Louis J. Mueller, Sr. opened production of furnaces in Milwaukee, WI in 1857 in his own hardware shop.  Within 30 years he had patented warm air units and then began manufacturing steam and hot water boilers.  In 1954, L.J. Mueller Furnace Company was merged with Worthington Corporation and the name was changed to Mueller Climatrol Division of Worthing Corporation. 

Eventually, Mueller was acquired by Fedders and Mueller was dropped from the name.  Many acquisitions later, Climatrol ceased to exist.  

While the company may no longer exist and replacement parts are no longer available, the advertisements for Mueller Furnaces have lived on in nostalgia catalogs and newspaper collections.