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Rock Energy Systems, maker of specialty furnaces designed to burn used vehicle waste oil, was acquired last month by WILsquare Capital, a private equity firm that operates out of Creve Coeur, Missouri, a suburb of St. Louis, the companies announced in a joint statement.

Rock Energy President and CEO Jeff Townsend, who joined Rock Energy System’s board of directors in 2011, confirmed on Friday the acquisition. In an email, he told The 69���� that under new ownership, Rock Energy Systems is working to develop a range of new products in addition to waste oil heaters; diesel-fired, infrared and radiant heaters; and large fans the company has made for years in Janesville through its subsidiaries Lanair, Clean Burn, Energy Logic and MacroAir, but he didn’t disclose details.

Neither Townsend nor WILsquare Capital disclosed financial details of the acquisition, which closed Sept. 15. Rock Energy Systems employed about 90 people after an acquisition of Tennessee-based company Energy Logic in late 2019.

Energy Logic is just one company Rock Energy absorbed through a pair of buyouts in the 2010s. Rock Energy’s growth has always been tied to the market for used motor oil and an environmental management framework for waste oil the Environmental Protection Agency set in place decades ago.

Rock Energy Systems was established in 1976.

The market for used waste oil has begun to shrink in recent years as vehicles become more efficient and more electric cars are produced. But Rock Energy has continued to expand its niche market by buying up market share through acquisition of competitors.

Over the past five years, Rock Energy Systems has moved into emerging heater technologies such as infrared heaters and radiant heating units, such as the type seen heating the air along the sidelines at college and professional football games.