How do I bleed radiators?

[mr j]

Hi everybody. My sister has a problem. I've noticed that most of her rads are hot at the bottom and cold at the top. When I bleed them some air comes out and then nothing, no more air. After the air finishes coming out it is not followed by water, as I'd expect. Nothing at all happens. Should I be bleeding them with the system OFF as I am doing? Or should I turn the system ON and then try bleeding. The rads are fed by a combi boiler and this "air problem" has been prevalent since the combi was installed about a year ago. Any ideas? Thanks, mr J

[Gel]

Sounds like air lock if youre sure lbleed nut fully unwound. Leave system on to pump water through, and take bleed nut all way out and wait. Be ready to fit back in as soon as you feel heat at top of rad/hear gurgling, but leave open until you see some dirty water dribbling out.

[Steve Walker]

"The rads are fed by a combi boiler... "

Mr J, no more water is coming out because it's a sealed system, and some of the thermostatic valves are closed. There's no incoming water arriving in the radiator to displace the air.

You need to open the thermo rad valves, and then find the mains valve which lets water into the system. It'll be under the boiler, in some kind of short flexible loop. Open it, wait for pressure to be indicated @ 1 bar before closing, and then go back and bleed some more. You might have to repeat this cycle a few times.

[mr j]

thanks, ill try that. mr J

[EricP]

Is the system at the correct pressure? It sounds like it is almost empty.

[mr j]

Ive just rang her and she's topped it up using the filling loop. It was nearly on empty. Tomorow I shall bleed her rads for her. Is it positive that I bleed with the pump pumping water round the rads? mr J

[John Plant]

It won't matter that thermostatic valves may be closed as water will pass up the other pipe to displace the air. As it is a closed system the pressure head is the same at both ends of the radiator.

Remember to check the pressure after bleeding.

John

[Dave Plowman (News)]

I'd be concerned where the water is leaking to.

[mr j]

I don't believe any water is leaking anywhere. She had the combi fitted when she qualified for a Government grant and from what she tells me (A) the system was never bled in the first place as the fitters told her that it "bled itself automatically" and (B) the rads have never worked at proper temperature since the combi was fitted. A lot of 'effing and blinding from the lads who fitted it but no aftercare.She was never told about the filling loop and she now tells me that the instructions suggest regular checking of the pressure and how to top up the system using the filling loop. I think it's a fine system (it cost her nowt) but it just needs bleeding. Everytime I bled it I knew nothing about refilling the system using the filling loop. To recap,tomorrow when I bleed it, should I bleed it with the system on then? - and fill the system accordingly. mr J

[Harry Bloomfield]

mr j formulated on Saturday :

The answer is in your final sentance, the mention of 'combi boiler'. If you release air from these systems, then you need to replace it with water. As a combi is a sealed system, as you bleed the air out the pressure in the system will gradually be lost.

What you need to do is find the filling loop, which will probably be somewhere near the boiler and top it up. The filling loop allows you to top up the system water with water from the water mains. A gauge on the boiler will indicate the system pressure, which should be between 1 and

2 bar once it is filled up and fully bled.

[Roger Mills (aka Set Square)]

No, bleed it with the boiler off - but check that you have about 1 bar pressure in the system before you start - topping up with the filling loop, as necessary. As you bleed air and/water out, the pressure will drop. So check it frequently during the bleeding operation and top up when it gets low. When you've finally finished, make sure that the cold pressure is about

1 bar and then disconnect the filling loop.

[mr j]

Cheers. mr J

[John Rumm]

Have a read through the filling and bleeding procedure here:

[Owain]

It's possible that the system wasn't properly flushed through and the air is actually gas resulting from corrosion. This might need some inhibiter added. Also, the radiators probably haven't been balanced properly.

Both are covered in the FAQ.

Owain

[Andrew Gabriel]

coming out!

but on the other hand, the hotter the water, the more air will come out of solution.

[Mike Harrison]

Yes, but before it has time to heat up too much - you don' want scalding water coming out!

[mr j]

Thanks for all your help on sorting out my sisters rads. I went round earlier and bled the system, topping up with the filling loop as I went along. All rads now flow hot top and bottom. Thanks for helping. mr J

[cs]

So if you miscalculate and the water starts spraying out of the completely unscrewed bleed nut, how are you gonna get that bleed nut back in? Even small amounts of water pressure can be really hard to fight, I've been there...

[Roger Mills (aka Set Square)]

Turn off both valves a bit rapidly?

[Christian McArdle]

It matters little on a sealed system at 1 bar pressure, although it is safer off, as any water spray will be cold. On a gravity system, you need to turn it off, as the pressure within the pipework can be sub ambient pressure, so it sucks air in.

Christian.