This article is a general introduction to domestic heating systems and the factors that are (or should be) taken into account in their design in a new house. Anyone considering the purchase of an existing house should take these same factors into account when judging the adequacy of the existing heating system.
The size of a heating plant (that is, its output in Btu per hr) will depend on (1) the average outside winter temperatures, (2) the size and shape of the house and the locations of the rooms relative to the sun, and (3) the amount of heat that's lost through the walls, roof, and floor of a house and the amount of cold air that leaks into the house.
The basic point that must be decided on before any heating system can be designed is the difference between the design outdoor temperature and the design indoor temperature, the latter usually being assumed to be 70°F. In fact, the most comfortable indoor temperature will depend not only on the temperature of the air but also on the humidity, the amount of physical activity being performed, the temperatures of the walls, ceilings, and floors, which will differ from the air temperature, as well as on psychological factors that are both extremely important and extremely difficult to pin down in a formula. Women, for example, usually feel most comfortable at room temperatures that men consider too high. How is this subjective fact to be measured?
A number of experiments have shown that a temperature of 68°F -- not 72°F -- is probably the optimum room temperature for most people who are working. In experiments with Students, for example, it was found that the students did their best work when the room temperature was 68°F. When the room temperature was increased above 68°F, the students become increasingly restless and dull-witted; at 74°F they were doing 15 % less work than they had been doing at 68°F. When the temperature was decreased below 68°F, the amount of work accomplished fell off again.
These experiments involved sedentary labor. When the average person is sitting quietly in a room, the metabolic processes of his body produce about 6 2/3 BTU of heat per minute. When this heat is dissipated from the body at the same rate at which it's produced, the skin temperature will be somewhere between 82°F and 89°F and the individual will feel “comfortable.” If his skin temperature should fall below 82°F, he will feel chilly. On the other hand, if his skin temperature should rise above 89°F, he will feel warm. Thus, it's not the temperature of the air within a room that's of basic importance for human comfort but the balance that exists between body temperature and the physiologically perceived room temperature.
Whenever the skin temperature is within the normal comfort range of 82 to 89°F, about 92 % of the body’s excess heat is dissipated from the skin by a combination of convection and radiation. Only about 8 % of this excess body heat is dissipated by respiration and the evaporation of moisture from the skin. As the body temperature increases, however, the body begins to perspire and the proportion of body heat dissipated by the evaporation of this perspiration increases until, when the skin temperature is much above 89°F, it amounts to about 30 % .
Several conclusions follow. If the air within a room is too dry, as is usually the case during the winter, the average individual will feel that the room is a bit on the cool side, even though the air temperature is within his comfort range, because the amount of moisture evaporating from his skin will be greater than if the air were more humid. A draft of air through a room has the same cooling effect, since the draft will increase the rate of evaporation as it strikes the skin, which is why a drafty room always feels a bit cooler in hot weather and a bit chilly in cold weather.
On the other hand, maintaining the relative humidity at a fairly high level, 50 % and more, increases the apparent air temperature because the humidity inhibits the evaporation of moisture from the skin. The amount of humidity can't increase indefinitely, however, because the excess moisture will begin to condense on cold surfaces, with potentially harmful consequences. (For details of this process, see CONDENSATION.)
Of the 92 % of body heat that's lost through convection and radiation at the low end of the comfort range, about half is lost through convection and half through radiation. The amount of body heat lost through convection depends on the air temperature in the room, and this factor can be controlled by the heating plant through the setting of a room thermostat.
The amount of body heat lost through radiation, however, depends entirely on the temperature of the surrounding walls, floor, and ceiling -- the room air temperature has nothing whatever to do with this radiation loss. The reason is that heat radiates from a warm object (like the human body) to nearby cold objects, and never vice-versa. The greater the difference in temperature between the two objects, the greater the rate at which heat is transferred from the warmer object to the colder object. It follows that in a room in which the walls, floor, and ceiling are cold, the rate at which the body gives up its heat will be high. In the ordinary, uninsulated wood-frame house, for example, the rooms on the north side of the house are much colder during the winter than the rooms on the south side of the house. The rooms are cold, not because “cold” is being radiated into the house but because the heat being radiated by the occupants and any other warm objects in the room is being transferred at a high rate to the walls. The walls, in turn, are transferring this heat to the outdoors with equal rapidity.
