Mr Lunny is brings the inquiry's attention to an email from John Martin of heating contractor company Green Energy Technology.

The email, sent in March 2014, talks about how the company is very busy installing 99 kilowatt container units.

It says: "We have five 99 kilowatt container units to be completed by April and another six later this summer all on pellets.

"It can be difficult to find a site where chips can be delivered and stored easily.

"RHI = Hectic Industry!!"

Mr Lunny suggests that if Mr Johnston's aim was to incentivise the growth of willow, but that he was hearing that it was lots of small boilers burning pellets, he should have brought this up with DETI.

"It would have been well known within the industry that it was all 99 kW boilers going in so they would have been very clear there was another way to do this," says Mr Johnston.

"But whether the industry, or whether the department felt that that was the right way to go, you know, this is me coming up with a very personal view of how ... biomass can be supplied and how Northern Ireland can meet renewable heat targets."

In the summer of 2015, Mr Johnston's friend Gavin - mentioned earlier - asked him if there were going to be changes in October that year.

DETI had realised by that stage that the cost of the scheme was escalating and it planned to introduce cost controls in the autumn to mitigate against the increase.

Image source, Getty Images

Mr Johnston told Gavin what he understood the changes to be and said it was because the poultry sector had been "exploiting" the scheme - Mr Lunny says that use of that term suggests he was aware of "abuse" of the scheme.

The witness disagrees, saying he'd been told that the budget for the scheme had been used up, mostly because so many poultry farmers were claiming from it.

Image source, RHI Inquiry

Inquiry panellist Dr Keith MacLean (above) asks Mr Johnston tough questions about what prompts him to ever bring up issues to department officials.

Mr Johnston repeats his argument that the RHI scheme was not his number one concern, since he had only been working in Afbi for "a short period of time" when he attended the event at which the scheme's overgenerosity was discussed.

"If I had known at the time that I would have been sitting here I would have paid a lot more attention to these presentations, but it wasn’t my focus at these events," says Mr Johnston.

Mr Johnston says "it's a little unfair" to suggest he was best placed to inform Deti about the over-generous RHI scheme, as detailed at the June 2013 presentation he attended.

Mr Lunny suggests that given DETI officials were not at the event Mr Johnston should have told them of what he had seen that exposed the flaw in the scheme.

"I honestly would've expected that they would've known or the information would've come [from] elsewhere," replies the witness.

He says he "never considered" that there may have been any expectation on him to inform Deti about how the initiative was operating.

In a presentation at the June 2013 RHI scheme event, one biomass boiler installer - Connel McMullan of the firm Alternative Heat - outlined the generous returns and the short payback times that the initiative offered.

He gave an hypothetical example of a 99kW boiler running for 4,000 hours a year - that would generate an income of £3,529 a year.

He also used case studies, from a hotel to a poultry house, that showed the payback time for the boilers was very short - about three years.

Image source, Getty Images

Inquiry barrister Mr Lunny says that information "would've been useful for DETI to know" and given that Mr Johnston was at the event he suggests that those details should've been passed to the department.

"This would not have been my focus," says the witness, saying he believes the scheme was meant to be generous to an extent in order to generate initial uptake.

"I wouldn't have come back from this thinking: 'I've got a lot of messages to go back to DETI with.'"

The RHI scheme was the subject of an event held by the energy group Action Renewables in Belfast in June 2013.

Dr Edmund Ward (below), from the scheme's administrator Ofgem, officials is being discussed sent an email - titled 'FYI: Brief feedback from RHI including feedback on Deti (very negative) and Ofgem (generally very positive)' - was sent the day after the event.

Image source, RHI Inquiry

Towards the end of the email, he says that there were no speakers from Deti at the event - this was "heavily criticised" by speakers at the time, he adds.

Mr Johnston says he does not recall any criticism of Deti from the speakers.

"I would normally be in and out [during the event]," he says.

