1. Conduction Cooling Loads

2. What You Need to Know
Describe the components that make up a cooling load
Understand the fundamental differences between heating and cooling loads

3. What You Need to be Able To Do
Calculate simple conduction cooling loads
Evaluate systems to identify energy savings

4. Terms Cooling load Total Equivalent Temperature Differential (TETD)
Storage effect
Time Lag
Thermal mass

5. Cooling Load “The amount of energy that must be
removed from a space in order to maintain the space within the comfort zone.”
I. Cooling Load
A. “The amount of energy that must be removed from a space in order to maintain the space within the comfort zone.”
B. Same essential definition as heating load, only in the
opposite direction
C. Notice we are still building on our definition of comfort.

6. Good News! Same ‘R’ values Same ‘U’ values
Same conduction heat transfer
Same convection heat transfer
Same radiation heat transfer
I. Good News!
A. Same ‘R’ values
B. Same ‘U’ values
C. Same conduction heat transfer
D. Same convection heat transfer
E. Same radiation heat transfer
II. Fundamentally, it is not any different from our study in heating loads

7. Cooling Load Components
roof
lights
partition
wall
people
infiltration
glass solar
equipment
glass
conduction
exterior
wall
floor

8. Sensible and Latent Gains
load
latent
load
cooling load components
conduction through roof, walls, windows,
and skylights
solar radiation through windows, skylights
conduction through ceiling, interior
partition walls, and floor
people
These load components contribute sensible and/or latent heat to the space. Conduction through the roof, exterior walls, windows, skylights, ceiling, interior walls, and floor, as well as the solar radiation through the windows and skylights, all contribute only sensible heat to the space.
The people inside the space contribute both sensible and latent heat. Lighting contributes only sensible heat to the space, while equipment in the space may contribute only sensible heat (as is the case for a computer) or both sensible and latent heat (as is the case for a coffee maker). Infiltration generally contributes both sensible and latent heat to the space.
The cooling coil has to handle the additional components of ventilation and system heat gains. Ventilation contributes both sensible and latent heat to the coil load. Other heat gains that occur in the HVAC system (from the fan, for example) generally contribute only sensible heat.
lights
equipment/appliances
infiltration
ventilation
system heat gains

9. Major Differences from Heating Loads
Peak conditions
Heat storage effect
Consideration of both latent and sensible gains
More unique sources of heat gain
I. Major Differences from Heating Loads
A. Peak conditions are usually different
1. Depends on the amount of glass and the
direction of glass, but usually from noon to
5:00 P.M.
2. This means that ΔT is variable!
B. Heat storage effects
1. The total heat is not immediately transferred
into the space. In other words, there is a time
lag between when the heat is generated and
when the heat actually reaches the room.
Sometimes, the heat doesn’t reach the room until
after the peak occurs!
C. Consideration of both latent and sensible heat gains
D. More unique sources of heat gains

10. Time of Peak Cooling Load
east-facing
window
roof
heat gain
One of the more difficult aspects of estimating the maximum cooling load for a space is determining the time at which this maximum load will occur. This is because the individual components that make up the space cooling load often peak at different times of the day, or even different months of the year.
For example, the heat gain through the roof will be highest in the late afternoon, when it is warm outside and the sun has been shining on it all day. Conversely, the heat gain due to the sun shining through an east-facing window will be highest in the early morning when the sun is rising in the east and shining directly into the window.
Determining the time that the maximum total space cooling load occurs will be discussed later in this clinic.
mid
a.m.
noon
p.m.
mid

11. Storage Effect (thermal lag)

12. Thermal Mass Dilemma

13. Time Lag solar effect 12 6 12 6 12 time lag B A mid a.m. noon p.m. mid
The walls and roof that make up a building’s envelope have the capacity to store heat energy. This property delays the heat transfer from outdoors to the space. The time required for heat to be transferred through a structure into the space is called the time lag.
For example, the heat that is transferred through a sunlit wall into a space is the result of sunlight that fell on the outer surface of the wall earlier in the day. Curve A shows the magnitude of the solar effect on the exterior wall. Curve B shows the resulting heat that is transferred through the wall into the space. This delay in the heat gain to the space is the time lag. The magnitude of this time lag depends on the materials used to construct the particular wall or roof, and on their capacity to store heat.
mid
a.m.
noon
p.m.
mid

14. Conduction – Sunlit Surfaces
Total Equivalent Temperature Difference (TETD) is used to account for the added heat transfer due to the sun shining on exterior walls, roofs, and windows, and the capacity of the wall and roof to store heat. The TETD is substituted for T in the equation for conduction.
Q = U  A  T
TETD

15. Conduction Gains (Walls and Roofs and doors)
Q = U x A x TETD
where TETD is the Total Equivalent Temperature Differential, which accounts for
Temperature difference
Mass
Color
Solar Gain
I. Conduction Gains (Walls and Roofs)
A. The formula for this is:
Q = U x A x CLTD where CLTD is the Cooling Load
Temperature Differential
B. The cooling load temperature differential accounts for
1. The storage effect
2. Unstable and highly variable temperatures

16. Step #1 – Select Wall Type

17. Step #2 Select Sun time Select color of wall Select wall orientation
D = dark
L = light
Select wall orientation

18. Step #3 – Read Value of TETD

19. Same Steps for Roofs

20. For Windows Btuh = (U x A x TD) + (A x SC x SHGF) A= Area
TD = outdoor design – indoor design temp.
SC = shading coefficient
SHGF = solar heat gain factors

21. EQ Credit 8.1 - Daylighting
ASHRAE Standard 90.1
10% lighting load credit for harvesting
10% lighting load credit for occupancy sensors

22. Heat Gain from People A function of activity
Always contains both sensible and latent components

24. Equipment - Office Best obtained from manufacturers
Can be reduced by using Capture Hood
Usually is sensible, but may have a latent component

25. Equipment Loads (ASHRAE Fundamentals)

26. Ventilation Load Must consider both sensible and latent loads
QS = 1.1 x CFM x (T2 – T1)
QL = .68 x CFM x (W2 – W1)
QT = QS + QL