Dirty systems will typically mean that radiators don't get warm or take a long time to do so; boilers and pipes can be noisy, pumps can perform poorly, valves can get stuck and energy can be wasted, so bills will be increased.
1 bottle of Salamander CG3 Cleaner will treat around 10 single panel radiators.
If a system is particularly dirty, doubling the dosage will increase the concentration and mean the clean can be carried out quicker.
There is no set rule to determine this.
An average UK home's central heating system (3 bedrooms, 8-10 single panel radiators) contains around 100L of water.
Without protection, a central heating system will start to corrode from the inside. Debris such as corrosion deposits and limescale will start to build up. This will stop system water flowing effectively as well as stopping heat being distributed equally in a radiator. Once these deposits get into components, they can rapidly bring about failure of boilers and pumps, costing lots to fix and repair.
If you are unsure or not confident in doing this, then we ALWAYS recommend that you seek a competent professional (plumber or gas engineer), to do this for you.
The Gas Safe Register contains a list of all professionals who are licensed to carry out work on central heating systems. Their website is https://www.gassaferegister.co.uk.
BS7593 is the code of practice preparation, commissioning and maintenance of domestic central heating and cooling water systems. This is a guideline for installers to follow when installing and servicing central heating systems.