Your boiler is relatively new, so if it was fitted to a new system, or to an old system which had been properly cleaned, and inhibitor was added, it should not suffer from sediment unless there is some other fault such as a leak.
If the F&E tank has water in it (have a look) but nothing, or very little, squirts out when you bleed the highest radiator in the house, then there is usually a blockage of corrosion sediment in the 15mm F&E pipe where it joins in a Tee to the main circulating (22mm) pipes. This is usually close to the pump and/or cylinder and/or boiler. The blockage is usually black iron oxide and can be detected by holding a magnet to the copper pipe, it will be attracted to the iron at the point of blockage.
If the Tee is a compression joint it can be undone and cleaned out, if it is soldered it will have to be cut out and replaced. You will not be able to solder wet pipes so replace with brass compression fittings. Drain the system before cutting or undoing the Tee.
The usual cause is lack of anti-corrosion chemical (inhibitor) in the circulating water, or possibly the system has been pumping-over and introducing fresh dissolved air into the water.
Assuming the problem is caused by corrosion sediment, you should clean it out from the rest of the system by using a sediment-loosening cleaning chemical such as Sentinel X400. After you have cleaned, drained and rinsed the system, you should add an Inhibitor chemical such as sentinel X100 on final fill.
Also see FAQ 12, 14, 20, 24, 26 and others
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