Complex sentence: A favorite design puzzle of Furness's was to install a large window over a fireplace, diverting the hidden flues into the walls flanking it, as he did at the First Unitarian Church of Philadelphia (1883-86).
Split into simpler sentences: A favorite design puzzle of Furness's was to install a large window over a fireplace, diverting the hidden flues into the walls flanking it. Furness placed a large window over the a fireplace at the First Unitarian Church of Philadelphia (1883-86).