Evening all.
Just wondered if anybody knows anything about central heating?
We have a new boiler (a condenser boiler but not a combi).
It sounds to me like eitehr there is air in the system or not enough water.
We have a feed and expansion tank in the loft (controlled by a ball cock)which only seems to have a tiny amount of water in it . Should that tank have a good amount of water in it? Does anybody know?
Thank you.

I'm not a plumber, I've just had experience of my system here for 21 years.

Tank in roof should be 'full'. Whatever its level is, somewhere below its overflow. This acts as a header for the whole system, allowing the water to expand and contract. So the normal level should be enough below the overflow to allow expansion without overflowing, but when cold, not normally allow more in, or it will overflow next time when hot.

If the tank is empty, it is likely the valve operated by the ballcock has stuck shut. This seems quite common as it operates rarely. However, unsticking it may then cause it to leak even when the tank is full. So be prepared to replace the shut-off washer. It is the same as what used to be found in loo cisterns before we all went push-button.

Fill the tank. Then you'll need to go around all the radiators and bleed them until all the air is out of each. This can be done without the system running. If it needs a lot of water to fill it, you'll probably need to add some Fernox or similar corrosion inhibitor. Like the cooling system in the car, if just plain water is used it will corrode its was out of the radiators which are just steel.

Thank you. I had a feeling that the entire capacity of the header tank would be expansion space and that there should be a decent amount of water in it.
You have confirmed my suspicions!
Thanks again.

Final word on this (I hope).
Ball cock arm was a bit sticky and pump was on its way out!