There are many synonyms for the phrase "like a furnace" that can be used to describe extreme heat or intensity. One common synonym is "blazing," which conveys the idea of intense heat or fire. Another good option is "scorching," which implies burning heat that can cause damage. "Boiling" is also a great choice, as it suggests heat that is at or near the boiling point. Additionally, "torrid" and "sweltering" are good synonyms for "like a furnace," as they both evoke the image of extreme heat. Overall, there are many different ways to describe intense heat, and each of these synonyms can help add depth and nuance to your writing.