In July 2012, Mr Johnston shared a document that outlined the subsidies on offer from the RHI scheme - which would open in Novembr that year - with a man called Gavin, who appears to have been considering installing a biomass boiler system at his premises.

Gavin responded by saying that the offer of 5.9 per kW of heat produced was "pretty good as my costings as coming in at about 4p".

Image source, RHI Inquiry

Mr Lunny says Mr Johnston would therefore have known that the scheme could give a claimant "free fuel and something on top of that".

He says to Mr Johnston: "All of that would've been known to you before you started your work with Afbi."

In March 2012, Mr Johnston gave a talk to the Ulster Farmers' Union about the RHI scheme, which was to be introduced in November that year.

Comparing two presentations Mr Johnston had given on renewables, one shows that a claimant could save £35,000 a year by installing a biomass boiler.

Image source, Getty Images

The payback for that boiler, without the RHI scheme, takes three-and-a-half years.

But in the March 2012 presentation the scheme is factored into the equation - applicants could earn £22,000 in annual RHI income and payback takes just two years.

Mr Johnston admits that this would give some a "very good return".

Mr Johnston exchanged WhatsApp messages about the RHI scheme in December 2016 with Mark Anderson, an academic specialising in renewable energy at Ulster University.

That was the month the scale of the scheme's vast overspend emerged to the public.

The men were discussing a suggestion that legislation could be introduced to retrospectively cut the subsidies on offer to mitigate the damage to the public purse.

Image source, Getty Images

In one message, Mr Johnston asks: "Can they do that - alter agreements which have been done in good faith based on the lucrative economics?"

Mr Lunny says he wants to examine Mr Johnston's understanding of just how lucrative the scheme could be during the time it was open to applicants.

Sir Patrick says that Mr Johnston's intention of promoting the use of willow chips as a fuel was being "frustrated" and the way the RHI scheme was set up was "not benefitting what you wanted to do".

Mr Johnston says he had expected it to be changed by Deti in the next phase of the scheme's development that would encourage greater use of larger boilers that could burn that fuel.

Image source, RHI Inquiry

The inquiry chair makes the point that phase two took much longer to implement that was planned.

The witness says he knows "things don't roll very quickly in government circles" but that "to me seemed OK".

Sir Patrick replies: "That's an answer you need to think about, particularly when the waste of public money is concerned."

Sir Patrick puts it to Mr Johnston that it was in his interest to see larger boilers - from 500kW to 999kW - installed because it was his intention to promote the use of willow chips as a fuel, rather than wood pellets.

He asks why he didn't press DETI to urge encouragement for the installation of larger boilers.

Image source, RHI Inquiry

Mr Johnston says he reckoned that his self-interest would have been less important than the department's target of getting Northern Ireland to produce 10% of its heat through renewable sources by 2020.

That target was one of the main reasons why the RHI scheme was set up in the first place.

Biomass boiler installers were regular attenders at the Afbi events and Mr Johnston says he had "great interest in trying to stay abreast of what was happening within the sector".

Many of the boilers installed through the RHI scheme were of the type and size - from 20kW to 99kW - that fell into the most lucrative subsidy rate and Mr Johnston is asked if he ever asked the installers why that was.

He replies that it was "probably quite clear to me" that there was a "sweet spot" where the "incentive was best" and therefore people stuck to installing boilers eligible for the top subsidy rate.

Afbi promoted biomass as a fuel and put on numerous events at its research facility in Hillsborough in County Down, many of which Mr Johnston was involved in.

The RHI scheme was a topic that came up on a regular basis, he says.

Image source, Getty Images

"[It] would very often have been mentioned in answer to the question: 'Is there any government help to implement these kind of low-carbon heating technologies?'

"The answer is RHI."

A debate has sprung up about Northern Ireland's carbon footprint and whether burning willow is really good for the atmosphere.

Sir Patrick questions Mr Johnston about whether cutting down willow trees reduces emissions.

