Suggestions for firing and cleaning a furnace that apply regardless of the fuel used may be given as follows:

* U. S. Bur. Standards, Circ. 55.

+ L. P. Breckenridge and S. B. Flagg. U. S. Bur. Mines, Tech. Paper 97.

The fire should be attended to regularly, and not left until it has burned low and heat is needed throughout the house. Often the need can be anticipated and by attention at the proper time trouble can be avoided. In addition, economy of fuel is more likely to be obtained by fairly uniform rates of burning than by attempting to supply a large amount of heat in a short period and then suddenly checking the fire.

The size of the coal fired should be as nearly uniform as possible. Using coal of uneven size prevents an even flow of air through the fuel-bed and increases the tendency of the fire to burn through in spots.

One should try to keep the fuel-bed free from air holes, as they cause waste of fuel and may prevent the heater from maintaining the desired temperature.

Excessive shaking of the grates should be avoided and thus the amount of coal lost by falling into the ash pit reduced. Ordinarily, the shaking of the grates should be stopped as soon as bright particles begin to drop through or, under some conditions, as soon as light from the fuel-bed begins to show in the ash pit.

When the demand for heat is urgent Or the fire must be built up quickly, the fuel-bed must be kept uniformly thick, but not too thick, using the coarser part of the coal and all the draft available. Air will then flow freely through the entire fuel-bed and burn the coal at a maximum rate. Under such conditions, the firings should be made at frequent intervals and small charges used, so that the fresh fuel will only for a short time chill the temperature of the fire pot. When heavy firings are made the fresh fuel not only increases the resistance to flow of air through the fuel-bed, so that the rate of combustion is lowered, but it acts as a cold blanket to screen the heating surfaces from the radiant heat of the fuel-bed.

In mild weather it is well to leave on the grates a layer of ashes under the active fuel-bed. This layer will increase the resistance to the flow of air through the fuel-bed and facilitate the maintenance of the low rate of combustion required in such weather; also, it will cut off some of the grate surface.

Clinkers should be kept worked out of the fuel-bed, for they obstruct the flow of air through it, clog the grates, and may break the parts of shaking grates.

Heating surfaces, or flues as they are sometimes called if the heater be a boiler, must be swept clean so that they will readily absorb heat. Soot cuts down the heat-absorbing power of any heating surface very rapidly, and, therefore, should not be allowed to accumulate. Ashes should not be allowed to pile up under the grates in the ash pit, for they will seal off the air from part of the grate surface and may cause the grate-bars to be burned and warped.

Coal as free from slate and ash as possible should be purchased.