Worcester Bosch produce the market leading Greenstar range high quality, solidly built, high efficient condensing boilers. All Greenstar boilers have Aluminium heat exchangers (Heat Engines) which have some advantages over Stainless Steel heat exchangers, but have one disadvantage, if not properly serviced at regular intervals a layer of aluminium oxide becomes compacted within the heat exchanger and becomes very hard to scrape away and clean. 

Since the internal state of the heat exchanger is not visible Worcester Bosch have deemed that the passing of the Fan Pressure Test be sufficient to determine if full servicing is required. This is very handy, too handy sometimes as over the years the need to strip and open up the heat exchangers is not needed, but the gaskets and seals of the gas burners and heat exchangers are never inspected or changed.

The photo adjacent is a heat exchanger gasket from a Greenstar Junior Boiler which had deteriorated and was allowing heat (fire hazard) and CO to escape inside the boiler. On this type of Greenstar boiler you can only inspect the electrodes by removing the Fan and Air/Gas Manifold.
I find the Greenstar CDi models the easiest to clean because they have a larger rectangle heat exchanger, the newer Greenstar Compact models the most difficult as there is little service space inside as they are designed to fit inside a kitchen wall cupboard and the Greenstar Junior, i, Si and Ri models not as bad as the Compact. More information can be found below.

Call Tony on 07831 623711