Expansion tank hot?

Greenstar Ri boiler open vented system. When boiler switched on constant flow of hot water to expansion tank through vent pipe. Circulation through rads still OK but seems reduced. Cold water feed ok and rad circlation pipes clear. Could this be partial blockage in heat exchanger if so how do you clear it? Service manual shows how it remove but only to brush out flue ducts. Any other causes? Have tried pump on lower speeds.