Two groups of people will be eligible;
All pensioners who get Pension Credit (Guarantee Credit)
People on most means-tested benefits (including Universal Credit, Income Support, Housing Benefit, Pension Credit (Savings Credit), and some people on Tax Credits) who also meet a High Energy Costs test.

This High Energy Costs test is new. The government will work out from the floor area of your house, its age and the property type whether you are likely to have high heating bills. This will be done using data from the Valuations Office Agency (VOA), which is used to set council tax bands.
Because the government is working out who is eligible, you no longer have to apply for the WHD if you live in England or Wales. The government should tell the energy suppliers who should be getting the WHD automatically.
It’s thought that the VOA holds the relevant data for 98% of housing in England and Wales. Where it doesn’t people will be contacted. Ofgem says:
If you are eligible, you should receive a letter from Government in November and December. In most cases, you will not need to do anything. Some people may be asked to contact a government helpline to confirm their eligibility.
The new Warm Home Discount scheme in Scotland
VOA data is not available in Scotland, so the government can’t decide automatically who will get the WHD. The good news though is that the criteria will now be the same across all suppliers and more people will get this than last year.
The application process for the WHD in Scotland will work in a similar way to the previous Scheme:
Pensioners getting Pension Credit (Guarantee Credit) will get the WHD automatically and will not have to apply
If you are of working age and on means-tested benefits, you will have to apply, probably in November. Details of how to do this have not yet been announced.