Chapter 5: Human Comfor & mechanical Systems Fundamentals
DEFINITIONS
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British Thermal Units (BTU): The amount of heat required to raise the temp of 1lbm of water by 1°
Coefficient of heat transmission: the overall rate of heat flow through any combination of
materials, including air spaces and air layers on the interior and exterior of a building assembly.
Reciprocal of the sum of all the resistances in the assembly.
Used to determine the size of a heating system for a building
U = 1/(R)
Conductance: The number of BTUs per hour that pass through 1sqft of homogeneous material of a
given thickness when the temp differential is 1°F. Reciprocal of Resistance.
Resistance: Number of hours needed for 1BTU to pass through 1sqft of material or assembly of a
given thickness when the temp differential is 1°F. Reciprocal of Conductance.
Conductivity: Number of BTUs per hour that pass through 1sqft of homogeneous material 1in thick
when the temp differential is 1°F.
Dew Point: Water vapor in the air becomes saturated and begins to condense to drops of water
Dry-bulb temperature: The temperature of the air-water mixture as measured with a standard drybulb thermometer
Enthalpy: The total heat in a substance, including latent heat and sensible heat.
Latent Heat: heat that causes a change of state of a substance, such as the heat required to
change water into steam.
How many BTUs to change 1lb of water from freezing to a boil?
212° - 32° = 180°
180° x 1btu = 180btu
Sensible Heat: Heat that causes a change in temperature of a substance but not a change of state.
Insolation: Total solar radiation on a horizontal surface.
Specific Heat: Number of BTUs required to raise the temp of a specific material by 1°f.
Latent heat of evaporation = 1000Btu
HUMAN COMOFRT
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Human Comfort is based on the quality of primary environmental factors:
Temperature, humidity, air movement, temperature radiation to and from surrounding surfaces,
air quality, sound, vibration.
Comfort factors:
Temperature
Humidity
Precipitation
Radiation
Air movement
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Human Metabolism
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Not efficient in conversion of food to energy. It must give off excess heat in order to maintain a
stable body temp.
Body’s heat production is measured in mets: (1.84 BTU/hr-sqft) the energy produced per unit o f
surface area per hour by a seated person at rest.
98.6°: no leat loss from conduction, convection or radiation
When temp rises above body temp, heat flow reverses and evaporation occurs
Adult at rest = 400 BTU/hr
Adult w/ moderate activity = 700 – 800 BTU/hr
Adult w/ strenuous activities = 2000 BTU/hr
People in cooling calculations = qp which is the number of people x BTUs per person
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Body loses heat in three primary ways:
Convection: transfer of heat through the movement of a fluid medium, either a gas or liquid.
When air temperature surrounding a person is less than the body’s skin temp (around 85°). The
body heats surrounding air which rises and is replaced with cool air
Evaporation: When moisture changes to a vapor as a person perspires or breaths
Radiation: transfer of heat energy through electromagnetic waves from one surface to a colder
surface
Conduction: transfer of heat through direct contact
If body cannot loose heat one way, it must loose it another.
If air temp is above the body temp there can be no convection transfer b/c heat always flow
from high level to a low level (second law of thermodynamics)
Air Temperature
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A measure of stored heat energy
Temperature is never transferred, only heat energy
Primary determinant of comfort
General comfort range: 69°F – 80°F
Tolerable range: 60°F – 85°F
Dry-bulb temperature is measured with a standard thermometer
Wet-Bulb temperature is measured with a sling psychrometer
Effective temperature: An index of thermal sensation, not a measure of actual thermometer temp
but a measure of a combination of several comfort indexes
Dry bulb temp
Relative humidity
Radiant energy
convection
Psychrometer: A device that consists of a thermometer with a moist cloth around the bulb.
Thermometer is swung rapidly in air, causing moisture in cloth to evaporate.
In dry air moisture evaporates rapidly and acquires latent heat, which produces a low wet-bulb
temp.
