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Technical Note (National Research Council of Canada. Division of Building Research), 1953-12-08
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Summer Cooling by Means of Ventilation
Hutcheon, N. B.
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DIVISION OF BUILDING RESEARCH
NATIONAL RESEARCH COUNCIL OF CANADA
'f
E
C
1HIN ][ CAlL
NOTE
No.
170NOT FOR PUBLICATION
PREPARED BY N. B. Hutcheon CHECKED BY
FOR INTERNAL USE
APPROVED BY
PREPARED FOR General Information DATE Dec.
·8, 1953.
•
SUBJECT Summer Cooling by Means of Ventilation
Summer cooling by means of ventilation alone has very definite limitations in guaranteeing comfort conditions within buildings during extreme hot weather in most parts of Canada. The reasons for this are not apparent to many people although the fact may be brought home by experience, frequently only after substantial outlay for expensive mechanical equipment.
It must first be realized that in heating or cooling a room or a building using a cirCUlating air stream as a heat carrier, the room can neither be heated nor cooled by introducing
air which is always exactly at room temperature. In order to
heat a room, and to maintain it at a given temperature, a
defini te amount of heat must be supplied at a rate which balances
the heat loss. Heating by means of an air stream introduced
into a space can be achieved thep, provided that the air ·introduced
is hotter than the room. The amount of heat, which is governed
by the amount the air stream temperature exceeds room temperature and by the air quantity, must be sufficient for the particular
situation. For a given amount of heat, the air quantity can be
large and the temperature only a small amount above room tempera-ture, or, alternatively, the air quantity can be small prOVided
the air is much hotter than room temperature. Similarly, in
cooling a room by means of air, the air introduced must be at a temperature below room temperature so that it can pick up heat before being carried out of. the room at room-air .temperature. As before, either a large amount of" air only slightly below room
temperature, or a small amount of air very much colder than room tempera:ture can be used to provide a given amount of cooling (i.e., to carry off a given amount of heat).
Fortunately, in air-heating ｳｾ tems it is quite feasible
to deliver air for heating at a temperature as much as 100 degrees
above room temperature without any great difficUlty. This makes
it possible to use moderate rates of air circulation. In summer
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2
-below room temperature. Assuming a difference of only 10 degrees,
the amount of air required in summer, for the same amount of
heat carried, will be ten times as great as in winter. If summer
loads are considered to be roughly half as great as winter ｬｯ｡､ｾＬ
in Canada, then at the same 10-degree cooling differential, five times the winter quantities will be required for summer cooling.
On the average, in Canadian buildings of good
construc-tion, about
5
British thermal units of heat will be requiredper hour to make up the heat losses for each cubic foot of space
in winter. For the 100-degree rise acceptable in heating, the
air change provided by the heat-carrying air can readily be
calculated. It is found to be about
3
cubic feet per cubic footof space, or three air changes per hour.
The outside-air ventilation requirement for freshness
is usually taken as 10 cubic feet per person. In a crowded
building such as a school, where only about 200 cubic feet of space are provided per occupant, 10 cubic feet per minute per occupant represents a fresh-air requirement of three air changes
per hour. This is equal to the air handling capacity required
for air heating. It may be seen, therefore, that considering
winter conditions, an adequate system for air heating or
ventila-ting, separately or together, seldom needs to have a capacity in
excess of three to five air changes per hour. When the space
provided is as much as 2,000 cubic feet per person, as may be the case in many laboratories, homes and similarly occupied
spaces, the air change in fresh air need not exceed about one-third
of an air change per hour. The se ventilation rates may now be
considered further in relation to the needs in summer, for cooling by ventilation with outside air only.
It is not unreasonable to assume that the summer cooling
load may be as much as half the winter heating load. At a 10-degree
difference between entering air and room air this will require ventilation rates five times greater than the rates prOVided by
a system designed for air heating in water, or for normal
ventila-tion requirements for freshness, even for crowded spaces. It must
be recognized also that this applies to the case where air is
available at a temperature 10 degrees below the maximum permissible
room temperature. When outside-air temperatures rise above the
comfort level, it is obvious that the indoor temperature, at a
level 10 degrees higher, will be still more uncomfortable. To
､･｣ｲ･｡ｾｴｨｩｳ differential to
5
degrees requires that the ventila-tion rate be doubled (i.e., to as much as thirty air changes per hour).The dilemma of summer cooling by ventilation is thus
apparent. Even though ventilation capacities are made many
times greater for summer use than for heating or normal ventila-tion at other times of the year, they can only provide comfort so long as the temperature of the air available for ventilation is substwatially below the maximum permissible for comfort.
