Darla Scott, the Queen of Steam for Broomhall Brothers Mechanical Contractors, knows steam boilers from their Hartford Loops on up, and she's as comfortable in a discussion of steam-boiler theory as she is with a wrench in her hand. To know steam boilers, of course, is to love them. And Scott does, both the residential and commercial breed. One of her favorites is a grand old boiler in a building near 20th and York; she sends a Christmas card to it every year. But then, our pal in Bonnie Brae whose boiler was saved by Scott feels pretty sentimental about her, too: He has her picture in his basement.