Optimizing HVAC Life-Cycle Performance

by Grahame E. Maisey, P.E. and Beverly Milestone, LEED AP
Building Services Consultants, Inc.

Last updated: 02-20-2007

Introduction

HVAC system design, construction, and ongoing adjustments require large capital investments. Yet, current HVAC system design practices often do not meet an owner's expectations for energy or maintenance efficiency, nor occupants' requirements for comfort. Without HVAC design optimization, even HVAC systems in sustainable buildings with extensive commissioning may fail to meet predicted energy efficiency and suffer deteriorating energy and maintenance performance over a short period. A Life-Cycle Performance Assessment will allow a HVAC system to be selected and sized for the whole life cycle performance, not just start-up performance.

HVAC design optimization for life-cycle performance is an engineering practice that considers a system's energy and maintenance performance and occupant impacts, throughout the life of the system. This practice can produce overall construction savings of up to 5%* and annual operation savings of over 50%*, as well as contribute to increased occupant productivity. For most buildings, sustaining HVAC system performance throughout the life of the system has proved elusive. As such, long-term efficiency and performance must be addressed at the beginning of a project as part of an integrated, or "whole building", approach so that the HVAC design process both includes and optimizes long-term operating and energy performance strategies. See also WBDG Functional/Operational Branch.

This Resource Page summarizes the process for optimizing HVAC life-cycle performance, and includes a case study on the long-term performance characteristics of HVAC systems and their long-term effects on occupant productivity.

Description

HVAC system selection is usually made on a first cost/first operating efficiency basis. However, the object of any HVAC system should be to provide optimum indoor comfort at reasonable costs over the life of the building and system. As such, by optimizing the life-cycle performance of HVAC systems—rather than focus on their first costs and efficiencies—building owners will be able to maximize their capital investment in HVAC systems and controls and produce buildings that are initial and life-cycle cost effective. The goal of HVAC life cycle performance optimization is to provide an indoor environment that allows for maximum occupant effectiveness while minimizing energy, maintenance and operating costs. See also Functional—Meet Performance Objectives.

A. Process for Optimizing HVAC System Life-Cycle Performance

HVAC system life-cycle performance optimization begins at the very beginning of a project. Close team collaboration is required to develop the client's short, medium, and long-term goals, objectives, and expectations related to:

Indoor thermal comfort for occupant effectiveness (which includes considerations such as ambient temperature control, radiant temperature control, humidity, ventilation, air cleanliness and purity, lighting quality, visual stimulation, sound control and acoustics);
Functional performance of HVAC systems; and
Performance requirements for present and long-term operation of HVAC and energy systems.

NOTE: Actual long-term operating characteristics can be very different from initial operating characteristics; and long-term needs can be very different for each building situation, particularly when long-term goals are introduced.

These performance expectations must be written down plainly in the design documents so what the clients want cannot be misinterpreted or misrepresented. These Detailed Design Intent Documents (DDI) are developed early in the design process and include a detailed narrative of the performance-based design intent (the client's immediate and long-term goals, objectives, and aspirations, including HVAC performance expectations and requirements) with a basis of design and details of system selections (the design team's detailed responses in system and component selection and sizing) as they relate to long-term operation and maintenance requirements. Every how and why of system performance criteria and selection is documented for the design, construction, and operating personnel. The DDI takes into account future foreseeable changes and modifications in the building usage and describes how the proposed systems will work through these scenarios. The DDI becomes the living document that chronicles the building planning, design development, construction, operation, modifications, and performance.

The next step in the HVAC life-cycle performance optimization process is providing supervision throughout HVAC system design and installation, and during testing, balancing, start-up, and turnover. See also Project Management Project Planning & Development and Project Delivery & Controls. Revisiting the project throughout the first year's operation ensures that the HVAC system is finely tuned to operate optimally through all the seasons.

In this way, HVAC life-cycle performance optimization provides continuity throughout the design, construction, and operation stages of the building, and should be an ongoing process throughout the life of the building, providing annual certification where necessary.

To measure the effect of a new indoor environment on client satisfaction, an evaluation of occupants' effectiveness/satisfaction in their current space(s) is necessary to establish a baseline for comparison. A post-occupancy evaluation of facility performance and user satisfaction with the new environment will determine if the client's expectations for improvement have been met and, if not, what needs to be adjusted.

Degradation of HVAC system performance during the first five years of a building's operation is a major problem facing most buildings. HVAC design optimization assures that system and component selection are made with long-term performance as one of the parameters. This means the systems' operation and performance expectations will be maintainable throughout the life of the building.

