Most engineers will carry out the minimum action to insure the system is cleaned. Carrying out a cold and hot flush would be the bare minimum. If you add chemicals such as the sentinel X300 or Fernox F3 to this protocol the job would satisfy the building regulations. Once the system is cleaned and flushed you can add an inhibitor to protect against corrosion and scale, this will extend the life of the system and ensure maximum efficiency. On most old conventional systems, this method wouldn't be enough to protect the system, especially if it's being converted to a combi boiler. In this case a powerflush would be a must as the old boilers were more robust when working with sludge. New combi boilers have smaller parts and path ways which are more prone to blockages and leaks. Adding a magnetic filter on the return pipe will give added protection, but if the system is heavily sludged these filters will need to be regularly cleaned manually, as the space inside is not unlimited.
A simple test can be carried out to inspect the condition of the system water. Take out some water from a bleed point on a radiator into a clean clear glass. If it's black and full of sludge then drain the whole system and carry out the test again, this time after running the heating for an hour so the sludge is disturbed. If there's not much improvement, it may be a good idea to get a power flush done. You can get a free test on the water sample, with the results emailed to you within 24 hours. Another strong indicator would be if you have cold spots on the radiators, or some radiators are not heating up. The magnet test is also a good method to determine the state of the system. Using a strong magnet place it on the copper pipe, as copper is non-magnetic it will not stick, unless there is a lot of sludge in the system.
So if you want to protect your expensive appliance, we always recommend a power flush to be sure all debris and sludge is removed from the system. The central heating system will always benefit from having clean water circulating inside. This cost may be expensive when added to the installation and the boiler cost, but in the long run it's cheaper and an added peace of mind.