The cast iron firebacks get warmth from the heat of the fire which then spits back into the room by the process of radiation. Since stainless steel firebacks radiate less heat than cast iron firebacks due to their smaller size, but reflect more light.

How effective is a fireback?

Radiator effect: how efficient is a fireback? The efficiency of a wood-burning fireplace to heat air is typically 10%, but with a fireback this may increase to 15%. This is less efficient than a stove or fireplace insert in heating air, but efficiency is a complicated concept with open hearth fireplaces.

Where does a fireback go?

Firebacks are heavy iron or steel plates sized in proportion to fireplaces and are placed against the wall at the back of the fireplace.