Large difference between wet-bulb and dry-bulb indicates relative humidity
In moist air, less moisture evaporates from the cloth, so wet-bulb temp is higher
Humidity
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Relative humidity is the ratio of the percentage of moisture in the air to the maximum amount the air
can hold at a given temp without condensing.
Comfortable between 30% - 65%
Tolerable between 20% - 70%
Dew point: temperature that water vapor in the air becomes saturated and condenses into droplets
As temp drops air can no longer hold as much water vapor and vapor condenses
Important in summer months b/c as temp rises, body can lose less heat through convection and
must rely on evaporation
However, as humidity rises it is more difficult for perspiration to evaporate, hence one feels hotter.
Air Movement
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Tends to increase evaporation and heat loss through convection
This is why one will feel comfort in high temps & humidites if a breeze is present
Also explains windchill
Surface Temperature
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If surface areas of surroundings are colder than skin (85) the body loses heat through radiation
If surrounding surfaces are warmer, body gains heat.
The rate at which radiation occurs depends on surface temperatures of body and adjacent, the
viewed angle and emissivity
Viewed angle: angle formed between the measuring position and the outer edges of the object.
Emissivity ( ): the ability of a matereial to absorb and then radiate heat.
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Thermal conductivity (k): ability of a material to transmit or conduct heat or electricity. Is the
amount of heat transmitted in one hour thru one sq ft of a 1” thick material for each degree
fahrenheit
The ratio of the radiation emitted by a given object or material to that emitted by a blackbody at
the same temp.
Shiny objects have very low emissivity (shiny foil on insulation to reduce heat transfer)
Mean radiant temperature (MRT): A weighted average of the various surface temps in a room and
Is the average radiant temperature of surroundings and is independent of air temperature the
angle of exposure to the occupant to these surfaces as well as any sunlight present
To determine effects of surface temps on comfort, all room surfaces with their temp and
positions must be taken into account.
As MRT is low, comfort zone shifts towards higher ambient temperatures
Operative Temperature: an average of the air temp of a space and the mean radiant temp (MRT).
It can be measured with a globe thermometer
Globe thermometer: a thermometer inside a black globe which can account for both the air temp
and radiant effects from surrounding surfaces
Specific heat: ability of a material to store heat in relationship to the materials weight
Thermal lag factor: numerical representation of the time it takes radiant heat gain to become
absorbed into room and and become part of lad on the cooling system
Clothing
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Insulator moderating the effects of conduction, convection and radiation.
Clo: unit to quantify the effects of clothing on the person, equal to the typ business suit…0.15
Clo/lbm (.80 BTU)
Ventilation
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Required to provide oxygen and remove carbon dioxide, odors and contaminants
Building codes specify min. fresh outdoor air that must be circulated (in CFMs)
Mech systems are designed to filter and recirculate much of the conditioned air and also too
introduce a percentage of outdoor air
Where exhausting of air is required building codes specify minimum exhaust rates per square foot of
floor area or how often complete air change must be made. Must exhaust directly to outside. NO
recirculation
Toilets
Kitchens
Spaces where noxious fumes are present
MEASUREMENT SYSTEMS
Comfort Chart
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Shows the relationships among temperatures, humidity and other comfort factors
With tolerable humidity levels between 20 – 70% and preferred levels between 30% - 65%,
comfort chart show hat as humidity increases, air temp must decrease to provide same amount of
comfort as felt with lower humidity levels.
As temp drops below recommended levels, radiation in the form of sunshine or mechanical radiation
is needed to maintain comfort
The lower the temp, the more radiation is required.
As humidity and temperature increase, air movement is required to maintain comfort
Psychrometric Chart
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Shows air at different temperatures and humidities
Used to graph total of energy stored in air called enthalpy (sensible and latent heat)
To cool and simultaneously dehumidify requires removing both forms of stored heat so only
need to look at change in enthalpy
Psychrometry: the study of water vapor content of air
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Sling psychrometer: instrument that measures relative humidity or wet bulb temperature and is
composed of dry and wet bulb thermometers. Difference between two thermometers is compared
to determine relative humidity
A Graphical representation of the complex interactions between heat, air and moisture
A way to calculate the amount of heat and moisture that must be added or removed by the
HVAC system.