If done correctly, HVAC systems, in combination with lighting and acoustics, can increase occupant effectiveness by over 25%*, which can pay for a portion—if not all—of the construction costs. E-Source reports that just one year of the salaries of workers in a typical office building equals twice the entire cost of the building and 100 times the annual electricity bill. Also, a productivity increase of 10% creates a 30% increase in a company's bottom line.

B. Features of HVAC Life-Cycle Performance Optimization

HVAC life-cycle performance optimization differs from traditionally practiced HVAC design in three primary areas:

A Detailed Design Intent. Developing a written Detailed Design Intent as standard practice will improve communication between owner, designers, contractors and operators many times over. Because a detailed explanation is required for any of the decisions that are made, every answer is totally thought out. This document helps reduce construction costs, operating costs and increases occupant performance.
Long-term operational considerations. Including minor modifications, maintenance and operational efficiencies over the longer term. These will often cause a change in system selection because systems that are generally chosen by other methods will not compare as favorably to long-term and modification-friendly requirements. These help reduce long-term operating and modification costs considerably.
Detailed Post-Occupancy Evaluation, aka Facility Performance Evaluation. The standard measurements of air-flow in ductwork and temperatures of air and water systems only indicate a small part of the performance of a buildings HVAC system. Building Occupancy Data Analysis is a standard way of assessing occupant reaction to the indoor environment.

C. Three Common Sense Rules of Thumb of Long-Term Maintenance

To understand long-term operating characteristics it is important to understand the common sense rules of thumb governing long-term maintenance—because systems that are not maintained correctly cannot perform at their peak. These rules of thumb were what the old time steam designers considered essential design strategies for maintenance, but have since been forgotten by mechanical system designers.

The Rules of Thumb of Long Term Maintenance are:

All moving parts must be in plant rooms.
If it is not easy to maintain; it will not be maintained.
Maintenance manpower and effort will decrease over time.

The three rules of thumb above were used by many design/construct HVAC engineers in the years prior to World War II (WWII). As HVAC systems became much more sophisticated after WWII, design engineers became separated from construction, and construction became separated from maintenance, resulting in three degrees of separation from design to operation. Consequently, long-term maintainability became entirely the problem of the operating and maintenance staff. The case study below illustrates how application of the rules of thumb plays out for various HVAC systems.

Application

HVAC life-cycle performance optimization is applicable to any new building design project or any major remodeling project. It assures long-term performance of HVAC systems and supports the building commissioning process. HVAC design optimization is an important aspect of sustainable design and helps facilitate LEED® certification of an owner's project.

A. Case Study

Example: A 200,000ft² office block in Philadelphia

Typical total construction costs for an office block = $110/ft²*, or $22M.
Amortized over 20 years = $2.2M/year or $11.00/ft²*.
Typical HVAC construction costs = $11.50/ft²* or $2.3M.
Typical electrical construction costs = $9.60/ft²* or $1.92M.
Typical plumbing construction costs = $4.50/ft²* or $0.9M.

Initial energy costs = $2.50/ft²y*. Initial HVAC energy costs = $1.60/ft²y*
Initial maintenance costs = $1.50/ft²y*. Initial HVAC maintenance costs = $1.10/ft²y*
Occupant wages and benefits: Assume an average of $200.00/ft²y*.
Running costs of the building = $215.00/ft²y*.
Adding taxes and a business profit of 12%* gives an occupant productivity of $250/ft²y*.
A 1% change in occupant efficiency is worth (or costs) $2.50/ft²y.

Table 1. Maintainability, Flexibility, and Adaptability - 200,000FT² Office Block, Comparison of Four HVAC Systems*

	VAV WITH REHEAT	MULTI-ZONE MULTI-UNIT	UFAD (UNDER FLOOR AIR DISTRIBUTION)	OPTIMIZED HVAC SYSTEM PRODUCTIVE, FLEXIBLE, EFF.
Maintainability
#1 Rule of Thumb. All moving parts in plant rooms	Very bad, VAV boxes and coils hidden in ceilings.	All parts in plant rooms.	All parts in plant rooms.	All parts in plant rooms, apart from individual controls.
#2 Rule of Thumb. Easy to maintain	Difficult, VAV boxes & fan control.	Easy, but lots of moving equipment.	Easy to maintain. Floor grille positioning difficult.	Easy to maintain.
#3 Rule of Thumb. Maintenance requirements with aging	Large number of boxes and coils hidden in ceiling.	None	None	None
Intensity and level of maintenance required	High intensity, high level required.	Low level, low intensity.	Low level, low intensity.	Low level, low intensity.
Flexibility & Adaptability
Adding/Moving conference room	Can be expensive, depending on location.	Can be expensive, depending on location.	Costly and difficult. System not made for cellular offices at all.	Easy. System adaptable and flexible.
Rehabbing whole floor to office cells	Quite costly. Complete ductwork modifications possible.	Reasonably costly. Complete ductwork modifications possible.	Very costly and difficult. System bad with cell offices.	Easy & inexpensive. System adaptable and flexible.
Rehabbing building for reuse, medical offices, surgeries	Costly. Major modifications to ductwork, difficult to re-commission.	Reasonably costly. Major modifications to ductwork.	Total rehab. System bad with cell offices.	Inexpensive, adaptable & flexible.