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3
-When mechanical cooling of air is used, say to 50 degrees, for room conditions at 80 degrees (i.e., a 30-degree differential) the air quantities required may be of the order of ten air changes per hour for the cooling load assumed above. This is still quite large in terms of cost of air distribution equipment, in addition to the cost of equipment for cooling the air.
It is not practical to provide a mechanical ventilation system with ducts running to various parts of a building to
provide fifteen to thirty air changes per hour for sumner
ventilation. The system is costly. the space occupied by ducts
. is great, the system will not operate satisfac·torily at the low air quantities which are adequate at other times of the year, and, finally. the system cannot guarantee comfort in warm
weather. Mechanical ventilation systems provided in buildings
of average occupancy will seldom provide capacities in excess of five air changes per hour. which. as will be evident in the light of the above discussion, may contribute little to summer
cooling. The role of open windows and doors in summer, in
permitting high ventilation rates to supplement a "normal" ventilation system. is now apparent.
It may be quite feasible in many cases to provide
simple. low-cost mechanical systems in certain spaoes to
supple-ment a normal ventilation system in hot weather. Large, relatively
inexpensive, propellor fans, without ､ｵ｣ｴｷｇｲｫｾｾ can be used to
advantage to sweep large quantities of air through certain portions of buildings where suoh arrangements are acoeptable.
Where a high degree of oomfort must be maintained throughout the summer, it beoomes neoessary in many cases to
consider the use of relatively expensive and complicated
meohani-cal equipment for oooling. In most cases the cost of such systems
must be weighed against the length of the summer period during
whioh they will be advantageously used. In some commercial
spaoes, such as theatres, restaurants and crowded stores, this
period of advantageous use may be 6 months or more. In other
types of buildings, having greater volume per person and lower heat gains per unit volume, the need may extend only to one or two months.
There is a need, however, in almost all buildings, to
design and operate them in such a way as to alleviate the summer
heat oondition, to reduce the periods of discomfort when no
cooling is provided, or to reduce the amount of cooling required.
A building cannot be heated or cooled suddenly. A
large amount of heat is stored in the great weight of material
making up a building. Before heat from the sun can penetrate a
masonry wall it must first warm up the wall. As a result of this
the maximum rate of heat gain inside a building from sunshine
falling on walls and roofs may occur from 2 to 7 hours after the
. attainment of maximum temperature on the outside surface. The
heavier the construction the greater will be this lag in reaching
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.4
-a l-ag in cooling during the night. Much more direct and rapid
heating inside a building is obtained ｾｲｯｭ sunshine which enters
directly through windows, and from the heat given orf by people, lights, ventilation air, motors, machinery and so on, inside
the building. In any building in which the direct heat gain is
small, it becomes possible to take advantage of the heat storage capacity or the building by keeping the building closed during the day and then, when it becomes hotter inside than out, to open the building, ventilating it at a high rate during the cool
night period so that it will be cooled for the next day. This
method works well in most houses, where a large attic fan can be
installed. It is widely used in houses in the United States and
should be even more acceptable in Canada, where in general the
night temperatures are lower.
In bUildings having high direct heat gains, the
tempera-tures may rise rapidly during the early part or the day. As
soon as the inside temperature reaches that ｯｾ the temperature
of the air available for ventilation there is no advantage in
keeping a bUilding closed, since some cooling can then be provided by bringing in outside air.
Direct solar heat through unshaded windows is one of
the greatest sources or unwanted heat in summer. About 300
Bri tish thermal heat units fall per hour on each square foot of
area exposed at right angles to the sun's rays. This is more
heat than is emitted by one square foot of steam-heated radiator
surface. Heat will be received at this rate on black, flat roofs,
or on pitched roofs facing the sun. Windows, however, are usually
at an angle to the sun so that a heat gain of about one-third or
this amount per square ｾｯｯｴ of window will, on the average, be
more usual. Even this, however, is a very ｾｯｲｭｩ､｡｢ｬ･ amount or
heat. An office 20 by 20 by 10 feet, providing 4,000 cubic feet
ｯｾ space, will have a winter heat requirement on the average of
about 20,000 British thermal units per hour. Such an office
could readily have as much as 60 square. feet ｯｾ glass, through
which the summer heat gain with an exposure to the sun could readily be 6,000 British thermal units per hour, the equivalent
in summer of turning on one-third of the radiators.
