Crucial questions on the unfolding row answered
Q. What exactly is the Renewable Heat Incentive (RHI) and what is its aim?
A. A government subsidy scheme established in 2012 by then economy minister Arlene Foster to encourage businesses to switch from burning fossil fuels to greener sources like wood pellets.
Q. So what went wrong? Why did the scheme run into difficulties?
A. It was supposed to pay a proportion of fuel costs, but tariffs were set too high, meaning that for every £1 participants spent on fuel, they received £1.60 in subsidy payments. This created a "burn to earn" incentive - with applicants able to access free heat and make a sizeable profit while doing so. Claims of widespread abuse include a farmer allegedly set to pocket around £1 million in the next 20 years for heating an empty shed.
Cost controls to prevent such an overspend were incorporated into a similar RHI scheme in Great Britain, but were absent in the Northern Ireland model.
The total cost in Northern Ireland is projected to be more than £1 billion over the next 20 years. While the Treasury will pick up the bill for most of it, an expected overspend of around £400 million will not be covered and will come out of the block grant for public services in Northern Ireland.
Q. What happened when efforts to control costs in the scheme were announced?
A. A move to introduce a tiered payment scheme in 2015 was met by a flurry of applicants trying to get on to the old system before the date set for the changes. Almost 1,000 applied in three months - around the same number from over the previous three years. The deluge of new applications is a key factor in the massive overspend, and opposition politicians have demanded answers around the spike.
When the cost controls were introduced the scheme limped on for a further three months but, after the Treasury made clear it would not pick up the overspend bill, it was closed for good in February.
Q. What was Mrs Foster's role in the Renewable Heat Incentive scheme?
A. As economy minister, she was in charge of the department that developed the RHI from its inception in 2012 to 2015. Much of the scrutiny on Mrs Foster has focused on how she responded to concerns raised by a whistleblower during that time.
There was a flurry of claims last week about whether the individual raised concerns directly to Mrs Foster, or if she only outlined them after the DUP stalwart passed her on to meet officials.
The DUP published an email sent from the whistleblower to Mrs Foster last week that made no mention of her RHI concerns - the party cited it to demand an apology from those who said she should have done more. However, another email has since emerged, sent directly to Mrs Foster in 2013, that raised specific concerns about the scheme.
Q. Why has Mrs Foster's successor in the department been making headlines?
A. After she left the economy portfolio, DUP MLA Jonathan Bell took on oversight of the scheme and ultimately closed it earlier this year. He broke party ranks last week to level a series of allegations against Mrs Foster and party advisers in an explosive TV interview.
He claimed he tried to pull the shutters down sooner but was dissuaded by Mrs Foster and other DUP advisers. Mrs Foster disputed his account and said she acted to close it sooner.
Q. Why could the dispute threaten the power-sharing institutions at Stormont?
A. Northern Ireland is governed by a mandatory coalition between the two largest parties, the DUP and Sinn Fein. If either was to withdraw from the administration it would fold and fresh Assembly elections would be triggered.
Sinn Fein Deputy First Minister Martin McGuinness has warned of "grave consequences" if Mrs Foster does not temporarily step aside to allow a fully independent inquiry into the matter. He withdrew his approval for a statement made by Mrs Foster to the Assembly on Monday. The Sinn Fein veteran's move prompted members from all parties but the DUP to question the validity of Mrs Foster's appearance and they walked out en masse from the chamber.
Mr McGuinness has insisted he remains focused on securing a positive resolution, but unless the parties reach an accommodation on the way forward, the future of their partnership will come under intense pressure.
Compiled by Michael McHugh and David Young, Press Association