Table 1. This Table illustrates the effect of the Rules of Thumb of Maintainability on HVAC systems. These overlooked rules cost owners billions of dollars each year. Flexibility of systems is essential in the life-cycle of buildings because most buildings will change in use over time and require modification of HVAC systems.

Table 2. HVAC Systems Productivity - 200,000FT² Office Block, Comparison of Four HVAC Systems*

	VAV WITH REHEAT	MULTI-ZONE MULTI-UNIT	UFAD (UNDER FLOOR AIR DISTRIBUTION)	OPTIMIZED HVAC SYSTEM PRODUCTIVE, FLEXIBLE, EFF.
Comfort and Productivity
Outside Air Control	Poor	Good	Good	Very Good
Air Recirculation Control	Poor	Good	Good	Very Good
Air Movement Control	Poor	Good	Good	Very Good
Air Cleanliness Control	Good	Good	Good	Very Good
Air Temp Control	Good	Good	Good	Very Good
Radiant Temp Control	None	None	Good	Very Good
Personal Control	None	None	Some	Very Good
Summer Humid Control	Poor	Good	Good	Very Good
Winter Humid Control	Poor	Poor	Poor	Very Good
Room Noise	Low	Good	Low	Low
AHU Noise	Low	Good	Low	Low
Low Load Comfort	Poor	Good	Good	Very Good
Spring/Fall Comfort	Poor	Good	Good	Very Good
Summer Comfort	Poor	Good	Good	Very Good
Winter Comfort	Poor	Poor	Good	Very Good
Overall Comfort	Poor	Good	Good	Very Good
Productivity Affect %	1% Decrease	1% Increase	3% Increase	8% Increase
Productivity Finances
Product Change $/ft²	$2.50/ft² decrease	$2.50/ft² increase	7.50/ft² increase	20.00/ft² increase
80% Occupancy Effect	$0.4M/y decrease	$0.4M/y increase	$1.2M/y increase	$3.2M/y increase
80% Occupancy Effect 4/ft²	$2.00/ft² decrease	$2.00/ft² increase	6.00/ft² increase	16.00/ft² increase

Table 2. This Table illustrates detailed parameters for comfort and productivity assessment. Shown are the average long-term results that the systems produce in real world application. Maintainability is a big factor in the HVAC systems ability to retain initial productivity.

Table 3. Long-Term Energy Use Characteristics - 200,000FT² Office Block, Comparison of Four HVAC Systems*

	VAV WITH REHEAT	MULTI-ZONE MULTI-UNIT	UFAD (UNDER FLOOR AIR DISTRIBUTION)	OPTIMIZED HVAC SYSTEM PRODUCTIVE, FLEXIBLE, EFF.
Energy Use Profiles
Electrical Demand	High	High	Low	Min
Electrical Use	Ave	Ave	Low	Min
Gas or Heat Demand	Ave	Ave	Low	Min
Free Cooling	Ave	Ave	Ave	Max
Heat Recovery	Ave	Ave	Good	Max
Thermal Storage	None	None	Some	Max
Low Load Use	High	Ave	Low	Min
Low Load Efficiency	Poor	Ave	Good	Max
Spring/Fall Use	Medium	Ave	Low	Min
Spring/Fall Efficiency	Low	Ave	Good	Max
Summer Use	Medium	Ave	Low	Min
Summer Efficiency	Low	Ave	Good	Max
Winter Use	Medium	Ave	Low	Min
Winter Efficiency	Low	Ave	Good	Max
Overall Efficiency	Poor	Ave	Good	Max
Electrical Costs	Base Case	5% Increase	25% Savings	80% Savings
Gas or Heat Costs	Base Case	2% Increase	20% Savings	45% Savings
Renewable Energy Use	None	None	Good	Max
R.E. Adaptability	Poor	Poor	Decent	Easy

Table 3. Maintainability is a great leveler to systems that start out efficient but have high maintenance requirements.

