Fireplaces are in greater demand than ever now, with buyers of new houses willing to pay thousands of dollars more to have them. Yet most buyers won't know whether the fireplaces they are getting will function well until after they move in.

It is possible, however, to size up a fireplace long before.

For instance, a smaller fireplace is a more efficient heat producer -- in part because it pulls less air out of the room.

A fireplace opening that is high and wide relative to its depth is a good heat producer too. The firebox, the cavity that's visible beyond the opening, should have sides that slant at a sharp angle inward toward the back.After rising straight up for 15 to 18 inches, the back of the firebox should slant forward.

Side and back surfaces should be flat and angled so as to reflect the fire's heat into the room and to guide the smoke in a smooth flow toward the flue. Unfortunately, most contemporary fireplaces have squarish boxy fireboxes, which can cut the heat by up to two-thirds. Often, heat production can be increased by rebuilding the firebox.

The best lining for the firebox is a light color firebrick or soapstone, again, because they reflect heat readily.

A lintel, usually a heavy metal crosspiece, should stretch across the top of the fireplace opening to support the brick or stone structure above. The flange of the damper is often made to serve the function of the lintel.In such construction, the firebox ends abruptly at the top of the fireplace opening and no space is left above an behind to collect the smoke. So smoke from the fire is frequently sent billowing into the room.

Check to see that the fireplace has a damper set into the throat, the slot-like opening that provides a passage between the firebox and the smoke chamber above it.

It is usually easy to slide back into the firebox in a hunched sitting position to have a look. But before you tilt your head up to see the damper, you should put on safety glasses to protect your eyes from falling soot or debris.

The throat should be just four inches wide, although a 4 1/2- to 5-inch throat is often provided in very large fireplaces. Behind the throat there should be a smoke shelf that is at least eight inches deep. Without it, drafts from the flue will blow smoke into the room and the flue will not draw well.