3. Direct and indirect systems

There are two general types of heating systems in use today, known as the direct and the indirect. The former (Fig. 46) includes ordinary stoves and the more modern and familiar direct steam and hot water radiators located right in the room to be heated. Such systems not only warm up the air in the room, but also give off more or less heat by radiation to the walls, the furniture, and the occupants.

The latter or indirect system (Fig. 47) has no heating surfaces in the room, but instead supplies heated air through one or more registers. This air mixes with and warms the air in the room to the desired temperature. Furnaces placed in the basement, both of the "pipeless" and the more satisfactory piped type, as well as "indirect" steam and hot-water radiators are used in such systems.

Fans are not necessary in any of these indirect systems in the average home, but may be used if the owner desires to accelerate the air flow over the indirect heating surfaces. When fans are used, somewhat lower heated air temperatures are sufficient, since more air is sent into the rooms which are to be warmed than would be the case with a gravity flow system. There is no radiation effect in the actual rooms of the house when indirect systems are used, and some people require a slightly higher air temperature where there are no direct heating surfaces.

1 This does not refer to the use of thermal insulation or the use of weather-strips, valuable as they may be when properly and intelligently applied, but merely to the equivalent of good first-class construction.

4. Floor and ceiling temperatures

Any of the systems indicated in the preceding sections is capable of heating a house to 70 degrees Fahrenheit at the "breathing line" (an arbitrary level five feet above the floor), at which level temperatures are always taken in checking up a heating guarantee. There may, however, be a great difference between the "breathing line" temperature and the air temperature at other levels in the room. Tests at the University of Illinois, in actual rooms, when it was zero outside and 70 degrees at the breathing line showed temperatures as low as 60 degrees near the floor and 85 degrees near the ceiling. Even worse conditions at floor and ceiling may exist with stoves and "pipeless" furnaces, although the "breathing" line temperature is maintained at 70 degrees in all cases.

Section of elevation of a house,

Fig. 47. - Section of elevation of a house, showing installation details for warm-air heating system. Note location of the return air register and tapered transition fittings to reduce friction of air flow.

Modern warm-air furnace heating systems (Fig. 48), and "indirect" steam and hot-water systems, as well as direct steam and hot-water systems (see note, Fig. 48) using long, low, narrow radiators, will maintain much better air temperatures at floor and ceiling than those quoted at 60 and 85 degrees, respectively.

Such extreme conditions are intolerable and the home-owner will avoid much dissatisfaction and argument, as well as much personal discomfort by giving thoughtful consideration to the effect of type of heating system and heating units in the rooms on the room air temperatures at floor and ceiling, regardless of the fact that the system may maintain a "breathing line" air temperature of 70 degrees. Here are the basic principles: a) For indirect systems, the air supplied to the rooms for heating should enter at a relatively low temperature even in coldest weather. When it is zero outside, this temperature should never be above 175 degrees at any register face, and better be 150 degrees. The air supply from "pipeless" furnaces is usually heated far above these temperatures.

Curves of floor, breathing level

Fig. 48. - Curves of floor, breathing level, and ceiling temperatures for living-room from research residence. Note air temperature difference between ceiling and floor increases rapidly as indoor-out-of-door temperature difference increases. Direct steam and hot-water systems with long, low narrow radiators under windows may be operated with smaller air temperature differences between ceiling and floor than shown in the chart.

b) For direct systems, the heating surface should not be highly heated as is often the case with stoves. Even with direct steam radiators which are usually somewhat above 212 degrees, much better results will be secured with long, low, narrow radiators than with high radiators. Hot-water radiators are seldom operated at water temperatures above 170 degrees with an open tank system, and maintain better room air temperatures at floor and ceiling than steam systems using radiators of the same type. With pressure systems of hot-water heating water temperatures may run to 220 degrees.

Experience has shown that the cheaper the heating system, the greater the air temperature difference between floor and ceiling.....