Boiler owners facing reduced Renewable Heat Incentive payments following a court ruling are to have their appeal heard in full next year, it was confirmed on Friday.
Senior judges had raised the possibility of a narrow re-examination of the case focused on the flawed scheme’s cost over just one year.
But proceedings will instead be widened out to all areas of dispute about the potential bill for a 20-year period.
Lord Chief Justice Sir Declan Morgan listed the challenge for a two-day hearing in February.
He told parties in the Court of Appeal: “Both of you, despite encouragement you may have got from the bench, are quite content to have a go at everything.
“It’s your case, not ours, therefore you should be allowed to have a go at everything and we (will) just hear the appeal.”
More than 500 members of the Renewable Heat Association NI Ltd are challenging the Department of the Economy for cutting their tariff rates.
They claim it was an unlawful step taken against operators with a cast-iron 20 year guaranteed rate of return on their investments.
Set up to encourage businesses and other non-domestic users to move to green energy systems, the scheme was plunged into controversy after the potential cost to taxpayers emerged.
With operators legitimately able to earn more cash the more fuel they burned, the bill was projected at up to £490 million - a figure fiercely disputed by the Association.
According to its lawyers the overspend could end up being as low as £60m.
However, the Department responded by claiming the cost could actually have reached £700m without the new cost controls.
The debacle surrounding the initiative led to the collapse of Stormont’s power-sharing administration, and the establishment of an ongoing public inquiry chaired by retired judge Sir Patrick Coghlin.
Last year former Economy Minister Simon Hamilton set out revised 2017 RHI Regulations at the centre of the legal challenge.
The Association’s lawyers claim the reduced rates was an unlawful move against boiler owners who had payments guaranteed for 20 years.
But the Department countered that it had legal authority to bring in revised tariffs essential to stop public money “haemorrhaging” for at least a decade.
In December a High Court judge identified a clash between the private interests of the boiler owners and the public interest asserted by the Department.
He ruled that introducing the capped tariffs did not represent an abuse of power, and concluded there was a compelling case for the reduced payments.
That determination will now be subjected to a further full scrutiny by the Court of Appeal.