Window shading is an important factor in maintaining
summer comfort. Briefly, it is more ･ｾｦ･｣ｴｩｶ･ to stop solar
radiation outside the window by solar overhangs, awnings or solar screening, than to stop it once it has passed through the
glass. Up to
75
per cent of the heat can be stopped by outsideshading, but seldom more than
25
to50
per cent by insideshading de vi ce s.
Heat gains through wall s and roors in summer can be markedly decreased by using light-coloured outside surface
finishes. Light paints are as much or more ･ｦｾ･｣ｴｩｶ･ than
5
-The proper use of ventilation is also important. Just as cool air can be used to carry off heat in summer, so
too, hot ventilating air brings in heat. Using the figures
previously given, hot air brought in at 10 degrees above room temperature at the rate of three air changes per hour may result in a heat gain of 20 per cent or more of the total to
be expected. Ventilation, either through a mechanical system
or by windows, or both, should be used judiciously in all
buildings without mechanical cooling. Briefly, ventilation
should be decreased when outside air temperatures are above those inside, and increased when the inside temperature rises
above that outside. Ventilation with cool night air should,
where possible, be maintained throughout the night to carry off the heat still reaching the inside of the building from heavy
walls and roofs which have been exposed to the sun, and to "empty" the bUilding of heat for the next day.
People also give off a rather remarkable amount of
heat. It is entirely possible in a crowded space such as an
auditorium to have enough heat from the people alone to heat it fUlly in winter, without any heat from the heating system. In summer, then, a hall filled with people may provide, in an extreme case, the equivalent in heat gain of turning on the
heating system at full capacity. Since spaces are to be used,
there is little that can be done to reduce the heat gain from people, but it is important to realize why and when extreme o verbeating, with resulting discomfort, can arise.
Thus far, nothing has been said of moisture and
humidi ty. A building can have moisture gains, leading to
increased humidity, from many sources. These moisture gains,
occurring at the same time as excessive heat gains, may lead to high humidity combined with high temperature and thus may
contribute to discomfort. In complete air conditioning, both
temperature and humidity are controlled by simultaneous cooling
and dehumidification. The question of ventilation without
mechanical cooling need not be complicated by a further consi-deration of humidity, except to note that ventilation may be required at times to carry off moisture alone, or heat and moisture together, in order to reduce humidities.
The effect of humidity on summer comfort is not marked
until temperatures rise. As the air temperature rises approaching
body temperature, the human body, in order to give off heat, must pour out perspiration on the skin to increase cooling by
evaporation. When the humidity in the air is high, the cooling
effect by evaporation is reduced. The effect of air velocity
becomes most marked in increasing cooling effect on the body, by increasing evaporation rate when the skin is wet by
perspira-tion. High air velocity over the skin, even though the air is
above the comfort condition, reduces the feeling ot! discomfort, so long as it is not completely saturated, and is not extremely
· ...Ｌｾ ...
6
-hot. The use ｯｾ individual disk or propellor ｾ｡ｮｳ in rooms,
to increase air velocity around the occupants, even without
other cooling, can be very ･ｾｾ･｣ｴｩｶ･ in minimizing ､ｩｳ｣ｯｾｯｲｴＮ
in It is interesting to note, in considering disk ｾ｡ｮｳＬ
that/the sizes used in ｯｾｾｩ｣･ｳＬ up to 12 inches in diameter, the
air handled may be ｾｲｯｭ .500 to 1.000 cubic ｾ･･ｴ per minute.
The larger of these rates ｯｾ air circulation in an office 20 by
20 by 10 feet <4.000 cu. ft.} amounts to fifteen air changes
per hour. The extent of the air circulation thus provided is
seldom realized.
Finally, sensation of air mo vement is only partl y a ｾｵｮ｣ｴｩｯｮ of velocity; the temperature of the air relative to
skin temperature is also important. A person is conscious of
air movement in rooms largely in terms ｯｾ the heating or cooling
sensations produced on the skin. For this reason "draughts" are
experienced at low velocities i£ the air which is moving is quite cool; while when temperatures are high and little cooling of the skin is produced, quite substantial velocities may not
be noticed. This leads to frequent complaints in summer that
air circulation devices are not working, when in ｾ｡｣ｴ they are, .
in terms of air being moved, although the cooling produced is ,inade quate •
