The DUP has faced mounting criticism on social media after party chairman, Lord Morrow, was filmed joking about the botched RHI scheme which is set to cost taxpayers £490m.
And the DUP's political opponents said that Lord Morrow's "frivolous remarks" showed just how "arrogant and out of touch" the party was with public opinion.
Lord Morrow and Arlene Foster were in Omagh on Friday to lodge papers for March's Assembly election.
As DUP members posed for photographs, it was remarked that it was a "lovely sunny day". Lord Morrow responded laughing, "It's hot. Yes, the boilers are all working."
Mrs Foster replied: "Stop it, stop."
The exchange was filmed by a BBC cameraman.
Members of the public, including former DUP supporters, vented their anger at the party on social media.
Alliance leader Naomi Long said: "For most people, the RHI scandal is no laughing matter. Lord Morrow's remarks will leave a bad taste with the electorate.
"We are talking here about a significant amount of public money and a scandal which has led to the collapse of the political institutions. There is nothing remotely amusing about it.
"This episode once again shows the DUP's arrogance in dealing with stuff that really matters to people."
TUV leader Jim Allister said that Lord Morrow's "frivolous remarks" showed that the DUP "just didn't get it" about the RHI scandal, but "the public very much do get it".
Fermanagh and South Tyrone Ulster Unionist Assembly candidate Rosemary Barton said that Lord Morrow should "catch himself on". She added: "If it hadn't been for his party's handling of the RHI debacle and Sinn Fein's opportunism, we wouldn't be having these elections which will cost the taxpayer another £5m.
"It's time Lord Morrow and his colleagues dropped their Stormont Castle mindset and tried to understand the consequences of their actions given the financial difficulty some people are having when trying to heat their home."
The DUP did not respond when contacted for comment by the Belfast Telegraph.