Major Factors in HVAC Design

Thermal Insulation

A substantial reduction in heating and cooling loads can be made by the judicious use of thermal insulation in wall and roof construction. Addition of insulation results in an increase in thermal resistance R, or a reduction in the coefficient of heat transfer U of the walls and roof.
Any material with high resistance to flow of heat is called insulation. Many kinds of insulation materials are used in building construction. See Art. 12.3.
Note that the maximum overall conductance U encountered in building construction is 1.5 Btu/ (hr)(ft2)(F). This would occur with a sheet-metal wall. The metal has, for practical purposes, no resistance to heat flow. The U value of 1.5 is due entirely to the resistance of the inside and outside air films. Most types of construction have U factors considerably less than 1.5.
The minimum U factor generally found in standard construction with 2 in of insulation is about 0.10.
Since the U factor for single glass is 1.13, it can be seen that windows are a large source of heat gain, or heat loss, compared with the rest of the structure. For double glass, the U factor is 0.45. For further comparison, the conductivity k of most commercial insulations varies from about 0.24 to about 0.34.


Heating by natural convection is very common, because air very easily transfers heat in this manner. As air becomes warmer, it becomes less dense and rises. As it leaves the proximity of the heating surface, other cooler air moves in to replace the rising volume of heated air. As the warm air rises, it comes in contact with cooler materials, such as walls, glass, and ceilings. It becomes cooler and heavier and, under the influence of gravity, begins to fall. Hence, a circulatory motion of air is established, and heat transfer occurs.
When a heating device called a convector operates in a cool space, heat from the convector is transmitted to the cooler walls and ceiling by convection. The convection process will continue as long as the walls or ceiling are colder and the temperature difference is maintained.
Heating of building interiors is usually accomplished with convectors with hot water or steam as the heating medium. The heating element usually consists of a steel or copper pipe with closely spaced steel or aluminum fins. The convector is mounted at floor level against an exterior wall. The fins are used to greatly increase the area of the heating surface. As cool room air near the floor comes in contact with the hot surfaces of the convector, the air quickly becomes very warm and rises rapidly along the cold wall surface above the convector. Additional cold air at floor level then moves into the convector to replace the heated air. In this manner, the entire room will become heated. This process is called heating by natural convection.


The most common form of heat transfer is by radiation. All materials and substances
radiate energy and absorb radiation energy.
The sun is a huge radiator and the earth is heated by this immense source of
radiated energy, which is often called solar energy (sunshine). Solar-collector de

vices are used to collect this energy and transfer it indoors to heat the interior of
a building.
When radiation from the sun is intercepted by walls, roofs, or glass windows,
this heat is transmitted through them and heats the interior of the building and its
occupants. The reverse is also true; that is, when the walls are cold, the people in
the space radiate their body heat to the cold wall and glass surfaces. If the rate of
radiation is high, the occupants will be uncomfortable.
Not all materials radiate or absorb radiation equally. Black- or dark-body materials
radiate and absorb energy better than light-colored or shiny materials. Materials
with smooth surfaces and light colors are poor absorbers of radiant energy
and also poor radiators.
Much of the radiation that strikes the surface of window glass is transmitted to
the interior of the building as short-wave radiation. This radiation will strike other
objects in the interior and radiate some of this energy back to the exterior, except
through glass, as a longer wavelength of radiation energy. The glass does not efficiently
transmit the longer wavelengths to the outside. Instead, it acts as a check
valve, limiting solar radiation to one-way flow. This one-way flow is desirable in
winter for heating. In summer, however, it is not desirable, because the longerwavelength
energy eventually becomes an additional load on the air-conditioning
The rate of radiation from an object may be determined by use of the Stefan-
Boltzmann law of radiation. This law states that the amount of energy radiated
from a perfect radiator, or a blackbody, is proportional to the fourth power of the
absolute temperature of the body. Because most materials are not perfect radiators
or absorbers, a proportionality constant called the hemispherical emittance factor is
used with this law. Methods for calculating and estimating radiation transfer rates
can be found in the ASHRAE Handbook Fundamentals.
The quantity of energy transferred by radiation depends on the individual temperatures
of the radiating bodies. These temperatures are usually combined into a
mean radiant temperature for use in heating and cooling calculations. The mean
radiant temperature is the uniform temperature of a block enclosure with which a
solid body (or occupant) would exchange the same amount of radiant heat as in
the actual nonuniform environment.

Thermal Criteria for Building Interiors

There are three very important conditions to be controlled in buildings for human
comfort. These important criteria are dry-bulb temperature, relative humidity, and
velocity or rate of air movement in the space.
Measurements of these conditions should be made where average conditions
exist in the building, room, or zone and at the breathing line, 3 to 5 ft above the
floor. The measurements should be taken where they would not be affected by
unusually high heat sources or heat losses. Minor variations or limits from the
design conditions, however, are usually acceptable.
The occupied zone of a conditioned space does not encompass the total room
volume. Rather, this occupied zone is generally taken as that volume bounded by
levels 3 in above the floor and 6 ft above the floor and by vertical planes 2 ft from
Indoor design temperatures are calculated from test data compiled for men and
women with various amounts of clothing and for various degrees of physical exertion.
For lightly clothed people doing light, active work in relatively still room
air, the design dry-bulb temperature can be determined from

When temperatures of walls, materials, equipment, furniture, etc., in a room are
all equal, t = tr = 75F. With low outside temperature, the building exterior becomes
cold, in which case the room temperature should be maintained above 75F to
provide the necessary heat that is being lost to the cold exterior. In accordance with
Eq. (13.22), the design dry-bulb temperature should be increased 1.4F for each
1F of mean radiant temperature below 75F in the room. In very warm weather,
the design temperature should be decreased correspondingly.
Humidity is often controlled for human comfort. Except in rare cases, relative
humidity (RH) usually should not exceed 60%, because the moisture in the air may
destroy wood finishes and support mildew. Below 20% RH, the air is so dry that
human nostrils become dry and wood furniture often cracks from drying out.
In summer, a relative humidity of 45 to 55% is generally acceptable. In winter,
a range of 30 to 35% RH is more desirable, to prevent condensation on windows
and in walls and roofs. When design temperatures in the range of 75F are maintained
in a space, the comfort of occupants who are inactive is not noticeably
affected by the relative humidity.
Variations from the design criteria are generally permitted for operational facilities.
These variations are usually established as a number of degrees above or below
the design point, such as 75F DB +- 2F. For relative humidity, the permitted
variation is usually given as a percent, for example, 55% RH  5%.
Design conditions vary widely for many commercial and industrial uses. Indoor
design criteria for various requirements are given in the Applications volume of
the ASHRAE Handbook.

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