When the scale of the overspend emerged, public and political concern rocketed.

As the minister in charge of the Stormont department that set up the RHI scheme, the DUP leader Arlene Foster faced calls to resign from her role as Northern Ireland's first minister in December 2016.

Image source, PA

She resisted, and Sinn Féin's Martin McGuinness then quit as deputy first minister in protest at the DUP's handling of what had by then become a full-blown political crisis.

That move brought about the collapse of the Northern Ireland Executive. Now, well beyond a year-and-a-half on from that, Northern Ireland remains without a devolved administration.

You can find much more detail on the RHI scheme in our need-to-know guide.

The budget of the RHI scheme ran out of control because of critical flaws in the way it was set up.

Claimants could effectively earn more money the more fuel they burned because the subsidies on offer for renewable fuels were far greater than the cost of the fuels themselves.

At one point the estimate for the overspend was set at £700m if permanent cost controls aren't introduced - temporary cuts have pulled the budget back on track for now.

Whatever the scale of the bill, it will have to be picked up by the Northern Ireland taxpayer.

It's been a while so let's go back to basics...

The Renewable Heat Incentive scheme - or RHI for short - came to the fore of the Northern Ireland public's knowledge in late-2016... and the fallout from the scandal attached to it is still being felt in the region's politics today.

Image source, Getty Images

The scheme was set up by the Northern Ireland Executive in 2012, as a way of encouraging businesses to switch from using fossil fuels to renewable sources for generating their heat.

Those who signed up were offered financial incentives to buy new heating systems and the fuel to run them.

First of all, an admission - us cash-for-ash nerds have been missing this...

The Renewable Heat Incentive Inquiry resumes at Stormont shortly after a two-month summer break and there are a fascinating few weeks ahead as it steps up a gear in its public evidence hearings.

Image source, PA

The witness roster includes some big names, with former Democratic Unionist Party (DUP) minister Jonathan Bell this week and party leader Arlene Foster expected back in the near future so get your popcorn ready.

Today's session sees the former Department of Enterprise, Trade and Investment (DETI) permanent secretary Dr Andrew McCormick take the hotseat.

The questions begin at 14:00 BST and we'll have a live stream and a text commentary of the best bits.