Ten boilers ran virtually round-the-clock during a massively-overspent public scheme to encourage green energy in Northern Ireland, a watchdog said.
The Renewable Heat Incentive (RHI) for businesses - which collapsed political powersharing in a spending row - paid subsidies worth at least £50,000 per machine.
Each burned wood for more than 21 hours a day last year, figures in an Audit Office report revealed.
The devices were installed before November 2015 when the scheme was modified to significantly reduce the taxpayer bill.
Comptroller and auditor general Kieran Donnelly said: "I continue to have significant concerns about the operation of this scheme and the serious systemic weaknesses in controls that have facilitated the possibility of funding that is at best not in line with the spirit of the scheme and at worst is fraudulent."
Mr Donnelly's report into Stormont's Economy Department said:
:: Of 1,687 boilers enrolled in the scheme prior to November 2015, 39% are using them for more than half the available hours in a year "more than 12 hours a day and 7 days a week all year round" with an RHI subsidy per boiler in 2016/17 of at least £28,000.
:: Ten are being used for 90% or more of available hours, creating a subsidy per boiler of at least £50,000 in 2016/17.
:: There is a considerably different pattern of usage in the revised scheme after November 2015. Of the 251 installed and operating; 96% were being used for 30% or less of the hours in a year and only one used for more than 50%.
:: The Economy Department estimates the scheme will cost £24 million in 2017/18 compared to £52 million had action not been taken to control costs.
:: The cost to the Northern Ireland budget in 2017/18 is projected to fall to £2 million compared to around £30 million had the old rates continued to stand.
:: Out of 2,128 boilers in the scheme, 1,419 were part of multiple installations at 437 single sites.
The auditor said: "I am concerned at the extent of the use of multiple boilers which allowed applicants to claim a considerably higher level of subsidy payments than would have been payable for installations with a single boiler of a more appropriate size greater than 100kW (or 200kW after November 2015)."
The state-funded RHI was supposed to offer a proportion of the cost businesses had to pay to run eco-friendly boilers, but the subsidy tariffs were set too high, and without a cap, so it ended up paying out significantly more than the price of fuel.
This enabled some applicants to "burn to earn" - getting free heat and making a profit as they did so.
An independent inquiry has been launched in a bid to determine who was responsible for the flawed scheme.