CHAPTER 4
HVAC LOAD MANAGEMENT
QUESTIONS AND ANSWERS
This Chapter offers guidelines for load management in order that building HVAC systems be
more economical to install and less costly to operate. Strategies include techniques for
architectural design to reduce loads, strategies for managing ventilation, designing efficient
lighting, and appliance load control.
More importantly, load control is considered in the context of Indoor Environmental Quality
(IEQ). Many strategies can effectively reduce loads, but have a negative impact on the quality of
space and on occupant productivity. Similarly, there are strategies which might actually increase
loads, but be economically advantageous due to improvements in occupant productivity
associated with higher IEQ.
The Chapter uses an Example building to demonstrate techniques. It is a square floor plan, three
story office building of 30,000 ft2 detailed in compliance with ASHRAE Standard 90.1, Climate
Zone 4. The base load, energy consumption and utility cost are estimated for comparison with
results from modifications. Various modifications are made to the design, and new loads, energy
and utility estimates performed. Cost difference is also estimated between the base and
modification for life cycle cost analysis.
Since the range of load management measures is fairly comprehensive, they can be applied to
different buildings. A good class project assignment consists of having students place
themselves in the position of a corporate manager of design and construction. In this role they
are assigned to providing guidance to the architect and engineer of a proposed building for which
they have submitted a preliminary design. The assigned building can be composed by the
instructor or could be an actual building on campus.
CHAPTER 4
HVAC LOAD MANAGEMENT
QUESTIONS AND ANSWERS
4.1 Rank these cooling loads in order of their magnitude for typical midrise office building
(highest to lowest): wall, conduction; window conduction; window solar; roof.
4.2 Rank these loads in order of their magnitude for a 1 acre single-story air-conditioned
warehouse (highest to lowest): wall, conduction; window conduction; window solar;
roof.
4.3 If the air-conditioning system for a 200,000 ft2 building is estimated to be 400 ft2 per
ton and to cost $4000 per ton, then what will be the system cost?
4.4 If, through load management, the system capacity for the building in Question 4.3 is
reduced by 10%, will the system cost be reduced by 10%? If not, why?
4.5 As an outside research exercise, briefly describe the methodology of energy estimating by
(a) The Degree-Day Method, (b) Equivalent Full Load Hours (EFLH), (c) Bin Analysis,
(d.) Hour-by-Hour Energy Simulation.
4.6 Why is the average annual cost per kWh higher for air-conditioning than for lighting
or appliances?
4.7 If the average annual heating efficiency were 90%? and gas cost were $0.50, what would
be the cost per mmBtu of heating load?
4.8 If the cost per mmBtu from Question 4.7 were used to analyze savings by load
management, rather than the value used in this chapter, would the load management
options be more or less attractive economically? By what percent?
4.9 If a load management option were suspected to create discomfort in the building, what
would be your recommendation on possible implementation? Why?
4.10 If a commercial office building could, hypothetically, be faced with all the windows
oriented in one direction what would the best orientation be? Why? What would the
worst orientation be? Why?
4.11 What sort of external shading devices could be considered to mitigate solar loads on
south walls? East and west walls?
4.12 Using tinted or reflective glass can reduce solar heat gains. What is the primary caution
in applying these features?
4.13 Why is passive solar heating more effective for residential buildings than for
commercial buildings?
4.14 A five-story 120,000 ft2 office building could have plan dimensions of 100’ X 240’ or
150’ x 160’. Which would you recommend for the sake of load management? Why?
4.15 If the architect insisted on the plan for Question 4.14, how would you counsel
him on orienting the building?
4.16 Example 4.1 compares two options for orienting glass on the Example Building.
Assume that natural gas is used for heating and electric produced by coal is used for
cooling. How would the economic results change if the social cost of CO2 were
included in the analysis at $25 per ton? Use values from Table 1.1.
1.1 we can estimate that approximately 2 lbs. CO2 will be released for each kWh if the
electric is produced by a mixture of gas fired and coal fired plants. CO2 will be 29,000
4.17 If the architect insisted on having more glass than the prescriptive percentage
guidelines in ASHRAE Std. 90.1, what issues should he explain to the owner? List
possible pros and cons.
4.18 How would you counsel a building owner who wants to double the wall insulation levels
cited as prescriptive requirement in ASHRAE Std. 90.1?
4.19 You are designing an environmentally responsive house for yourself (Midwest), and are
willing to use different materials on walls facing different orientations. What would be
your strategy for thermal mass; that is, which walls (north, south, east, west) would you
make heavy thermal mass? Which light thermal mass? Why?
North walls have very low air-conditioning loads compared with other orientations and that
the effects of increasing thermal mass are minimal; use light thermal mass.
For other orientations, a lot depends on the occupancy schedule. South walls with low
thermal mass exhibit maximum load in the afternoon and contribute fully to the peak block
4.20 What building types benefit most from controlling ventilation loads? Why?
4.21 Name three methods for managing ventilation loads. Which would be most appropriate
for a building with constant occupancy? Which would be most appropriate for a
building with varying occupancy?
(1) Heat recovery from exhaust is most effective for buildings with constant occupancy and
4.22 Why is LED lighting so effective at reducing cooling loads compared with fluorescent?
4.23 In life cycle economic evaluation different load management concepts should have
different life periods for analysis purposes. What would you recommend as the period
of analysis for a building envelope option? A mechanical system feature? A lighting
system?
4.24 As lighting becomes more efficient over time, what will be the effect on feasibility of
lighting load management concepts such as occupancy and daylight controls?
4.25 You are Energy Manager for a large corporation. Last year you received an award by
saving 10% of the headquarters energy usage by such things as lowering thermostats
3°F in winter, raising thermostats 3°F in summer, and removing lamps in fixtures,
which reduced illumination levels by 20%. The building now costs $1.80 per ft2 per
year, a reduction of 10% from the prior year’s $2.00 per ft2 per year. Did you really
deserve an award? Why or why not?
