Modified grates may also improve efficiency. Another type uses water which heats in the grate and rises to a radiator above the fireplace where heat is distributed into the room. Either type can be designed to work with glass doors. Prefabricated fireplaces are double-walled steel units designed to be encased in the masonry of the chimney. Intakes near the base draw cold air in from the room, circulate it through the fireplace walls, and exhaust it, warmed, back into the room through vents above the fireplace.
Repairs are difficult as the unit is sealed within masonry.
It is cheaper to install, and with a brick or stone face resembles the more expensive masonry fireplace. Forced air vents may help them serve more than one room, or they may be connected to a central heating system. With glass doors and an outside source of combustion air, these units are quite efficient. You have to be building or remodeling to install a prefabricated or zero-clearance fireplace. However, if you already have a standard fireplace you can install a fireplace insert.