Table 4. Installation & Modification Costs - 200,000FT² Office Block, Comparison of Four HVAC Systems*

	VAV WITH REHEAT	MULTI-ZONE MULTI-UNIT	UFAD (UNDER FLOOR AIR DISTRIBUTION)	OPTIMIZED HVAC SYSTEM PRODUCTIVE, FLEXIBLE, EFF.
Installation Cost
Distribution Systems	$1.1M	$1.1M	$0.8M	$2.1M
Central Units	$0.25M	$0.2M	$0.25M	$0.25M
Cooling Plant	$0.6M	$0.6M	$0.5M	$0.4M
Boiler Plant	$0.15M	$0.15M	$0.15M	$0.05M
Controls	$0.2M	$0.15M	$0.15M	$0.2M
Extra Building Costs	0	0	$0.65M	$0.25M
Total First Costs	$2.3M	$2.2M	$2.5M	$3.6M
Annual Mortgage Cost	$0.23M	$0.22M	$0.25M	$0.36M
Cost/ft²year	$1.15/ft²year	$1.10/ft²year	$1.25/ft²year	$1.80/ft²year
Upkeep Costs
Ave. Maintenance 10 years	$0.85/ft²year	$0.82/ft²year	$0.78/ft²year	$0.38/ft²year
Operating Costs	$0.32/ft²year	$0.30/ft²year	$0.29/ft²year	$0.15/ft²year
Total Upkeep Costs	$1.17/ft²year	$1.12/ft²year	$1.07/ft²year	$0.53/ft²year
Conference Room Move
Total Costs	$0.05M	$0.03M	$0.1M	$0.01M
Total Ft² Cost	$0.25/ft²	$0.15/ft²	$0.50/ft²	$0.05/ft²
Frequency of Move	Every two years	Every two years	Every two years	Every two years
One Floor Rehab. Cost
Total Floor Rehab	$0.25M	$0.2M	$0.5M	$0.05M
Total Ft² Cost	$1.25/ft²	$1.00/ft²	$2.50/ft²	$0.25/ft²
Frequency of Rehab	Every ten years	Every ten years	Every ten years	Every ten years
Complete Rehab. Cost
Total Costs	$1.1M	$1M	$2M	$0.2M
Total Ft² Cost	$5.50/ft²	$5.00/ft²	$10.00/ft²	$1.00/ft²
Frequency of Move	Every twenty years	Every twenty years	Every twenty years	Every twenty years

Table 4. Installation cost of an optimized life cycle performance HVAC design is a worst-case scenario that could be reduced to within 10% of the UFAD costs with close design integration. HVAC system flexibility and adaptability is an expensive omission during the life cycle of a building. Designing distribution systems that are very flexible and adaptable will save modification costs and time.

Table 5. Operating Costs - 200,000FT² Office Block, Comparison of Four HVAC Systems*

