IT'S the scandal often cited as the cause of Stormont's collapse in January, yet we still don't know for sure where the blame lies for the green energy scheme that saw public money literally go up in smoke.
Launched in 2012, the Non-Domestic Renewable Heat Incentive was ostensibly an effort to encourage businesses to use renewable energy rather than burn fossil fuel.
A corresponding scheme operated in Britain over a similar period with little or no controversy, but the regionally tailored RHI - developed under the watch of the then Minister for Enterprise, Trade and Investment, Arlene Foster - was badly flawed.
Claimants of Northern Ireland's bespoke scheme, for which take-up was initially sluggish, were paid £1.60 for every £1 worth of wood pellets they burned.
There was also no limit on the amount that could be claimed, which meant businesses were effectively incentivised to burn as much woodchip as they liked with an assurance that they'd be reimbursed for their fuel plus a premium.
It was said that an one case, a single farmer stood to earn £1m over the 20-year duration of the scheme, while the scale of payments threatened to cost the cash-strapped Stormont executive up to £500m over the same period.
When it became apparent that the British Treasury would not pick up the entire £1 billion-plus bill for the Northern Ireland scheme, cost controls were imposed in the autumn of 2015.
However, before they became effective there was a huge spike in applications, suggesting word got out that the RHI's largesse was about to be curtailed.
It was closed completely to new applicants in February 2016 by Mrs Foster's successor Jonathan Bell.
In an interview with The Irish News last October, Mrs Foster defended her oversight of the scheme, arguing that it had been developed by her officials and she could not be expected to be across "every jot and tittle" of her department's business.
However, as public outrage grew and political pressure increased, the DUP leader said she was "sorry that the initial scheme did not contain cost control measures and that there were fundamental flaws in its design".
Subsequently, Mrs Foster's former special adviser Andrew Crawford, whose brother was a poultry farmer, resigned after a senior civil said he had pressed to delay RHI cost controls.
Mr Crawford said an inquiry would show that he acted "with complete integrity" at all times.
After some initial reluctance, the then finance minister Máirtín Ó Muilleoir succumbed to growing pressure and announced the establishment of a public inquiry into RHI in January.
The inquiry team, headed by retired judge Sir Patrick Coghlin, began its work in February.
It has since been engaged in an evidence gathering process in preparation for the oral hearings, issuing more than 500 statutory notices to people and organisations compelling the production of documents and witness statements.
While it was initially suggested that the probe could report within six months, the chairman said in April that it is "simply not possible" to set a timeframe.
Its remit is to focus on the design and implementation of the botched scheme; how it was administered, promoted and supervised; the circumstances around the imposition of costs controls; and the scheme's subsequent suspension.
Some nine months after its work began, the inquiry hearings finally begin tomorrow at Stormont.
The oral evidence sessions will run every day from 10.15am - 4.30pm, with an hour for lunch. On Fridays, however, they will wind up at 1pm.
This week's proceedings are expected to taken up almost entirely by an opening scene-setting statement from the senior counsel to the inquiry, David Scoffield QC.
The first witness, Alison Clydesdale from the Department of the Economy, is scheduled to give evidence on Tuesday November 28.
Between now and Christmas, it's entirely civil servants and consultants from Cambridge Economic Policy Associates (Cepa) who will give evidence.
Arguably the most significant witness before the festive break is Fiona Hepper, who was the director of the renewable energy team in Mrs Foster's department.
The 'star' witnesses – including Arlene Foster, Jonathan Bell and Andrew Crawford – are not expected to take the stand until well into the new year.
They are among 30-odd individuals or organisations that have been granted 'enhanced participatory rights', which entitles them legal representation at hearings and potentially for their lawyer to scrutinise the evidence of other witnesses. They are also privy to relevant parts of the inquiry’s witness statement bundle.
The sessions in the Parliament Buildings' Senate Chamber will be open to the public during oral hearings, while the proceedings will also be streamed on the inquiry website – www.rhiinquiry.org – where daily transcripts and evidence will also be displayed. On some occasions, the proceedings will be closed to the public for legal reasons.
The cost of the inquiry is expected to be significant.
