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, Pacemaker

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 the initiative 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 weren't introduced - temporary cuts have since 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.

The Renewable Heat Incentive scheme - or RHI for short - came to the fore of the Northern Ireland public's knowledge in late 2016.

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 people 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.

As the lime trees lining the mile-long Prince of Wales Avenue leading up to Stormont hill lose the last of their leaves and a cold wind whirls across the estate, you can tell the end is nigh.

Yes, we're into the final week of public hearings at the Renewable Heat Incentive Inquiry and it promises to be a captivating one.

You should settle back in your armchair with a bowl of popcorn for today's evidence, with two former Stormont ministers due to give their sides of the cash-for-ash story.

Image source, PA

The Democratic Unionist Party's (DUP) Simon Hamilton is first up - he was the economy minister when the RHI debacle blew up in December 2016.

And later we'll hear from Máirtín Ó Muilleoir of Sinn Féin - he was the finance minister at that same time.

You can watch it on our live stream that starts here shortly and we'll keep you updated with a text commentary on the best bits.