How does the water in the central heating system get dirty?

Essentially, it's the reaction between the different metals used within the system. This causes a build up of ferrox oxide. (As previously stated, rust within the radiators, and heat exchanger, if cast iron). Power flushing could well help, though I would give careful consideration to this process, on an old system ! Pipe-work and fittings can become compromised and as a result leaks may well appears! Try a basic chemical clean, run hot, then a thorough cold flush. Add inhibitor. If you can afford the product, have a magnetic filter added! Adrian Southsea plumbing

The dirty water in your heating system is called magnetite. It is essentially your radiators rusting and breaking down internally. The water goes reddish to begin with, but over time turns black. Your heating system should have had a preservative called inhibitor added to it when installed, every few years, or if any alteration has been made to the heating system (whichever comes first). This inhibitor stops the corrosion happening in the system. Unfortunately, once the heating system is sludged up (with magnetite) the only way to effectively remove it is to powerflush the heating system. Depending on the size of the heating pipework (i.e 8-10mm or 15mm to each radiator) and number or radiators will depend on price, but expect to pay a days labour for an engineer to flush it with a powerflushing machine. Hope this helps