Roofs, External Walls & Conduction through Glass
The equation used for sensible loads from the opaque elements such as walls, roof, partitions and the conduction through glass is:
H = U * A * (CLTD)
Where
• H describes Sensible heat flow (Btu/Hr)
• U = Thermal Transmittance for roof or wall or glass. See 1997 ASHRAE Fundamentals, Chapter 24 or 2001 ASHRAE Fundamentals, chapter 25. (Unit- Btu/Hr Sq-ft °F)
• A = area of roof, wall or glass calculated from building plans (sq-ft)
• CLTD = Cooling Load Temperature Difference (in °F) for roof, wall or glass. For winter months CLTD is ( Ti - T0 ) which is temperature difference between inside and outside. For Summer cooling load, this temperature differential is affected by thermal mass, daily temperature range, orientation, tilt, month, day, hour,latitude, solar absorbance, wall facing direction and other variables and therefore adjusted CLTD values are used. Refer 1997 ASHRAE Fundamentals, Chapter 28, tables 30, 31, 32, 33 and 34.
Solar Load through Glass, Skylights and Plastic Sheets
Heat transfer through glazing is both conductive and transmission. It is calculated in two steps:
Step # 1
The equation used for sensible loads from the conduction through glass is:
H = U * A * (CLTD)
Where
• H = Sensible heat gain (Btu/Hr)
• U = Thermal Transmittance for roof or wall or glass. See 1997 ASHRAE Fundamentals, Chapter 24 or 2001 ASHRAE Fundamentals, chapter 25. (Unit- Btu/Hr Sq-ft °F)
• A = area of roof, wall or glass calculated from building plans (sq-ft)
• CLTD = Cooling Load Temperature Difference (in °F) for glass. Refer 1997 ASHRAE
Fundamentals, Chapter 28, tables 30, 31, 32, 33 and 34.
Step # 2
The equation used for radiant sensible loads from the transparent/translucent elements such as window glass, skylights and plastic sheets is:
H = A*(SHGC)*(SC)*(CLF)
Where
• H = Sensible heat gain (Btu/Hr)
• A = area of roof, wall or glass calculated from building plans (sq-ft)
• SHGC = Solar Heat Gain Coefficient. See 1997 ASHRAE Fundamentals, Chapter 28, table 35
• CLF = Solar Cooling Load Factor. See 1997 ASHRAE Fundamentals, Chapter 28, table 36.
Partitions, Ceilings & Floors
The equation used for sensible loads from the partitions, ceilings and floors:
H = U * A * (Ta - Tr)
Where
• H = Sensible heat gain (Btu/Hr)
• U = Thermal Transmittance for roof or wall or glass. See 1997 ASHRAE Fundamentals, Chapter 24 or 2001 ASHRAE Fundamentals, and Chapter 25. (Unit- Btu/Hr Sq-ft °F)
• A = area of partition, ceiling or floor calculated from building plans (sq-ft)
• Ta = Temperature of adjacent space in °F (Note: If adjacent space is not conditioned and temperature is not available, use outdoor air temperature less 5 ° F)
• Tr = Inside room design temperature of conditioned space in °F (assumed constant usually 75°F)
Ventilation & Infiltration Air
Ventilation air is the amount of outdoor air required to maintain Indoor Air Quality for the occupants (refer ASHRAE Standard 62 for minimum ventilation requirements) and makeup for air leaving the space due to equipment exhaust, exfiltration and pressurization.
