Northern Ireland’s cash-for-ash scandal started almost a decade ago as a way to save the planet, veered into greed, cronyism and dysfunction, and will now reach a denouement in the place where it all started: Stormont.

Sir Patrick Coghlin, chairman of the public inquiry into the region’s bungled green energy scheme, is due to publish his report on Friday at the grand estate outside Belfast that hosts the devolved government’s assembly and executive.

The report is expected to deliver a damning verdict on the politicians and civil servants implicated in a saga of incompetence and apparent sleaze at the heart of Stormont, a scandal that toppled a power-sharing government in 2017 and paralysed politics for three years.

The renewable heat initiative (RHI) started in 2012 as a well-intentioned UK-wide effort to reduce carbon emissions by switching from fossil fuels to renewable sources.

But Northern Ireland lifted cost controls, turning wood pellet boilers into a de facto licence to print money – £700m over 20 years, according to one early estimate – in the mistaken belief British taxpayers would pick up the tab.

“Some senior figures thought it was legitimate to milk the treasury teat even when a perverse scheme such as cash-for-ash was running out of control,” said Sam McBride, author of Burned: The Inside Story of the Cash-for-Ash Scandal.

“The exposure of that culture will leave a long legacy. Like the boy who cried wolf, when in years to come Stormont goes to the treasury with a genuine claim for increased funding – especially around a health service which in places is collapsing – its claims will be met with deep scepticism.”

Coghlin, a retired court of appeal judge, chaired hearings that lasted over a year, interviewed dozens of witnesses and amassed more than a million documents.

The long-awaited report is expected to criticise Arlene Foster, the first minister and head of the Democratic Unionist party (DUP), because she approved the scheme and ignored warnings about perverse incentives and cost overruns.

Foster and other DUP ministers barely read important documents, if at all; civil servants cut corners and ignored signs of problems; unelected special advisers essentially ran Stormont and in some cases helped relatives access – arguably loot – public funds.

“I’m accountable but I’m not responsible,” Foster told the inquiry in 2018 when asked about a special adviser’s possible criminal conduct.

The scandal’s origins lie in a 2012 decision by Northern Ireland’s Department of Enterprise, Trade and Investment (since renamed the Department for the Economy), then run by Foster, to make the subsidy more valuable than the cost of wood pellets used to heat boilers. Nor did it cap the total subsidy.

Word spread that boilers meant profits, and the more boilers and the more you burned, the richer you became. The result was a scramble to install boilers and run them 24/7. Farmers, went the joke, were using oven gloves to open sheds.

A whistleblower reported abuses to Stormont officials in 2013 and 2014, to no avail. The scheme continued and was extended until the belated realisation that the UK treasury would not foot the bill.

The debacle prompted Sinn Féin to pull the plug on its power-sharing administration with the DUP, creating a damaging three-year political vacuum that ended in January when political parties agreed to restore the assembly and executive.

Drastic revisions to the scheme’s terms have turned an envisaged overspend of hundreds of millions of pounds into an underspend.

The pendulum may have swung too far back. Many RHI claimants say they signed up in good faith and borrowed money to install boilers, assuming the scheme was properly managed and would benefit them and the environment, only to find themselves branded unethical and greedy – and then financially marooned when tariffs were slashed.

A farmer in county Down who spoke on condition of anonymity said he borrowed £75,000 on the basis his boiler would generate about £6,500 per year, but that had dwindled to £2,000. “More than likely we will have to sell some of our other assets to try and pay off that RHI loan,” he said. “We have been scapegoats in all this.”

A county Antrim farmer said he was receiving just 10% of the original tariff, a fraction of what farmers in the rest of the UK received, miring him in debt. “I’m stressed financially, emotionally, psychologically.”