system.

Safety first

Bleeding radiators and topping up the pressure are jobs most homeowners can do themselves. Anything involving the boiler itself, its gas supply or its flue must only be done by a Gas Safe registered engineer. If your boiler is showing a fault code, keeps losing pressure, or you're worried about it in any way, get it checked.

If you ever smell gas, leave the property, don't use any electrical switches, and call the National Gas Emergency number on 0800 111 999.

Need a hand?

If your radiators still aren't right, or you'd rather someone else did it, our family-run team has been looking after homes across Hove, Brighton, Worthing and Sussex since 1980. We're Gas Safe registered and happy to help with everything from a stubborn radiator to a full system clean.

Call us on 01273 417004 (Hove) or 01903 629483 (Worthing), email dorringtonplumbing@gmail.com, or book online here.