Hsensible = 1.08 * CFM * (T0 – Tc)
Hlatent = 0.68 x CFM x ▲WGR
Hlatent = 4840 x CFM x▲W Lb
Htotal = 4.5 * CFM * (ho – hc)
Htotal = H sensible + H latent
Where
• H sensible = Sensible heat gain (Btu/hr)
• Hlatent = Latent heat gain (Btu/hr)
• Htotal = Total heat gain (Btu/hr)
• CFM = Ventilation airflow rate in cubic feet per minute
• To = Outside dry bulb temperature, °F
• Tc = Dry bulb temperature of air leaving the cooling coil, °F
• ▲WGR = Humidity Ratio Difference (Gr H2O/Lb of dry air) = (WO – WC)
• ▲WLB = Humidity Ratio Difference (Lb H2O /Lb of dry air) and = (WO – WC)
• WO = Outside humidity ratio, Lb H2O per Lb (dry air)
• WC = Humidity ratio of air leaving the cooling coil, Lb H2O per Lb (dry air)
• hO = Outside/Inside air enthalpy, Btu per lb (dry air)
• hC = Enthalpy of air leaving the cooling c oil Btu per lb (dry air) Refer to 1997 ASHRAE Fundamentals , Chapter 25, for determining infiltration
People
The heat load from people is both sensible load and the latent load. Sensible heat is transferred through conduction, c onvection and radiation while latent heat from persons is transferred through water vapor released in breathing and/or perspiration. The total heat transferred depends on the activity, clothing, air temperature and the number of persons in the building.
H se ns ib le = N * (H S ) * (CLF)
H la t en t = N * (H L)
Where
• H se ns ib le = Total Sensible heat gain (Btu/hr)
• H la t en t = Total latent heat gain (Btu/hr)
• N = number of people in space.
• HS, HL = Sensible and Latent heat gain from occupancy is given in 1997 ASHRAE
Fundamentals Chapter 28, Table 3 (Btu/hr per person depending on nature of activity)
• CLF = Cooling Load Factor, by hour of occupancy. See 1997 ASHRAE Fundamentals,
Chapter 28, table 37.
Note: CLF = 1.0, if operation is 24 hours or of cooling is off at night or during weekends.
The sensible heat influence on the air temperature and latent heat influence the moisture content of indoor space.
Lights
The lights result in sensible heat gain.
H = 3.41 * W * F UT * F BF * (CLF)
Where
• H = Sensible heat gain (Btu/hr)
• W = Installed lamp watts input from electrical lighting plan or lighting load data
• F BF = Lighting use factor, as appropriate
• F BF = Blast factor allowance, as appropriate
• CLF = Cooling Load Factor, by hour of occupancy. See 1997 ASHRAE Fundamentals,
Chapter 28, Table 38.
Note: CLF = 1.0, if operation is 24 hours or if cooling is off at night or during weekends .
Power Loads & Motors
Three different equations are used under different scenarios:
a. Heat gain of power driven equipment and motor when both are located inside the space to be conditioned
H = 2545 * (P / Eff) * F UM * F L M
Where
• H = Sensible heat gain (Btu/hr)
• P = Horsepower rating from electrical power plans or manufacturer’s data (HP)
• Eff = Equipment motor efficiency, as decimal fraction
• F UM = Motor us e factor (normally = 1.0)
• F UM = Motor load factor (normally = 1.0)
• Note: F UM = 1.0, if operation is 24 hours
b. Heat gain of when driven equipment is loc ated inside the space to be conditioned space and the motor is outside the space or air stream
H = 2545 * P * F UM * F L M
Where
• H = Sensible heat gain (Btu/hr)
• P = Horsepower rating from electrical power plans or manufacturer’s data (in HP)
• Eff = Equipment motor efficiency, as decimal fraction
• F U M = Motor us e factor
• F L M = Motor load factor
• Note: F UM = 1.0, if operation is 24 hours
c. Heat gain of when driven equipment is located outside the space to be conditioned space and the motor is inside the space or air stream
H = 2545 * P * [(1.0-Eff)/Eff] * F UM * F L M
Where
• H = Sensible heat gain (Btu/hr)
• P = Horsepower rating from electrical power plans or manufacturer’s data (HP)
• Eff = Equipment motor efficiency, as decimal fraction
• F U M = Motor us e factor
• F L M = Motor load factor
• Note: F UM = 1.0, if operation is 24 hours
Appliances
H = 3.41 * W * F u * F r * (CLF)
Where
• H = Sensible heat gain (Btu/hr)
• W = Installed rating of appliances in watts. See 1997 ASHRAE Fundamentals, Chapter 28; Table 5 thru 9 or use manufacturer’s data. For computers, monitors, printers and miscellaneous office equipment, see 2001 ASHRAE Fundamentals, Chapter 29, Tables 8,9 & 10.
