Here are some warning signs to look out for, they could be a result of sludge/corrosion build up in the heating system:
Cold spots on the radiator
Radiators require frequent bleeding
It takes a long time for the radiators to warm up
A noisy pump or boiler
The pump fails
There are many benefits to having your heating system flushed. If the pipework and radiators are clogged with rust and corrosion then this will put great strain on the pump and even the boiler itself. After flushing, you should notice that the radiators produce heat more evenly, they warm up much quicker and in some cases the whole system will be quieter.
Most boiler manufacturers advise homeowners to have their system flushed before installing a new boiler. The new boiler won’t work efficiently if the pipework is clogged. You may also find that any guarantee is invalidated if the cause of any failure is due to an excessive build up of rust and corrosion.
This depends on the size of the system and how clogged it is. Most systems can be fully flushed in a day.