Mr Ellis gave presentations at DARD events at which the RHI scheme was promoted.

In one presentation he used the example of a particular poultry farm in County Tyrone that had two 99kW biomass boilers and four 60kW boilers installed in June and September of 2013.

Mr Lunny shows a slide used by Mr Ellis that demonstrates the boilers were "on target to generate more than £23,000 in RHI tariff income per annum".

Image source, RHI Inquiry

The time to recoup the cost of the heating system is shown as less than three years.

"You weren't making a secret of what you knew about the very short payback periods?" asks Mr Lunny.

Mr Ellis agrees, saying that DARD felt this farm was a particularly good example of how a participant had learned how to use the scheme most effectively.

Mr Ellis had started work on biomass in 2006, which as inquiry panellist Dr Keith MacLean was "long before [the RHI scheme] was a twinkle in anybody's eye".

The witness explained earlier that his role at DARD was included overseeing the development of renewable heating systems at CAFRE colleges.

Heating a horticulture glasshouse at the Greenmount campus using oil was costing £70,000 a year - a "pretty horrendous" sum, says Mr Ellis.

That prompted the college to opt for biomass system, and more followed to replace oil boilers at buildings at the various campuses around Northern Ireland.

Image source, CAFRE

Mr Lunny takes the witness on to DARD's involvement in the RHI scheme.

It was promoted at 58 renewable energy events run at CAFRE colleges from November 2011 to October 2015.

These were attended by 2,358 people.

Mr Ellis tells the inquiry that he noted there was potential for wasteful use of heat through the RHI scheme on some farms in order to collect cash but he believed there were deterrents in place.

He explains that mushroom houses could have continued to be heated after one crop had been harvested and before another was added to the house.

Image source, Getty Images

The same could have applied to poultry units when some chickens had been moved out and before more were moved in, he adds.

But he says that was a "risky" strategy because it could have been picked up in audits by those running the scheme, who would have had expertise about the various industries.

"If a specific unit was seen to be running temperatures over and above the norm for the industry, I thought that that would be picked up... and there would be questions as to why," he adds.

Many farmers, particularly those in producing mushrooms, were installing 99kW biomass boilers through the RHI scheme because of the layout of their farms, says Mr Ellis.

The 99kW boilers happened to be eligible for the most lucrative subsidy that was on offer.

Image source, Reuters

"Most mushroom houses suited having an individual 99kW boiler, so it made some sense that you could reduce your energy inefficiency by removing pipework connecting multiple houses," he says.

That would also protect against a major crop failure in the event of the breakdown of a single boiler serving multiple houses, Mr Ellis explains.

He agrees when Sir Patrick observes that it would have been useful if someone at DARD had told DETI about that, which then could've been taken into consideration when calculating the subsidies on offer.

DARD officials carried out calculations to find out how "lucrative", as they described it, the RHI scheme could be to potential claimants.

One calculation, based on a real-life example of a biomass boiler costing £25,000, offered £11,328 in subsidies in a year over the 20-year lifetime of the scheme.

Image source, Getty Images

That meant the cost of the heating system could be recouped in just over two years.

Sir Patrick wants to know why that calculation was not provided at DETI - Mr Ellis says it wasn't his role to pass it on and he doesn't know if that was ever done.

But he adds that he doesn't see any reason why it shouldn't have been shared with the other Northern Ireland Executive department.

Inquiry chair Sir Patrick Coghlin intervenes to ask Mr Ellis about his recollection that he spoke to Mr Hutchinson about the significant heat requirement in the mushroom farming industry.

"I don't recall anything significant being said back to me," the witness replies.

Image source, RHI Inquiry

Asked whether that caused him worry that DETI hadn't taken it into account, Mr Ellis says it didn't strike him.

"I can only says that it was probably a lack of experience, a lack of confidence on my part to take that forward," he says.

Mr Lunny probes the witness's knowledge of the scheme's tariffs at the time it opened.

He's particularly interested in the tariff for 20kW to 99 kW boilers, which formed the majority of installations under the scheme.

"We had no information or idea of how the tariffs had been developed or put together - we had no access to any information," says Mr Ellis.

He adds that he was never "fully aware of" the rate of return available through the scheme but he "made the assumption" that DETI had technical experts and economists who would've ensured the scheme was watertight.

He claims he told DETI's Mr Hutchinson that scheme claimants from the agricultural industry would've needed to use their heating systems for long periods of time - that was a crucial detail that hadn't been considered when setting up the initiative.

Asked about contacts with DETI officials at DARD promotional events, Mr Ellis says he was busy delivering lectures and giving guided tours.

"The way things worked out our paths didn't cross," he says.

Image source, RHI Inquiry

His main contact with DETI's Peter Hutchinson (above) - the main official who was working on the RHI scheme, almost single-handedly at times - was to clear up queries of a technical nature, often from farmers.

That was usually on the phone because he "found that sometimes emails weren't responded to".

Mr Ellis accepts that he would've been in a good position to give Stormont's Department of Enterprise, Trade and Investment (DETI) - which was running the RHI scheme - feedback, given his level of contact with the agriculture industry and his expertise of it.

As a renewable energy technologist, Mr Ellis promoted energy systems that could be applied across the agricultural sector, which was the main industry that benefitted from the scheme.

He also had responsibility for overseeing the installation of renewable energy systems - such a biomass boilers and wind turbines - at CAFRE campuses.

He tells the inquiry he would've been in frequent contact with installers of biomass boiler as well as fuel supplies as he would've invited them to promotional events that were run by DARD.

Image source, RHI Inquiry

Mr Lunny wants to explore a number of areas with Mr Ellis. They include:

• his work history
• his understanding of the RHI scheme
• the involvement of DARD's College of Agriculture, Food and Rural Enterprise (CAFRE) in promoting the scheme
• CAFRE's interactions with certain people regarding the scheme, including boiler suppliers and installers, the poultry producer Moy Park, the Ulster Farmers' Union and other government officials

Mr Ellis had been a civil servant from 1984 until last December, when he retired.

He spent much of his career at the Department of Agriculture and Rural Development (DARD) working in horticulture but during the time of the RHI scheme he was a renewable energy technologist.

Image source, RHI Inquiry

You can find his written witness statement of evidence on the inquiry's website, external.

He takes the oath and the inquiry counsel Donal Lunny begins asking the questions.

BBC News Northern Ireland

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, more than a year 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.

We're back!

After a couple of weeks off, the Renewable Heat Incentive (RHI) Inquiry returns with an intriguing week in store.

Image source, AFP

We'll be hearing from Democratic Unionist Party leader Arlene Foster and her former adviser Andrew Crawford over the next few days.

But first up is Cathal Ellis, who had an involvement in the RHI scheme during his time as an official at the Department of Agriculture and Rural Development.