If you receive the correct combination of income-related benefits, including tax credits, pension guarantee credit, income support, ESA or JSA, then you could have your old boiler replaced with a brand new A-rated boiler for little, to no cost.
As well as receiving the correct combination of benefits, your current gas, oil or LPG boiler must be at least over 5 years old. If it is less than 5 years old, the scheme makes an assumption that is is more cost effective to repair your old boiler, than it is to have it replaced on the scheme.
Some of the benefits mean that you will qualify for a boiler grant automatically, with no other requirements. With some others you may need to have an extra qualifying component such as being responsible for a child under 16, be of a certain age etc. You can view the full criteria here.