To determine the dew point of the moisture in the air to avoid condensation of interior surfaces and
inside the building.
EXTERNAL AND INTERNAL LOADS
A building must resist either the loss of heat to the outside during cold weather or gain of heat in hot weather.
Any excess heat gain or loss must be compensated for with passive energy conservation or with mechanical
heating or cooling
• External factors that cause heat loss: air temp and wind
• External factors that cause heat gain: air temperature and sunlight.
• Internal factors: people, lights and equipment
• Heat is transferred between the outside and the inside through conduction, convection and radiation
Conduction: transfer of heat through direct contact
Convection: transfer of heat through movement of air
Radiation: transfer of heat energy through electromagnetic waaves from ne surface to a colder
surface
Heat Loss Calculations
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Heat is lost through the envelope and through infiltration
Every material has its own unique property of conductivity
Degree Day: temp difference between a 24 hur ext temp and an intreior temp of 65°
Design Day: A day hotter or colder than 98% of the rest of the year
Conductivity (k): amount of heat lost through 1sqft of a 1” thickness of a material when the
temperature differential is 1°F
Conductance (C): the same property, but when the material is a thickness other than 1”
Resistance (R): is the number of hours needed for 1 BTU to pass through a material of a given
thickness when temperature differential is 1°F
Conductance and resistance are related by:
R = 1/C
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Values for k, C and R are given in standard reference texts & in the American Society of Heating,
Refrigerating and Air-Conditioning Engineers (ASHRAE) Handbook of Fundamentals
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U-value: combined conductance of an assembly
Is reciprocol of the sum of resistances
When building assembly consists of more than one material, the value used to calculate heat
loss is the coefficient of heat transmission (U). However the value of U is not the sum of all
conductances if individual materials. Instead must be calculated by :
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U = 1/R
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For an entire area of one type of material, this value is multiplied by the total area to get the total
heat loss. This formula is:
qc = UAt
qc = U(A)(TINSIDE – TOUTSIDE)
qc = U(A)24(DD)
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In order to calculate the heat loss for an entire room or building, the heat losses of all the different
types of assemblies must be determined and added.
Example: if summation of the R-values (R) of the assembly is 20.15, then the overall coefficient of
transmission is:
U = 1/R
= 1/20.15 BTU/hr-ft2-°F
= 0.05 BTU/hr- ft2-°F
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The value for t is determined by subtracting the outdoor design temperature from the desired
indoor temperature in the winter, (70°). Outdoor design temperatures vary with geographical region
and are found in the ASHRAE Handbook or are set by local codes
Important aspect of heat loss calculations and the use of the psvhchrometric chart is to determine
the dew point of the moisture in the air to avoid condensation of interior surfaces and inside the
building.
Example: air at 70 and 35% relative humidity has a dew point of 41F. Moisture will condense
on surfaces at or below this temperature
Example 2: if outdoor temp is 0F and indoor temp is 70F somewhere inside the wall assembly
the temp is 41F or less. Water vapor from inside the building permeating the construction would
condense on this surface, damaging the wood construction and possibly negating the
effectiveness of the insulation.
vapor barrier must be placed on warm side of the insulation.
Heat Gain/Cooling Load Calculations
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Heat is produced by radiation of the sun on glazing, by building occupants, lighting and equipment.
Each factor varies with occupancy:
Residence = dominated by gains from envelope & glazing
Large office = large no. of occupants, large no. of lights and equipment.
Heat loss through bldg envelope calculated similar to heat loss using the overall coefficient of heat
transmission and the area of the building assembly (q = UAt). However temp differential is not
used directly. Instead a value known as design equivalent temperature difference (DETD)
DETD accounts for air temp differences, sun effects, thermal mass storage, colors and daily
temp range. Values are published in ASHRAE.