	VAV WITH REHEAT	MULTI-ZONE MULTI-UNIT	UFAD (UNDER FLOOR AIR DISTRIBUTION)	OPTIMIZED HVAC SYSTEM PRODUCTIVE, FLEXIBLE, EFF.
First Year
Mortgage Cost	$1.15/ft²year	$1.10/ft²year	$1.25/ft²year	$1.80/ft²year
Energy Cost	$1.60/ft²year	$1.60/ft²year	$1.20/ft²year	$0.45/ft²year
O&M Cost	$1.10/ft²year	$1.05/ft²year	$1.00/ft²year	$0.50/ft²year
Total Annual Cost/ft²	$3.85/ft²year	$3.75/ft²year	$3.45/ft²year	$2.75/ft²year
Total Annual Costs	$0.77M/year	$0.75M/year	$0.69M/year	$0.55M/year
Total Costs to Date	$0.77M	$0.75M	$0.69M	$0.55M
Productivity to Date	$0.4M Loss	$0.4M Gain	$1.2M Gain	$3.2M Gain
Total Benefit to Date	Base	$0.81M	$1.68M	$3.82M
Tenth Year
Mortgage Cost	$1.15/ft²year	$1.10/ft²year	$1.25/ft²year	$1.80/ft²year
Energy Cost	$1.80/ft²year	$1.80/ft²year	$1.40/ft²year	$0.45/ft²year
O&M Cost	$1.20/ft²year	$1.15/ft²year	$1.10/ft²year	$0.54/ft²year
Total Annual Cost/ft²	$4.15/ft²year	$4.05/ft²year	$3.75/ft²year	$2.79/ft²year
Average Annual Costs	$0.80M/year	$0.79M/year	$0.72M/year	$0.554M/year
Conference Rooms	$0.25M	$0.15M	$0.5M	$0.05M
Floor Rehab	$0.25M	$0.2M	$0.5M	$0.05M
Total Costs to Date	$8.5M	$8.25M	$8.2M	$5.64M
Productivity to Date	$4M Loss	$4M Gain	$12M Gain	$32M Gain
Total Benefit to Date	Base	$8.1M	$16.8M	$38.9M
Twentieth Year
Mortgage Cost	$1.15/ft²year	$1.10/ft²year	$1.25/ft²year	$1.80/ft²year
Energy Cost	$1.80/ft²year	$1.80/ft²year	$1.40/ft²year	$0.45/ft²year
O&M Cost	$1.20/ft²year	$1.15/ft²year	$1.10/ft²year	$0.54/ft²year
Total Annual Cost/ft²	$4.15/ft²year	$4.05/ft²year	$3.75/ft²year	$2.79/ft²year
Average Annual Costs	$0.815M/year	$0.80M/year	$0.73M/year	$0.556M/year
Conference Rooms	$0.5M	$0.3M	$1.0M	$0.01M
Floor Rehab	$0.5M	$0.4M	$1.0M	$0.01M
Reuse Rehab	$0.75M	$0.7M	$1.0M	$0.02M
Total Costs to Date	$18.05M	$17.4M	$17.6M	$11.52M
Productivity to Date	$8M Loss	$8M Gain	$24M Gain	$64M Gain
Total Benefit to Date	Base	$16.2M	$33.6M	$78.5M
Thirtieth Year
Total Mortgage Cost	$4.6M	$4.4M	$5M	$7.2M
Total Energy Cost	$10.5M	$10.5M	$8.1M	$2.7M
Total O&M Cost	$7.05M	$6.75M	$6.45M	$3.18M
Total Modification Cost	$2.25M	$1.75M	$4M	$0.5M
Total Costs to Date	$24.4M	$23.4M	$23.55M	$13.58M
Productivity to Date	$12M Loss	$12M Gain	$36M Gain	$96M Gain
Total Benefit to Date	Base	$24M	$49M	$119M

Table 5. Construction costs of a HVAC system is a small part of total system costs. Wages and productivity of the occupants represent the highest investment.

Relevant Codes and Standards

Energy Policy Act of 2005 (PDF 1.9 MB, 550 pgs)
AABC Commissioning Guideline
ASHRAE Hand Books
ASHRAE Guidelines for Commissioning HVAC Systems
CIBSE Commissioning Guides
NEBB Building Systems Commissioning

Additional Resources

WBDG

Building / Space Types

Applicable and relevant to all building types and space types

Organizations/Associations

ASHRAE—A leading organization in the development of standardized commissioning guidelines
Building Commissioning Association—A leading professional association for membership and certification of building commissioning practitioners
Commissioning Specialist's Association (UK)
U.S. Green Building Council

Related Agencies and Websites

California Commissioning Collaborative—A group of government, utility, and building-services professionals committed to developing and promoting commissioning practices in California
Energy Design Resources—Sponsored by Pacific Gas and Electric Company, San Diego Gas & Electric, Southern California Edison, and Southern California Gas
Federal Energy Management Program—Offers programs and resources for energy efficiency in operation of federal facilities
Oregon Department of Energy—Contains information on the benefits of Commissioning, case study, tool kit of new and existing commissioning application materials, and the full text of Commissioning for Better Buildings in Oregon
Portland Energy Conservation, Inc.

Publications

Air Conditioning Applications and Design by Arnold Jones. 1980.
Architecture and Energy by Stein. Anchor Press, 1978.
Commissioning for Better Buildings in Oregon by Oregon Office of Energy / PECI, 1997.
Creating the Productive Workplace by Ed. Clements-Croome. E & FN Spon, 2000.
Design for Maintainability Guidebook. Construction Industry Institute, 1997.
Energy and Environment in Architecture by Baker and Steemers. E & FN Spon, 2000
The Energy Design Handbook by Ed. Donald Watson. Washington, DC: AIA Press, 1993.
The Idea of Building by Steven Groak, E & FN Spon, 1992.
Indoor Air Quality Handbook by Ed. Spengler. McGraw-Hill, 2000.
Inside Out by Brown, Haglund, Loveland. New York, NY: John Wiley & Sons, Inc., 1982.
Practical Thermal Design in Buildings by Peter Burberry. Batsford Press, 1983.

* All cost information in this Resource Page is derived from the authors' personal experience assessing over 2,500 HVAC systems with instrumented tests and over 900 boiler and chiller plants with instrumented tests.