You know its time to bleed your boiler system if some of your radiators are cool but some are hot. Once you verify that your boiler is working, the cool radiators are most likely caused by excessive air in the system.
The purpose of bleeding your radiators is to release the air trapped in them. Once air gets in the system, it will get trapped in the radiators. By releasing (or bleeding) the air out of the radiator you make sure your radiators are able to fully heat-up.
There are different opinions on radiator bleed order. We usually start at the highest level of your home (or furthest away from the boiler) and work your way towards boiler. Remember, each radiator is independent, so you only need to bleed the radiators that don’t get hot.