There can’t be a lime scale build-up in your central heating side of your system. In percentage (%) terms; the sludge in your system (if present) is made up of 4% lime scale, 95% rust and 1% debris. Obviously these percentages are an estimate and the actual values will vary from system to system as different areas will have different water hardness values.There can be a build-up of lime scale in your hot water heat exchanger (plate heat exchanger) and only on the normal water side, these heat exchangers are only found in combi-boilers and not present in open vented systems.
The other possibility is in you hot water cylinder, which again is normal tap water side and NOT the central heating side. A standard power flush is not the same thing as having the lime scale flushed out of your hot water heat exchanger or hot water cylinder. When asking our members for a quote, you have to say which one you want “a central heating power flush” or “hot water heat exchanger flush”.
A central heating flush is to flush the boiler, pipes and radiators, while a hot water heat exchanger flush means removing the hot water heat exchanger; flushing it separately on both sides and refitting it into the combi-boiler. Getting both procedures done is preferable if you are having hot water problems on a combi-boiler system. You simply will not need a hot water heat exchanger flush, if you have an open vented system, because this system does not have one fitted.