A properly designed throat damper affords a means of regulating the fire.
The damper consists of a cast-iron frame with a lid hinged preferably
at the back so that the width of the throat opening may be varied from
nothing to 6 inches. There are a number of patterns on the market, some
of which are designed to support the masonry over the fireplace opening.
A roaring pine fire requires a full throat opening, but slow-burning
hardwood logs require but 1 or 2 inches of opening. Regulating the
opening according to the kind of fire prevents waste of heat up
the chimney. Closing the opening completely in summer keeps flies,
mosquitoes, and other insects from entering the house by way of the
chimney.
In houses heated by furnaces or other modern systems fireplaces without
throat dampers interfere with even heating, particularly in very cold
weather. An open fire must be supplied with air and the larger the fire
the greater the quantity required; a fireplace with a width of 5 feet
or more may pull air from distant parts of the house. This air that
is heated at the expenditure of fuel in the furnace is carried up the
chimney and wasted, but with a throat damper open only 1 or 2 inches a
slow fire of hardwood can be kept going without smoking the room, thus
reducing materially the waste of hot air.
[Illustration: Fig. 17.--Smoke dampers with lids hinged in the center do
not turn the up draft as well as do those hinged at the rear side.]