PETER Robinson advised Jonathan Bell not to go public with allegations against the DUP after he confided in him concerns about issues including RHI.
The former first minister warned Mr Bell it would "impact on his political future in the DUP" and advised having a "clear-the-air" meeting with Arlene Foster.
Mr Bell famously broke ranks with the DUP to claim in a BBC interview in December 2016 that party special advisers (Spads) delayed the introduction of RHI cost controls.
DUP leader Mrs Foster earlier this week told the RHI inquiry that Mr Bell pressed the "nuclear button" by giving the interview and "contributed" to Stormont's collapse.
In his witness statement to the inquiry, Mr Robinson said he was not involved in delaying cost controls and no problems with the energy scheme were brought to his attention.
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He said he first became aware of the scheme's incentive to produce heat for profit through media reports after he had left elected office in December 2015 and early January 2016.
He said colleagues would have known that special adviser (Spad) Timothy Johnston's role was to "gather information and advise me if there were difficulties on any aspect of their departmental programmes", but added: "No difficulty with the RHI scheme was reported to me."
Mr Robinson said that around February 2016 he met Mr Bell – then enterprise minister and in charge of RHI after succeeding Mrs Foster in the role – at Stormont parliament buildings.
He said Mr Bell "expressed the view that he was being marginalised and alienated", and told of an attempt by his Spad at the request of other Spads to "expunge" Mrs Foster's name from aspects of RHI documentation.
"He asked for advice and I strongly counselled him that in no circumstances should he tamper with official documentation or allow anyone else to do so," Mr Robinson said.
He said he suggested Mr Bell should have a "clear-the-air meeting with Arlene", given the range of issues Mr Bell had raised.
The ex-DUP leader also recalled a meeting at his Saintfield office in last March 2016 in which Mr Bell raised a "range of issues".
He said Mr Bell claimed he had been trying to close the scheme but was being hampered by Spads "that he believed were acting for the first minister", and raised issues that led to a "heated argument" with Mrs Foster.
"The primary issue causing Minister Bell upset related to a party decision that he believed would impact upon his electoral prospects in the upcoming assembly election and he expressed his intention to publicly air a number of the issues he had raised," Mr Robinson said.
"I suggested that if he dealt with the internal party matters publicly, it would likely have an adverse impact on his political future in the DUP."
Mr Robinson said he advised speaking to the party's director of elections and "sought to encourage him to resolve the other matters directly with the party leader".
Earlier this month former DUP Spad Timothy Cairns claimed at the inquiry that Mr Robinson "did nothing" about bullying allegations raised against Mr Bell.
Mr Robinson is not asked in his witness statement about the bullying allegations or claims that he did not properly address concerns.
In relation to the RHI scheme, Mr Robinson recalled receiving papers from Mrs Foster about the non-domestic scheme's launch in 2012 and the domestic scheme in 2014, and correspondence from Mr Bell in November 2015 about implementing cost controls.
"Except for the receipt of the aforementioned submissions, the issue was never raised with me for the whole of my time in office, whether in an executive, party, or any other context, and, apart from these communications, I never received nor sent any communications or had any interaction with any official, special adviser, ministerial colleague, MLA, politician or public body relating to or touching upon the RHI scheme," he added.