Mr Johnston answers that "there is not a short answer" to that question.

Inquiry panelist Dame Una O'Brien cuts in to make the point that producing willow trees locally can reduce transport and therefore reduce carbon emissions.

Sir Patrick gets a few laughs when he wraps up the slight digression by saying: "I don’t think the panel needs two or three chapters on whether or not this is a good idea."

Mr Johnston argues that he believed that DETI had a plan to develop the RHI scheme in order to make it more sustainable.

Inquiry chair Sir Patrick Coghlin puts it to him that he didn't know when the next phase of the scheme was coming down the track.

Image source, RHI Inquiry

"You didn’t know that GB had had these since 2011 and therefore one would have thought given your common interest in this kind of industry, you would have asked about tiering," said Mr Coghlin.

Mr Johnston said: "I didn't."

Image source, Getty Images

Asked whether he understood that without tiering, there was a likelihood for overcompensation from the RHI scheme Mr Johnston says he did.

"Tiering definitely would have [stopped overcompensation]," he says.

But he didn't raise any concerns with DETI about the absence of the cost control.

Mr Johnston attended a renewable energy presentation in 2013 that pointed out the "perverse incentives" associated with schemes without tiering.

He said he doesn't remember it word-for-word and that he would go to presentations to learn "as a whole" about biomass and renewables, not to focus on scrutinising "the mechanics of an RHI scheme" and he "wasn't looking for problems in it".

Mr Johnston says he never thought people would take advantage of the scheme and the prospect of people "burning biomass just to make money on it" didn't cross his mind.

"I didn't think that the incentive along would actually be enough to put in biomass boiler, buy a load of pellets and burn them all and make money - I never considered that would happen."

Mr Johnston was provided with information in 2012 about the RHI scheme by Peter Hutchinson, the DETI civil servant who was virtually running it single-handedly.

Questioned on the RHI scheme's subsidies, he says he a document that showed that the tariffs were not tiered - the absence of tiering was a critical flaw in the initiative.

Image source, RHI Inquiry

Tiering works by dropping the subsidy rate on offer once a certain limit of heat usage has been reached, with the intention of controlling costs by preventing a claimant from overusing their heating system to collect more cash.

"I was aware there was tiering in the GB scheme," he says, but adds that he didn't know when it sunk in that there was no tiering in the Northern Ireland scheme.

Mr Johnston says in his witness statement that to do his role "effectively" he had to have "many connections to people and bodies operating within the renewable energy and heat sectors" in Northern Ireland.

He says his interest in the scheme was on how it could promote the growth of biomass fuel crops and his contact with industry about the RHI scheme would've been focused on that subject.

It's put to him that because of his links to industry he would've been "in a reasonably good position" to provide feedback to Stormont's Department of Enterprise, Trade and Investment (DETI), which was running the scheme.

Image source, PA

He doesn't agree with that, stating that he never had any evidence of how the scheme was performing.

He claims he "would never have been shy of saying" of telling DETI about what the scheme was doing for biomass growth and the growth of energy supply in the small and medium business sector in Northern Ireland.

He says he hopes he was considered an honest broker by DETI when it came to giving the department information.

Chris Johnston is from the Agri-Food and Biosciences Institute (Afbi), which falls under Stormont's Department of Agriculture, Environment and Rural Affairs.

He joined Afbi in 2012 he has led the Environment and Renewable Energy Centre there, which carries out renewable energy research and development projects.

Image source, RHI Inquiry

In his witness statement,which you can find on the inquiry website, external, he says his job is to assist the agri-food industry, and other sectors, in maximising the potential of renewable energy".

He'll be asked questions about his knowledge of the scheme, Afbi's promotion of it and contact he had with third parties about the initiative.

His profile on the Afbi website, external reveals that he's big fan of Graham Linehan comedies - Father Ted and The IT Crowd are favourites - and he loves BBC Radio 4, skiing, rugby and lobster potting.