We have just fitted an air source heat pump to a property with radiators fed by an old 8mm pipe system called a micro bore system.
We knew this would be a challenge as there was going to be plenty of sludge in the existing radiator system.
Many of the radiators were upsized to modern double convectors. We then used a device called a low loss header which helped with flow through the 8mm pipes. Combining this with 2 Wilo pumps we got our required flow.
A power flush later revealed all the sludge we were expecting. The switching on of the system brought comfort heating immediately to the granite walls of the 3 bedroom house in St. Austell, previously heated by a back boiler and coal fire.
Roll on winter and the RHI!