For public building management units, HVAC system replacement is a systematic engineering project involving technical assessment, budget planning, regulatory compliance, and operational coordination. Unlike new construction projects that can be carried out at leisure on a blank site, replacement projects must accomplish the challenging task of "construction during continued operation." This article takes an engineering practice perspective to systematically analyze six key aspects of public building HVAC replacement -- from scientific assessment of replacement timing, performance diagnostics of existing systems, comparative evaluation of upgrade options, chiller selection and refrigerant transition strategies, non-disruptive construction planning during construction phases, to post-completion energy savings verification -- providing public building managers with a complete decision-making and execution framework.

1. Indicators for Determining HVAC Replacement Timing in Public Buildings

The decision to replace an HVAC system should not rely solely on "equipment age" as the only indicator but must comprehensively consider multiple quantitative and qualitative factors. Premature replacement wastes resources, while delayed replacement leads to escalating energy costs and sharply increased failure risks. Establishing a scientific replacement timing assessment system is the primary task of public building HVAC management.

Equipment Lifespan and Efficiency Degradation Curves

According to ASHRAE equipment lifespan statistics, the median lifespan of centrifugal chillers is approximately 23 years, screw chillers approximately 20 years, while cooling towers and air handling units have lifespans of 15 to 20 years[1]. However, lifespan data only represents the physical durability of equipment; what truly affects replacement decisions is the efficiency degradation curve. Generally, HVAC chillers experience relatively slow efficiency degradation during the first ten years of service (approximately 0.5% to 1% per year), but degradation rates accelerate significantly after fifteen years, with chillers over twenty years old potentially operating at only 60% to 70% of their original rated efficiency[2].

Key Replacement Indicator System

Combining international practical experience with domestic public works characteristics, the following indicators are recommended as replacement timing benchmarks:

  • Energy efficiency degradation rate: Actual operating COP below 60% of equivalent new equipment rated values, or annual electricity cost growth rate exceeding 5% for three consecutive years (excluding electricity price adjustments and usage changes)
  • Maintenance cost ratio: Annual maintenance costs exceeding 15% of equipment replacement cost, or single major repair costs exceeding 30% of replacement cost
  • Refrigerant compliance: Equipment using controlled refrigerants such as R-22, which should be planned for replacement before the refrigerant ban deadline per EPA refrigerant management regulations[3]
  • Spare parts availability: Critical components (compressor bearings, control boards, heat exchanger tube bundles) discontinued or lead times exceeding three months
  • Failure frequency: Unplanned shutdowns exceeding three times per year, or cumulative downtime affecting normal building operations
  • Regulatory compliance: Existing equipment failing to meet current building code[4] requirements for building energy conservation, or not meeting government-announced energy consumption standards

Establishing a Decision Matrix

Public building management units are recommended to establish a replacement decision matrix, scoring the above indicators by weight, with the replacement planning process initiated when the total score exceeds the set threshold. This decision matrix should be updated annually, serving as an objective basis for annual budget preparation and medium-to-long-term capital expenditure planning. For central government agencies, this decision matrix can also serve as supporting documentation for special project funding applications.

2. Existing System Performance Diagnostics and Energy Audit

After confirming the replacement need, the next step is a comprehensive performance diagnosis and energy audit of the existing system. This not only confirms the severity of problems but also establishes baseline data for subsequent solution design -- only by precisely understanding "the current state" can one scientifically predict "the post-upgrade benefits."

System Performance Diagnostic Items

A complete HVAC system performance diagnostic should cover the following technical items:

  • Chiller performance testing: Measuring actual COP values at different load ratios (25%, 50%, 75%, 100%), comparing against original manufacturer rated performance curves
  • Condenser water system diagnosis: Cooling tower heat rejection performance, condenser water temperature differential, water quality conditions, scale and biofilm accumulation
  • Air distribution system inspection: Air handling unit and ductwork system airflow degradation, filter pressure differential, coil efficiency, duct leakage rate
  • Piping system assessment: Chilled water piping insulation deterioration, valve leaks, hydraulic balance conditions, pipe wall corrosion or scale thickness
  • Automatic control system review: Sensor accuracy drift, control valve operation status, DDC controller functional integrity, communication interface compatibility

Energy Audit Methodology

Public building energy audits should follow ASHRAE's three-level energy audit framework[5]. Level 1 is a walk-through audit, primarily consisting of site inspection and utility bill analysis, taking approximately one to two weeks; Level 2 is a standard audit, adding equipment measurement and energy simulation analysis, taking approximately four to eight weeks; Level 3 is a detailed audit, including long-term monitoring and return on investment analysis, taking approximately three to six months. For major investment projects such as chiller replacement, at least a Level 2 energy audit is recommended to obtain reliable baseline data.

Core outputs of the energy audit include: annual HVAC energy consumption baseline (kWh/year), HVAC system energy use intensity (EUI, kWh/m²/year), sub-system energy consumption ratio analysis, identification and prioritization of energy saving opportunities, and payback period estimates. This data not only forms the foundation for replacement solution design but also serves as the comparison benchmark for subsequent verification of energy savings benefits.

3. Replacement Option Comparison: Full Replacement vs. Phased Replacement vs. Improvement Upgrade

Based on performance diagnostics and energy audit results, management units face three primary replacement strategy choices. Each strategy has its applicable scenarios with pros and cons, requiring careful evaluation based on the building's specific conditions and budget constraints.

