Arlene Foster said if she had known the scale of the difficulties around the RHI scheme in the autumn of 2015, she would have raised it during the Fresh Start Talks.
Arlene Foster defends Spads at RHI inquiry
Martin McGuinness knew about RHI whistleblower concerns, insists Arlene Foster
Those talks took place following a crisis at Stormont sparked by the murder of republican Kevin McGuigan in Belfast, and questions over whether the Provisional IRA was responsible.
The Fresh Start talks took place in September/October 2015, resulting in the Fresh Start Agreement in November 2015.
Mrs Foster said if she had known about the spiralling costs of the RHI scheme she would have raised it in those talks, when they were negotiating more money from the Treasury.
She told the inquiry she knew there was an issue with the RHI scheme in June 2015, but not the specifics.
"If we had of known there was such an issue, thinking about this over the last few days, with funding, then that's something that would have been dealt with in the Fresh Start Talks," she told the inquiry.
"During those political talks we had asked Treasury for assistance in relation to further funding, so if we had of known the scale of it before the Fresh Start Talks, we certainly would have raised it as an issue in those talks. There were absolutely no discussion about RHI in the Fresh Start Talks."
Mrs Foster said it would have been much harder in January 2016 to go back to the Treasury to ask for more money for Northern Ireland.
"After the talks were finished, and Treasury were not too happy that here again Northern Ireland had asked for more money after a political negotiation," she said.
"In January then it was difficult to ask for any space if you like, in relation to RHI.
"But if we had known before the Fresh Start Talks had taken place or during the Fresh Start Talks, we certainly would have tried to gain some money from the Treasury at that time, or some leeway."
Mrs Foster has defended the suitability of her party's special advisers.
She said the DUP is no different to other parties in how they appoint advisers.
"I don't think we were any different at all, if you look at the Ulster Unionists, the SDLP, certainly Sinn Fein put people into posts from their backroom team, we were no different," she told the inquiry.
"I will say this, all of the special advisers that the DUP appointed were people who had third-level education, and who had an ability to work within the system.
"I am not sure that that can be said about every other special adviser."
Inquiry chairman Sir Patrick Coghlin has voiced concern over how both the DUP and Sinn Fein appointed advisers.
Mrs Foster responded: "I'm not going to say there's not room for improvement."
Mrs Foster also revealed that discussions have taken place with other parties around governance at the powersharing institutions at Stormont, and that the appointment of special advisers was included in those discussions.
Mrs Foster told the inquiry that as a Stormont minister, you were "thrown in at the deep end".
She said that needs to be rectified when or if the Northern Ireland Assembly is reinstated.
Mrs Foster served as enterprise minister, finance minister, environment minister and also first minister at various times during devolution in Northern Ireland.
"When you become a minister, you are thrown in at the deep end," she told the inquiry.
Mrs Foster said there was no training day and you relied on advice, your own reading and senior civil servants.
"I think that is a shortcoming and is something that needs to be rectified when or if the Assembly gets up and running again."
Arlene Foster has said with hindsight she wishes she had removed Jonathan Bell as a minister in January 2016.
But she said there were concerns he would "go rogue" if replaced.
Mrs Foster said that Mr Bell "hero worshipped" Peter Robinson, and "never really accepted" her leadership of the DUP.
She became first minister in January 2016.
"It was a general view among senior members of the party that Jonathan was not a suitable candidate to be minister of enterprise, trade and investment," she told the inquiry.
"I have to say it is a great regret of mine that I didn't remove him from the post, but my decision at the time with counsel from other senior colleagues was to allow him to remain up until the election.
"The Assembly sat until March 19 so essentially you had until January to the middle of March and I was very clear that I wasn't going to reappoint him as a minister after the election."
Mrs Foster said: "As is clear from the fact I didn't appoint him as a minister in any department after the election, I wouldn't have appointed him into any department, I didn't think he acted in an appropriate way as a minster, I didn't think he took his ministerial role seriously enough.
"He saw it as a role where he was to be served rather than to serve, and therefore I thought he wasn't an appropriate person to serve as a minister.
"Clearly with hindsight I shouldn't have left him there, the view was expressed to me, how much harm can he do in three months. It would cause a lot of problems and he would go 'rogue' if he was replaced as a minister.
"Clearly the relationship between myself and himself deteriorated.
"He retreated and basically wouldn't speak to anyone, he did of course speak to the former first minister on many occasions and take his counsel rather than deal with the person who had been appointed leader by the party.
"He never really accepted my leadership, he hero worshipped Peter (Robinson) and had a real difficulty when Peter left."
When asked about a note from a whistleblower raising "very serious concerns" about people abusing the RHI scheme running boilers 24/7 all year round for financial gain, Mrs Foster said this note was the first she had heard of such concerns.
Mrs Foster told the inquiry the note was left with her, and she then passed it on to the head of the civil service.
"I knew that once I handed it to the head of the civil service, that it was being dealt with in the appropriate way," she told the inquiry.
"I was essentially acting as a post box so that it could be dealt with by the appropriate department.
"Before I had been handed this document, the head of the civil service had phoned me that morning telling me there were serious difficulties with the budget and the financing of the scheme and that we would have to take action to deal with it.
"Malcolm (McKibbin) took that very seriously, the head of the civil service."
Mrs Foster has denied claims previously heard by the RHI Inquiry that she had wanted the RHI scheme kept open.
Former DUP special adviser Timothy Cairns told the inquiry earlier this month that he felt a papertrail was being created to make it look as if Mrs Foster wanted the scheme closed, but efforts were going on behind the scenes to keep it open.
"That's clearly not the case," Mrs Foster told the inquiry.
"We knew of the seriousness of it, we wanted to close the scheme."
Arlene Foster said there was a "huge reaction" when the RHI scheme was closed down.
"There was a huge reaction when it was decided to close the scheme," Mrs Foster told the inquiry.
She recalled the announcement was put out in a press release by Mr Bell on a Friday afternoon.
"There was a huge reaction from individuals, companies, the National Trust, CBI, everyone seemed to be outraged that the scheme was closing," she said.
The RHI Inquiry also heard of an incident where special adviser Timothy Cairns removed a reference to the Office of the First and Deputy First Minister (OFMDFM) from a submission to close the green energy scheme.
The inquiry heard that this had been done to "curry favour" with Mrs Foster.
Reacting Mrs Foster described it as a "very strange episode".
Inquiry chairman Sir Patrick Coghlin said: "A ministerial submission was altered by a spad who felt he had the authority to do so?"
"I have no understanding why he took out OFMDFM at all," Mrs Foster told the inquiry, pointing out that it had "no import whatsoever".