Drill a series of small holes in the top of each radiator to allow the air to escape. As its in a radiator the air will be warm so will rise and make its way out of the radiator.

Seal up the holes afterwards with either plastacine, bogeys or blobs of weld.

You might have a gas in the pipes problem. To put it simply if you have corrosion in the system it will produce Hydrogen gas, other things can happen as well you could actually have bacteria in the system producing methane gas. Both quite common problems.
Buy some Fernox or equivalent Preventing Coldspots I don't work for them by the way. It was my specialisation once up on a time though.
Stick some in the hedder (careful it is for the heating and not the domestic water). drain off some of the system and or (depending how much gas/air is in the system) just vent the rads, get it into circulation anyway. Re-vent a few days later.
You could see if it is hydrogen or methane being produced but I would not recomend it. Once saw someone light the vent from a rad in a house on park lane London. The flame setfire to some early 19th century wall paper and damaged a picture. Result £12,000 damage and that was in the 80's!

If the set point of the central heating is too high, it may be kettling (the water is actually boiling in the boiler) it should be set to around 60 degrees C. If it is kettling you get a rumbling sound coming from the boiler, usually a sign that the pump has failed. It should be pumping the cooled return water back into the boiler.

Originally Posted by vinniethemanxcat

Is it a combi... sealed system, or does it have a small tank in the loft or airing cupboard which tops the system up via a ball- valve?

Was going to say the same thing - check the small tank in the loft - sometimes the ball cock sticks and not enought water gets through the system

Cant you just bleed the radiators? Small screw at the top right of the panel with a plastic directional hole. Unscrew with a container placed under to catch the water.

Pumping over is the first to check.

Basically, the oxygen dragged into the system by this reacts with the steel of the rads.

Iron.. Fe reacts with Oxygen.. O.

Normally this combines with 2 molecules of oxygen to form Fe O2...... rust/ iron oxide

In this case, with a smaller amount of oxygen available, it becomes Fe O.... black sludge/ magnetite, but if you drop this outside on the patio, the black sludge picks up the extra oxygen and turns into orange rust.

Basically, your rads are rotting from the inside out, especially your paper- thin, chromed steel designer towel radiator.

If it isn't pumping over, if it takes a while before the rads need bleeding, it could be small leaks on joints, eg. the rad valves on the suction side, check under the plastic caps for corrosion,or the pump valves.

Anywhere there is a green deposit is a cause for concern.

A check around before you call someone in isn't a bad idea.

What boiler is it?

Is the heating system sealed or is there a 4 gallon feed and expansion tank in the loft. "A tank in the loft" isn't the answer; my heating is a sealed system, there is a 50 gallon cold water storage tank in the loft.

Originally Posted by BONNACON

You could see if it is hydrogen or methane being produced but I would not recomend it. Once saw someone light the vent from a rad in a house on park lane London. The flame setfire to some early 19th century wall paper and damaged a picture. Result £12,000 damage and that was in the 80's!

Hold a glass upside down over the radiator air vent when you bleed the gas out. If it's hydrogen it will rise and fill the glass. Keeping the glass upside-down, try to light the gas in the glass. If it's hydrogen, it will ignite with a pop and burn upwards with a blue flame.

Loads to think about there then, and not all positive! In a nutshell, I probably bleed the hall rad about once a week. There's no smell of methane
(I understand hydrogen is odourless). The water that comes out is clear, but I'd expect any nasties to sink to the bottom. No sign of any visible corrosion.

Thanks for the replies chaps. Especially P_G.

Bollock-chops may be onto something. The boiler was set at full pelt!