Before i call out a plumber can anyone help?

If the radiators are old round top types then take the bleed screw out and push a small pin into the hole as sometimes the orifice will choke up with rust/sludge.

If you tried to bleed the Rad's and no air come out then the system needs filling.

If it's a closed system you will have to fill it via the mains water supply.

This should be next to the boiler it's a flexy pipe which joins the heating system with the water supply open the taps on the flexy pipe and watch the bar pressure dial on the boiler fill to 1 .1/2 bar

then close taps. It's also possible that the filler could be built in under the boiler.

If it's a gravity system where the system is filled by a header tank check that the tank has water in may be a stuck bullcock, when done re-bleed rads.

hope this helps.

Unfortunately as per many others you dont say what boiler you have(it really can help) and I think all of the answers you have had may well be correct in the correct application but one is missing for an older system it is possible if you have a tank fed system feed to get a blockage in the cold feed co

ming from the tank into the heating system. It is solid magnetite and would require a system drain down and replace the blocked pipework/fittings.

you need to flush the system the reason way there cold at the bottom is because there full of sludge which needs flushing. on one of your downstairs radiators you should have a drain off tap if you have connect a hose pipe on to it and open the tap the water will drain out after about 5 minutes open all the bleed nipples starting with up stairs till all the water runs out ofter the water stops coming out then close all the bleed nipples and drain tap and start filling the radiators remember to bleed the radiators staring with the furthest away once all done you should be fine. if you need help get in touch on dfrith99@yahoo.com

are both valves open (each side of radiator), did you switch off the heating controls before opening the bleed valve,if the heating is on then air will be pulled into the system when the bleed valve is opened?

Look at the following link - they might help:

http://www.abacusgas.com/air-in-system.htm

My guess is they are rusted inside. You may need to have them replaced.