the fire, and ashes must be removed; but the hot-water system has the
advantage of economy and cleanliness.

[Illustration: FIG. 16.--Fresh air from outside circulates over the
radiators and then rises into the rooms to be heated.]

11. Fresh Air. Fresh air is essential to normal healthy living, and
2000 cubic feet of air per hour is desirable for each individual. If a
gentle breeze is blowing, a barely perceptible opening of a window
will give the needed amount, even if there are no additional drafts of
fresh air into the room through cracks. Most houses are so loosely
constructed that fresh air enters imperceptibly in many ways, and
whether we will or no, we receive some fresh air. The supply is,
however, never sufficient in itself and should not be depended upon
alone. At night, or at any other time when gas lights are required,
the need for ventilation increases, because every gas light in a room
uses up the same amount of air as four people.

[Illustration: FIG. 17.--The air which goes to the schoolrooms is
warmed by passage over the radiators.]

In the preceding Section, we learned that many houses heated by hot
water are supplied with fresh-air pipes which admit fresh air into
separate rooms or into suites of rooms. In some cases the amount which
enters is so great that the air in a room is changed three or four
times an hour. The constant inflow of cold air and exit of warm air
necessitates larger radiators and more hot water and hence more coal
to heat the larger quantity of water, but the additional expense is
more than compensated by the gain in health.