There's an assumption that RHI scheme boilers "will last for 20 years" but the reality is somewhat different, says Mr Forgrave.

The subsidies on offer from the scheme last for a 20-year spell but they stop when the boiler bought through the initiative is no longer operating - in the case of it breaking down and being unable to repaired, for example.

His boilers are four years old and already showing wear and tear and need to be repaired, he says.

"That's only going to go one direction in the long-term - machines gives bother."

Two new, large poultry sheds - each able to house about 35,000 chickens - were built on Mr Forgrave's farm in autumn 2015 and he fitted two separate 99kW boilers in each them.

Inquiry chair Sir Patrick Coghlin wants to know why he chose that heating system arrangement rather than fitting one 199kW boiler, which would have been eligible for a lower RHI scheme subsidy.

"It comes down to commercial reality," the witness replies.

Image source, RHI Inquiry

"The answer is that two 99kW boilers were more lucrative," says Sir Patrick.

Again Mr Forgrave insists that he was being cautious about financing the new development: "The word lucrative at no point crossed my mind," he says.

"I'm not criticising you for doing this," says the chair. "Anyone, particularly farmers, have to view their activities economically."

The only type of biomass boiler that farmers talked about installing was the one that was eligible for the most lucrative subsidy on the RHI scheme, says Mr Forgrave.

But he denies that he installed 99kW boilers in order to collect the highest amount of money possible from the scheme.

All of Mr Forgrave's boilers run on separate heating systems - had they been linked together on the same system they would only have been eligible for a much lower subsidy.

"What we did glean from having casual conversation [in the poultry industry] was that the tariff for a 101kW boiler wasn't financially viable," he says.

Mr Forgrave says he borrowed £200,000 over 10 years to install three boilers but adds: "We had no confidence that this would work."

There were three key reasons for poultry farmers to move from fossil fuel heating systems to biomass boilers for their sheds, says Mr Forgrave.

It was better for the environment, it was better for the health of the birds he was producing and there was the financial incentive on offer through the RHI scheme.

Image source, RHI Inquiry

In February 2014, he installed three boilers - each heating two sheds - and an entire new heating infrastructure at a cost of about £208,000.

Mr Forgrave says installing those new systems on farms was a "complete step-change" in poultry production: "It was one of the biggest changes implemented in the poultry in the last 30 years."

The traditional system of heating chicken sheds involved using several direct gas heaters, which Mr Forgarve describes this as operating "like the old Superser heaters".

That caused health problems for the birds due to humidity issues, ammonia and carbon dioxide build up.

Image source, RHI Inquiry

A more modern option is a hot water system that works more like a domestic heating system - the fuel is burnt in a boiler that heats water that circulates in the shed.

Mr Forgrave says it was beneficial to Moy Park for farmers to use the hot water system but he had already switched before the company started promoting it.

"We were slightly ahead of the curve," he says.

Mr Forgrave's farm produces 1,275,000 chickens every year.

That figure is about double what he produced before he adopted a heating system using biomass boilers in 2014.

Image source, PA

He has 10 biomass boilers heating poultry sheds on his farm.

Some members of the inquiry team have been to Mr Forgarve's farm, at his invitation, to have a look at it.

Image source, Getty Images

Mr Lunny outlines the areas he wants to explore with the witness. They are:

• Mr Forgrave's farm set-up
• his involvement in the RHI scheme
• hot water heating systems for the poultry sector
• Moy Park and its promotion of hot water heating and the RHI
• payback under the scheme
• the potential for abuse of the RHI
• his role in the preparation of a paper on heat use for a Stormont department

The first new witness the inquiry has heard from in several weeks, Tom Forgrave affirms that he'll tell the truth in his evidence today.

The inquiry barrister Donal Lunny says there are "several reasons why" Mr Forgrave has been called to give evidence.

Firstly, he's an award-winning poultry farmer and a co-chair of a committee of poultry growers that supply the major producer Moy Park.

Image source, RHI Inquiry

Mr Lunny points out that about 62% of the poultry houses that supply Moy Park are on the RHI scheme, making it "potentially a very significant potential beneficiary" of the initiative.

The witness was also a chair of the Ulster Farmers' Union's poultry committee, which represents the views of poultry farmers, and a founder of the Renewable Heat Association, a group that represents RHI scheme claimants.

And finally, he's a claimant on the RHI scheme.

He's supplied more than 500 pages of documents to the inquiry as evidence and you can find his written statement on the inquiry's website, external.

BBC News NI

A key meeting that led to a disastrous delay in RHI cost controls was driven by an unelected DUP adviser rather than his minister, the inquiry was told.

The decision to push back the introduction of subsidy cuts by a month was taken at a 15 minute meeting in August 2015.

Image source, Pacemaker

The extra four weeks allowed an extra 800 boilers to be accredited to the scheme, adding a massive sum to the overall cost.

The inquiry heard the idea for the delay was at the behest of DUP adviser Timothy Cairns even though his minister Jonathan Bell (above) was in the room.

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, 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, 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.

It's a fine day at Stormont with views across the city to the sunlit Belfast hills.

We've been hearing from a lot of civil servants recently but today's inquiry witness is an influential figure in the agricultural sector.

Tom Forgrave is a poultry producer and former chair of the Ulster Farmers' Union's poultry committee.

The session starts shortly - stay with us for live video and commentary throughout the day.