A source reveals that the source of the fire that raged at 740 Park Avenue last week was a sauna in the apartment of Bernie-Madoff-feeder fat cat Ezra Merkin. And that’s caused fresh worries for two more of his neighbors. Recent purchaser Howard Marks has installed a sauna in his ongoing renovation. And long-time resident Israel “Izzy” Englander is currently combining two apartments and planning to install his own personal hot box. “The concern is that the board will make Marks remove [his] and stop Englander,” says the insider. This comes atop the water and smoke damage done to Merkin’s neighbors David and Julia Koch, Andrew and Dana Stone, Vera Wang and Thomas Tisch and his wife, the former board president Alice Tisch.