Sizing a Heating Plant
When a heating contractor designs the heating system for a new house, the main objective is to select a heating plant having a certain optimum heat output. That is, the heat output (in Btu per hr) must be sufficient to maintain the design indoor temperature within the house during cold weather while at the same time keeping the fuel consumption as low as possible.
Basically, the size of any heating plant is determined by the difference between two assumed temperatures—the design indoor temperature and the design outdoor temperature. The latter temperature isn't the lowest temperature likely to be encountered during any given winter but a statistic. It is obtained by examining the weather records for past years and then striking a reasonable balance between the coldest temperature that may occur during any given year and the average temperatures that usually do prevail during the winter. Typically, about 97½ % of the temperatures that do occur during any given year will be at or above the design outdoor temperature and about 21, % will be below. If the heating plant has been correctly sized, therefore, the heating plant will be unable to maintain the design indoor temperature for about 2½ % of the time. If, for example, the design indoor and outdoor temperatures are 70°F and 10°F, respectively, and the outdoor temperature drops to, say, 5°F for a few hours, the heating plant will be capable of heating the house only to 65°F during that period.
This may seem unreasonable to the reader, but it's plain wasteful to install an oversize heating plant merely because once every few years the outside temperature may fall as low as 5°F for a few hours. Besides, the lowest temperatures occur at night, when the thermostat is usually turned down anyway. Of course, if money is no object, the heating contractor can be told to go ahead and design the heating system around a lower assumed outdoor temperature.
The design indoor and outdoor temperatures having been determined, the heating contractor can proceed to calculate the total amount of heat that will be lost from the house per hr. The basic fact that must be taken into account here is that different types of wall construction lose heat to the outdoors at different rates. It should be obvious, for example, that in any given time period a lesser amount of heat will be lost through a 2-ft-thick masonry wall than will be lost through the thin sheets of paper that are pasted to the exterior wall screens of the traditional Japanese house.
A considerable amount of research has been conducted at universities and by different trade and engineering associations into the heat-transmission characteristics of different kinds of material and different types of construction. This research has been summarized in the form of heat-transmission coefficients that allow one to compare these materials and constructions directly with each other. Formulas have been developed into which these coefficients can be “plugged” when the heat loss is being calculated in square feet of wall area per hr. By examining the plans of a new house or the actual construction in an existing house, the heating contractor can determine what coefficients to use in order to determine the heat loss for that house. Separate calculations are required to determine the heat loss for the exposed wall areas and for the doors and windows in each room.
If the ceiling of a room is also the underside of a roof surface or an attic, another calculation is necessary. The heat loss through a roof is treated the same as the heat loss through a wall. As for the heat loss through an attic floor, the calculation must take into account whether the attic is heated or whether it's unheated and vented to the outdoors. The heat loss through the walls and floor of a basement, or through a slab-on-grade floor, must also calculated separately, the calculations being based on an assumed soil temperature, which will vary according to the latitude.
Another cause of heat loss is the crevices that always exist between doors and windows and their frames. In an old house these crevices can be large enough that a complete change of air occurs three or four times every hour. Even in a new house they may allow one or two air changes every hour. This air- infiltration loss through the crevices must also be calculated separately.
Finally, the individual calculations completed, the heating contractor adds them all together and comes up with a number that represents the total heat loss in Btu per hr. The required size of heating plant is determined by this number. A correctly sized heating plant will have a heat output that's sufficient to maintain the house at the desired indoor temperature without its either operating continuously or switching on and off at frequent intervals. If a heating plant operates continuously, it means it's undersized. If it switches on and off continually, it means it's oversize. In either case the fuel consumption will be greater than necessary and , in the games that homeowners play, one of the games is called minimizing the fuel bill.In addition to maintaining a desired air temperature, the heating plant has another function, which is to quickly heat a house that has gone completely cold. This quick pick-up is needed when the family returns from a winter vacation, or if a malfunction of some kind makes it necessary to shut down the heating system for a day or two. It is also needed on a regular basis in most households when, to conserve fuel, the thermo stat is turned down to about 62°F every night. Every morning, therefore, there is a start-up period during which the heating plant must operate continuously until the design indoor temperature is reached. If the capacity of the heating plant were just sufficient to replace only the heat lost as calculated above, then, from a cold start, the heating plant would not be able to achieve the desired indoor temperature. It would run continuously until a spell of warm weather allowed it to catch up with a decreasing air temperature.
For this reason boilers and furnaces are rated at both net and gross output. The net-output rating should be slightly above the calculated heat loss, and the gross-output rating should provide the additional heat required for morning pick-up.