As seems to happen rather too often in my (happily) DIY orientated life, this post has didn’t so much come to me as a flash of inspiration. Rather, my needy, half renovated Victorian cottage kept persistently reminding me about it – by sending gurgling, clanking noises through the radiators – typically rudely early in the morning or in the evening.

Radiators need bleeding because air gets trapped – typically in the highest radiators in the house. Why? well, because air can enter the central heating system – often getting trapped in the coil that runs through the hot water cylinder which is often the highest point in the system. In my case it’s most  likely because over the summer I have been working on my new bathroom, draining down the central heating system in order to re-route some of the radiator pipework. In re-filling the system, inevitably some air is left behind.

We knew our radiators needed bleeding for a couple of reasons. I guess most obviously, the radiators that have air trapped in them don’t heat up where the air pockets are. So typically the radiator will only feel warm towards the bottom, or in the case of our heated towel rail in the bathroom, the top 5 rungs didn’t heat up. Secondly, and almost more obvious in one sense, when the central heating kicks in we’ve experienced a bubbling, gurgling noise in the radiators that air is trapped in.

When we moved into our house, we had a new, pressurised central heating system installed, replacing the open vented system we previously had with a pressurised system. This is relevant to the bleeding issue because when you bleed your radiators in a pressurised system you’re releasing the pressure from the system – which then has to be topped up once you’ve finished bleeding the radiators – or half way through bleeding them if, like me, you had a serious amount of air to remove! Open vented systems – typically found in older central heating systems – have a header tank in the loft which fills the boiler and radiators and keeps them topped up. This sounds like less hassle and you read a few old school plumbers on forums saying they prefer open vented systems, but they cannot get to high pressure, can allow pollutants into the system and aren’t suitable for modern condensing boilders.

How to re-pressurise

central heating system pressure guage

The pressure guage on my system – showing a working pressure of 1 bar, and also, far right, the valve that you turn with a screw driver to top up the pressure

Step 1: So before you bleed your radiators it’s a good idea to find out where your pressure guage is – typically on the boiler itself and/or (as is the case in our house) attached to the pipework near the hot water cylinder.

Step 2: You then need to find the tap or screw that adds additional pressure into the system. When I lived in London I had a Vaillant combi boiler that had a top up valve located on the underside of the boiler.On my existing system the pressure valve is rather less conveniently located in the airing cupboard that houses the hot water cylinder – next to which is one of the two valves (see photo left) that I have to open with a flat head screwdriver to introduce more water into the system.

Have a look at what the pressure is when your system is cold. That way, when you bleed your radiators you then know what you need to top the pressure back up to.

Step 3: When the central heating has been on for a while, feel each radiator, and the chances are that the radiators that fail to heat up all the way to the top need bleeding.

Step 4: There’s lots of chat on the forums about whether you should bleed your system when it’s cold or hot. To me, this is a bit of a no-brainer. If you bleed it when it’s cold, you don’t get scolded by hot water, and more importantly you know exactly what the pressure was before you started, and what you need to top it up to. When the system is hot, the pressure obviously rises, which gives you an inaccurate, or moving pressure reading.

So when the system has cooled down, bleed each radiator in turn, holding a towel just below the bleed valve to catch any escaping water when the air has all been released.

What sort of bleed key should I use? Traditional radiators require a bleed key that fits the square bleed screw in the end of the radiator. More modern eg convector radiators have the same square screw, but conveniently they are also cut down the middle to accommodate a small, flat headed screwdriver.

Step 5: When you’ve bled all the radiators, top up the water into the system to the same pressure it was at before you started. If before you started the pressure seemed a bit low, check your boiler documentation or phone the supplier to find out what the working pressure should be.

Once you’ve topped up the pressure, it should pretty much remain the same. If it gradually drops, the chances are you’ve got a leak somewhere on your system, and you should get this checked over by a qualified plumber a) because it’s not great to have water escaping somewhere under your floor and b) because if the system is running at an unduly low pressure this might put your boilder under strain and damage it.