Civil servants starting new roles in a different Stormont department should read the relevant legislation as one of the "first things" they do, advises Mr Stewart.

There's a need for them to take responsibility for familiarising themselves with the "statutory duties that operate within their particular sphere of influence".

Image source, Getty Images

He says that when he started working at the education department he took two weeks out to read through a "two-feet high" pile of legislation related to the work.

Asked if he read the RHI scheme legislation when he joined DETI, he admits: "Not on day one... but eventually yes."

He admits he can't remember if he noticed the statutory duty and is "guilty of the same sin that I've attributed to others".

Image source, RHI Inquiry

In his witness statement, Mr Stewart accepts that there were governance and oversight failures on the part of DETI in the running of the RHI scheme.

The department took a "hands-off approach" and had an over-reliance on the scheme administrator Ofgem doing the right thing.

He admits to the inquiry that it was "absolutely" and "entirely my responsibility".

The decision to prioritise the domestic RHI scheme - however it was made - meant that proposals to introduce cost controls to the non-domestic scheme were put on hold.

Mr Stewart accepts that it would've been possible to set aside the expansion of the non-domestic scheme but to have proceeded with the cost control element to apply to both initiatives.

Image source, Getty Images

But he says he's not seen any evidence that it was ever considered.

"For whatever reason the connection wasn't made that cost controls are necessary and need to be equally applicable to both schemes," he says.

Both schemes were funded out of the same pot and protecting the budget should've been a priority, he adds.

After the RHI scheme - which was for non-domestic claimants - was open, DETI made moves in 2014 to follow it up with a similar initiative for domestic users.

Mr Stewart's written evidence to the inquiry is that it was a "high ministerial priority" and was "already well established" as that before he arrived in the department in August that year.

But the then DETI minister Arlene Foster (above) did not make a decision to push ahead with opening the domestic scheme instead of making crucial changes to the non-domestic one.

Image source, PA

Sir Patrick says the decision seems to "have appeared by itself" from somewhere within DETI, adding: "It doesn't reflect well on clear departmental responsibility or decision making."

The witness says he'd been told "as a matter of fact" by energy boss John Mills that it was the minister who'd made the call and he now accepts that he doesn't know who made the decision.

Mr Mills has told the inquiry that his claim that it was Mrs Foster's decision was "completely incorrect".

Communication between DETI staff involved in the RHI scheme "wasn't what it might have been", accepts Mr Stewart.

He says the main cause of that was "pressure" and the "sheer workload" that staff were having to handle.

Image source, Getty Images

The department had "become very reactive", particularly in the summer of 2015 when staff began a scramble to get a rein on the scheme's budget.

"It wasn't a case of proactively communicating for effective outcomes - it was responding in the quickest way possible, which was often email - often not the most effective."

One of the victims of DETI's lack of resources was the energy division director John Mills (below).

Mr Stewart says he hadn't encountered anything like the volume or complexity of work expected in a post at that level anywhere in the civil service.

He also says there was a "very high level of political and stakeholder interest" that Mr Mills had to handle.

Image source, RHI Inquiry

The witness agrees that there was too much responsibility put on Mr Mills and it "struck me as profoundly significant" when he asked for an early release from the post.

Sir Patrick notes that Mr Mills had been assured that he would have to work fewer hours than his predecessor - that was due to family reasons.

"It doesn't reflect well on the recruiter of John Mills," says the chairman.

Mr Stewart agrees with a suggestion made by David Sterling, the head of the Northern Ireland Civil Service, that the RHI scheme was a "project too far".

He says he remembers speaking to a counterpart in the civil service in the Republic of Ireland who asked him how many energy specialists that DETI had.

Image source, Getty Images

"He was very surprised when I replied: 'None'. His advice to me was: 'You need to do something about that.'"

DETI has had a "deficiency" in energy expertise and has not been able to address it, says Mr Stewart.

Image source, Getty Images

Mr Stewart admits that when senior DETI managers were "sitting in the wreckage of RHI" they concluded that inadequate resources within the department "played a part" in the disaster.

Many of the civil service witnesses have told the inquiry that DETI was short of the amount of staff that were needed to do the work that was necessary on the scheme.

And that's something that Mr Stewart says he's noted and agrees is correct.

Mr Stewart says the "can-do" attitude exhibited by DETI staff was a positive feature but he sounds a note of caution for the time whenever devolved government returns in Northern Ireland.

"We need a very honest conversation with the incoming executive to say: 'We need to do fewer things and we need to do them better'," he says.

He tells the inquiry that in the past there was a culture at Stormont of attempting to replicate the work of Whitehall departments, even though the civil service had much smaller teams to do that.

"We've got away that - if you want to keep want to keep doing that it's only a matter of time before we have another RHI," he adds.

A civil service scheme that gave staff the chance to apply for redundancy left Stormont departments without some of its most "senior people... and those with the greatest experience", says Mr Stewart.