• F u = Usage factor. See 1997 ASHRAE Fundamentals, Chapter 28, Table 6 and 7
• F r = Radiation factor. See 1997 ASHRAE Fundamentals, Chapter 28, Table 6 and 7
• CLF = Cooling Load Factor, by hour of occupancy. See 1997 ASHRAE Fundamentals,
Chapter 28, Table 37 and 39. Note: CLF = 1.0, if operation is 24 hours or of cooling is off at night or during weekends.
Conductive Heat Transfer
Conductive heat flow occurs in the direction of decreasing temperature and takes place when a temperature gradient exists in a solid (or stationary fluid) medium. The equation used to express heat transfer by conduction is k nown as Fourier’s Law and is expressed as:
H = k x A x ▲T / t
Where
• H = Hat transferred per unit time (Btu/hr)
• A = Heat transfer area (ft2)
• k = Thermal conductivity of the material (Btu/ (hr0F ft2/ft))
• ▲T = Temperature difference across the material (°F)
• t = material thickness (ft)
R-Values/U-Values
R = 1/ C = 1/K x t
U = 1/ ∑R
Where
• R = R-Value (Hr Sq-ft °F/Btu)
• U = U-Value (Btu/Hr Sq-ft °F)
• C = Conductance (Btu/hr Sq-ft °F)
• K = Conductivity (Btu in/ hr Sq-ft °F)
• ∑R = Sum of the thermal resistances for each component used in the construction of the wall or roof section.
• t = thickness (ft)
Notes: The lower the U-factor, the greater the material's resistance to heat flow and the better is the insulating value. U-value is the inverse of R-value (hr sq-ft °F /Btu).
Heat Loss by Conduction & Convection through Roof & Walls
Heat loss by conduction and convection heat transfer through any surface is given by:
H sensible = A * U * (Ti – To)
Where
• H = heat transfer through walls, roof, glass, etc. (Btu/hr)
• A = surface areas (sq-ft)
• U = air-to-air heat transfer coefficient (Unit- Btu/Hr Sq-ft °F)
• Ti = indoor air temperature (°F)
• To = outdoor air temperature (°F)
Heat Loss through Floors on Slab
The slab heat loss is calculated by using the following equation:
H = F* P * (T i -T o)
Where:
• H = Sensible heat loss (Btu/hr)
• F = Heat Los s Coefficient for the particular construction and is a function of the degree days of heating. (Unit- Btu/Hr Sq-ft °F)
• P = Perimeter of slab (ft)
• T i = Inside temperature (°F)
• T o = Outside temperature (°F)
Heat loss from slab-on- grade foundations is a function of the slab perimeter rather than the floor area. The losses are from the edges of the slab and insulation on these edges will significantly reduce the heat losses.
Heat loss through Infiltration and Ventilation
The heat loss due to infiltration and controlled natural ventilation is divided into sensible and latent losses. The energy associated with having to raise the temperature of infiltrating or ventilating air up to indoor air temperature is the sensible heat loss, which is estimated by:
H sensible = V * рair * Cp * (Ti – To)
Where:
• H sensible = Sensible heat loss
• V = volumetric air flow rate
• рair is the density of the air
• Cp = specific heat capacity of air at constant pressure
• Ti = indoor air temperature
• To = outdoor air temperature
The energy quantity associated with net loss of moisture from the space is latent heat loss which is given by:
Hlatent = V * рair * hfg * (Wi – Wo)
Where
• Hlatent = Latent heat loss
• V = volumetric air flow rate
• рair is the density of the air
• Wi = humidity ratio of indoor air
• Wo = humidity ratio of outdoor air
• hfg = latent heat of evaporation at indoor air temperature
