DIYbanter - Central Heating removing sludge and changing radiator

Hi,

I have over 25 years old central heating with Glow Worm Boiler. It is
working fine and there are two water pumps. I intend cleaning the
radiators and pipes and removing sludge from the system.

What treatment should I give to water circulating in the system? How
many days should I leave the chemicals to work?

There is some rust showing on some radiators and I would like to change
them with the new. They are old British standard threads and now we
have metric threads. Any problem it would cause?

House is a large 4 bedroom house with kitchen, garage, dining and bath
room. I intend putting power shower in the bathroom.

Your advice would be much appreciated. I shall be visiting my local
stores such as B&Q stores for assistance and Graham central heating
expert.

Rajinder