One way to keep the heater clean is to regulate the draft up the chimney so that ashes and bits of burning fuel are not drawn into it. Also, the fire should be kept burning with a clear (not smoky) blaze. Soot is unburnt fuel. =75. Operating Heating Stoves.= A stove is a device for holding the fuel and for permitting the heat to pass readily into the room. In the stove there is space below the fire for collecting ashes. There is an opening for fresh air to enter below the fuel, to aid combustion, and a damper above to act as a check draft when open, a chimney to carry off smoke, and one or two dampers in the chimney to regulate the draft. When a fire is being built, close the draft over the fire box and open the one below; open the damper in the chimney--this allows the free passage of the air up the chimney. =76. Care of the Stove.= Do not permit a large bed of ashes to accumulate in the bottom of a stove. A thin layer of ashes must be kept in the bottom of some wood stoves to keep the fire away from the metal bottom. The polish or finish of the stove is a matter of taste. Some stoves are made of iron, which does not need blacking; some must be blacked. Blacking keeps them from rusting. All must be kept free from dust and dirt, as this accumulates moisture and causes the stove to rust. Letting the stove get red hot warps it. It should not be permitted to get so hot. The grate (Fig. 3) in stoves holds the fuel so that air can flow up thru it. If the grate is clogged with ashes, this cannot happen. The grate should be shaken to make the ashes drop thru. A clean grate is important to the complete combustion of the fuel. Shaking after glowing coals begin to fall is a waste of fuel. When an attempt to shake the grate is made, it may suddenly refuse to move. In this case, something may have lodged between its parts, or it may have been shaken from its proper position. Shaking the stove too hard may displace the grate. The common remedy for a displaced grate is to let the fire go out, remove all ashes and cinders, and readjust the grate. Some kinds of soft coal form "clinkers," and these catch in the grate. In burning fuel that makes clinkers, shake the ashes from the fire several times a day. Remove all accumulations in the fire box daily. Clinkers are made from substances which melt and recombine, forming a different material which is quite hard and does not burn. Constant attention to the fire prevents clinkers from forming in large masses. CHAPTER XI GAS, ELECTRIC AND KEROSENE HEATERS =77. Kinds of Gas Heaters.= There are several types of gas heaters--those using an illuminating flame and reflector, those fitted with a Bunsen burner and an asbestos back, and those heating water in a device like a radiator. The last two burn with a blue flame. All gas stoves ought to be fitted with a flue for discharging the products of combustion. [Illustration: FIG. 39. Gas heater showing air mixer.]