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Heat gain through glazing (qr or SHGF) = multiply area of glazing by the design cooling load
factor (DCLF)
Values are published in ASHRAE
Heat gain from people (qp): number of people x BTU per person
Total sensible heat = multiply number of occupants by 225 BTU/hr
Occupants produce sensible heat and latent heat in form of moisture from perspiration and
breathing. 225 BTU/hr = sensible heat gain from occupants.
Heat gains through lighting (qi): = multiply total wattage load of bldg by 3.41.
One watt = 3.41 BTU/hr
Qi = 3.4(W)
Where: W = wattage of equipment
Fluorescent & discharge lights w/ elec ballasts = multiply fixtures individual BTU/hr by 1.25
Heat gain from equipment (qm):
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qm = 1500 x Bhp
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Where: Bhp = Brake horse power (2545 BTUs)
High mass materials – mitigates effects of heat gain from solar radiation & air temp
Masonry, concrete and tile slow transmission of heat.
Day: absorb heat energy and store it
Night: as temp drops below surface of the mass, energy lost to atmosphere instead of into bldg
Cooling load temperature differential: related to conduction and radiation
Factors including
Mass and storage capacity
Color
Orientation
Total heating load: using all factors
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roof)
QTOTAL = qc + qs + qi
Conductance (qc) may consist of several “sub” qc’s, one for each surface (wall, floor,
Infiltration
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Heat gain through infiltration when outside temps are high.
Transfer of air into and out of a bldg thru doors, cracks around windows, flues and vents.
Heat gain thru infiltration:
qi = V(1.08)t
Step 1: determine amount of air infiltration
Step 2: determine amount of heating/cooling required to bring air to desired
temperature
1.08 BTU-min/ft3 = Specific heat of air (amount of heat that air at a certain density can hold
V = calculated via air lost through cracks, doors openings, etc (estimated with use of tables)
Air changes: 6 – 10
Required to know number of air changes per hour (Qcfh)
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Qcfh = N x V
Where: N = number of air changes
V = building volume in ft3
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Crach method: based on the number of linear feet of crack
Example: 3’ x 6’ window = 3’ + 6’ + 3’ + 6’ = 18’
Double hung would have to add 3’ for intermediate
Amount of infiltration is determined by a table which considers windspeed and window type
Value is then multiplied:
Qcfh = LF x CFH/lin.ft.
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Finally, the amount of heating or cooling required may be calculated by:
qi = .018(Qcfh) )t
= .018(Qcfh) (TINSIDE – TOUTSIDE)
Temperature gradients: temperature change across a single material
tlayer = (RLAYER/RTOTAL)
CLIMATIC TYPES AND DESIGN RESPONSES
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Mechanical systems w/o regard to passive design strategies appropriate to local climate:
Increases cost
Wastes energy
Contributes to pollution
Ignores desirable regional characteristics of architecture
USA = four basic climate types/zones
Cool: Canada, Northern middle USA & Mountainous regions of WY & CO
Temperate: middle latitudes, NW & NE
Hot-humid: SE
Hot-arid: SoCal to SW Texas
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Cold climates: Minimize surface area to reduce heat loss
Cubical shape
Partial underground
Minimize Northern exposure
Minimize windows & doors
Landscaping & bldg design should block winter winds
Due to lack of direct sunlight in winter, passive solar not appropriate
Mechanical heating & active solar heating required
Temperate climates: significant heatloss in winter
Minimize Northern exposure
Block winter winds
Solar heat gain in winter desirable bldg length should be oriented East and West to max
Southern exposure.
Summer, same south face = deciduous trees for shading & mechanical devised (awnings)
To mitigate effects of daytime heating, provide nighttime venting
Solar heating – active and passive work well
Hot-humid climates: most difficult to design for w/o mechanical cooling
Plan form max natural ventilation
Narrow floor plans w/ cross ventilation
Large open windows, porches breezeways
Shade with vegetation
Double roof
Thermally light weight so as to not store daytime heating & release at night
Hot-Arid climates:
Shading from direct sunlight
Diurnal effect – wide variations bet day and night temp
Materials with high thermal mass
Evaporative cooling with pools
Roof ponds provide evaporative cooling & high thermal mass
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