"We didn't know who was going to leave until they left," he adds, saying that those who did depart "weren't necessarily the people that we wanted to leave".

Image source, RHI Inquiry

People with long experience and vast knowledge and expertise were allowed to go because they were "closest to retirement and therefore cost least when it came to compensation".

He says lessons need to be learned from the voluntary exit scheme.

It was a "bad thing" for the three key staff working on the RHI scheme to leave DETI within six months of each other, admits Mr Stewart.

The department's energy team boss Fiona Hepper moved on in November 2014, while the two most hands-on staff members - Joanne McCutcheon and Peter Hutchinson - were allowed to depart in spring 2015.

Image source, Getty Images

Asked if it should've been allowed to happen, Mr Stewart says: "Looking at the consequences now, all concerned wish that we'd done it differently."

"It was clearly sub-optimal to have that amount of staff turnover in that time."

But he adds that even had there been better succession planning there was a "very, very short supply" of experienced policy staff to step into the vacant roles.

Energy has been "by far the most complex, technical and difficult" subject that Mr Stewart has ever had to deal with during his time in the civil service.

One thing that led to the problems in the RHI scheme was that the three key staff who were working on it left DETI within the space of a few months.

Those who replaced them were "inexperienced and ill-prepared" for what they were taking on, as the inquiry panellist Dr Keith MacLean puts it.

Image source, Getty Images

One even told the inquiry that he knew "absolutely nothing" about the scheme when he started working on it.

Mr Stewart tells the inquiry that staff arriving in the department to work on energy matters, regardless of their experience elsewhere in the civil service, need to "get to grips with the technical complexities".

"It's not enough simply to transfer the information - you have to have a process that ensures that's converted into understanding."

Mr Stewart did not receive a formal handover from his predecessor when became DETI's deputy permanent secretary.

He believes that was standard practice across the Northern Ireland Civil Service - any handover was typically "ad hoc".

Inquiry chair Sir Patrick Coghlin picks up on that: "These were people who were generalists would not necessarily have any experience in the area to which they were being transferred."

Image source, RHI Inquiry

Mr Stewart accepts that it's not satisfactory and says it's persisted "for as long as I can remember".

"The effect on any continuity of knowledge and experience must have been disastrous," observes Sir Patrick.

The witness says it was certainly disastrous in a case like the RHI scheme, in which there had been a considerable turnover of staff in a short time.

Mr Stewart says he had limited involvement in the RHI scheme in much of his first year at DETI, which he admits "looks very surprising now" given that it fell under his brief.

But he explains that there was a "lack of visibility of the issue generally in the department".

The "first day brief" he received when he joined the department listed 53 policy issues that were "worthy of attention" - of those there were 17 energy matters, of which the RHI scheme was the 17th.

He says there "simply wasn't any visibility of what we now know to be the very serious flaws and issues in the scheme and very significant the risks associated with it".

Chris Stewart, a vastly experienced civil servant who's worked across several Stormont departments, affirms that he'll tell the truth in his evidence and he settles down for the first of two days at the inquiry.

Since 2014 he's been the deputy permanent secretary at the Department for the Economy - formerly DETI - which set up and ran the RHI scheme.

Image source, RHI Inquiry

Over the course of today and tomorrow we're likely to hear some intriguing evidence, given that he had significant involvement in dealing with the scheme as it became clear it was running out of control.

His first statement has been published on the inquiry's website, external and he's also provided a second one, given to the inquiry just last week, external.

The inquiry's senior counsel David Scoffield QC will be posing the questions.

BBC News NI

Finance officials should have taken more care to establish how the budget of the RHI scheme worked, the inquiry was told.

They were too "hands-off", accepting assurances from others when they should have done more to interrogate finances.

Image source, RHI Inquiry

They failed to "join up the dots" because there was no handover process for staff taking up responsibility.

Emer Morelli (above), a senior official in the Department of Finance, admitted assurances were taken "at face value".

BBC News NI

An independent inquiry into the RHI scandal was established in January last year by the then finance minister Máirtín Ó Muilleoir.

He ordered it in the wake of the huge public concern and what was then a developing political crisis surrounding the scheme.

The RHI Inquiry began in November and Sir Patrick Coghlin (below), a retired Court of Appeal judge, is its chair and has been given full control over how it will operate.

Image source, Pacemaker

It will look at:

• the design and introduction of the RHI scheme
• the scheme's initial operation, administration, promotion and supervision
• the introduction of revised subsidies and a usage cap for new scheme claimants in 2015
• the scheme's closure

For more information on the RHI Inquiry, you can read our handy Q&A.

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 Democratic Unionist Party (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, 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.

The most recent estimate for the overspend was set at £700m, if permanent cost controls aren't introduced.

The massive overspend bill 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... 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.