The deciding factor is evidence of sludge or restricted circulation, not simply the age of the heating system.
In Belfast, this is usually a circulation and water-quality question. A power flush can remove iron-oxide sludge and debris from a wet central-heating system, but the symptoms need diagnosing first because trapped air, faulty valves, pump problems and poor balancing can produce similar complaints.
For the question “Can a power flush protect a replacement boiler in Belfast”, an engineer should compare radiator temperature patterns, system water condition, pump performance and circulation around the full circuit. Cold bottoms and dark system water point more toward sludge; cold tops are more commonly trapped air. A flush will not repair a failed valve, undersized pipework or a mechanical boiler fault, so it should be recommended for evidence of contamination rather than automatically.
After a successful clean, the system should be rinsed, protected with inhibitor and balanced correctly. Where appropriate, a magnetic filter can help capture circulating debris. BMK can assess the heating system in Belfast first and only recommend power flushing where the symptoms and water condition justify it.