Option 1: Full Replacement

Full replacement means replacing the entire HVAC system in a single project -- including chillers, cooling towers, air handling units, piping, and control systems -- all with new equipment. The advantages of this approach include: highest system integration, maximum energy efficiency improvement, and simplest subsequent maintenance management. Disadvantages include: highest initial investment, greatest impact on building operations during construction, and the one-time budget requirement placing pressure on public agency financial planning. Full replacement is suitable when equipment has been in service for over twenty years, all subsystems are generally deteriorated, and the piping system has severely degraded.

Option 2: Phased Replacement

Phased replacement divides the overall upgrade project into multiple years or phases. A typical phased strategy is: Phase 1 replaces chillers and cooling towers (the core equipment with the largest energy consumption), Phase 2 updates air handling units and ductwork, and Phase 3 upgrades the automatic control system and optimizes operating strategies. Advantages include: distributed budget pressure, reduced impact on building operations, and the ability to adjust subsequent phases based on earlier phase results. Disadvantages include: total project costs typically higher than a one-time full replacement (due to repeated temporary works and mobilization costs), and the need for careful integration planning between phases.

Option 3: Improvement Upgrade

Improvement upgrade retains existing major equipment while targeting performance bottlenecks for partial improvements. Common improvement measures include: adding variable frequency drives to chilled water and condenser water pumps, replacing high-efficiency cooling tower fill, upgrading the automatic control system to a smart energy management platform, and improving building envelope insulation to reduce HVAC loads. This approach is suitable when equipment is still in mid-economic life, main equipment condition is acceptable, but system operating strategies have significant room for improvement. Improvement upgrade typically has the shortest payback period (two to five years) but limited energy savings potential (typically 10% to 25%).

The selection among these three options should be based on life-cycle cost analysis (LCCA), considering the total cost of ownership over the equipment's service life in the spirit of procurement regulations[6], rather than relying solely on initial investment amount as the decision criterion.

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4. Chiller Selection and Refrigerant Transition (R-22 Phase-Out Response)

The chiller is the heart of a central HVAC system, and its selection directly affects the next fifteen to twenty years of energy consumption and operating costs. The refrigerant transition issue further elevates aging chiller replacement from an "efficiency consideration" to a "regulatory compliance" necessity.

R-22 Refrigerant Phase-Out Timeline and Response

A large number of existing public building chillers in Taiwan still use HCFC refrigerant R-22. Per the Montreal Protocol and Taiwan EPA refrigerant management regulations[3], R-22 production and import have been completely banned. Market R-22 stockpiles are declining annually while prices continue to rise, making R-22 unit maintenance costs increasingly burdensome. More critically, the Kigali Amendment to the Montreal Protocol[7] further sets reduction timelines for high-GWP HFC refrigerants, meaning medium-to-long-term regulatory direction must also be considered when selecting replacement refrigerants.

Next-Generation Refrigerant Options Analysis

Current refrigerant options for public building chillers mainly include:

  • R-134a: GWP of 1,430, currently the most mature option for large centrifugal chillers, but its high GWP means long-term control under the Kigali Amendment
  • R-513A: GWP of 631, a lower-GWP alternative to R-134a, usable in partially retrofitted centrifugal chillers with approximately 3% to 5% performance loss
  • R-1233zd(E): GWP of only 1, an HFO low-pressure refrigerant for next-generation low-pressure centrifugal chillers with excellent energy efficiency but higher equipment unit cost
  • R-32: GWP of 675, suitable for small-to-medium screw chillers with approximately 5% to 10% better energy efficiency than R-410A, but mildly flammable (A2L classification)
  • R-515B: GWP of 293, a next-generation low-GWP option gradually gaining support from mainstream equipment manufacturers

Chiller Type and Capacity Selection

Public building chiller selection should consider: the building's peak cooling load and part-load characteristics, number of chillers and capacity ratio (recommended at least two chillers for both redundancy and part-load efficiency), variable frequency drive adoption (variable-speed centrifugal or screw chillers significantly outperform constant-speed units at part-load conditions), and IPLV (Integrated Part-Load Value) requirements -- public buildings operate at part-load conditions 70% to 80% of the time, making IPLV a better indicator of actual operating efficiency than full-load COP[8].

Per ASHRAE Standard 90.1[5] recommendations, water-cooled centrifugal chillers should have a minimum COP of no less than 6.1 (corresponding to EER of approximately 20.8) at full-load conditions, with IPLV no less than 9.7. Public works specifications should set efficiency thresholds above these baseline requirements.

5. Non-Disruptive Construction Strategy and Risk Management

The greatest challenge in public building HVAC replacement is that the building must maintain normal operations during construction. Courts cannot suspend hearings, hospitals cannot stop treating patients, and government offices cannot cease operations. Therefore, "non-disruptive construction" is not an optional enhancement but the core premise of project planning.

Temporary HVAC Solution Design

Common strategies for maintaining basic building HVAC needs during chiller replacement include:

  • Rental temporary chillers: Installing rental chillers outside the mechanical room, connected to the existing chilled water system via temporary piping to maintain cooling supply. This approach requires advance planning for temporary piping routes, electrical connection points, and condenser water discharge methods
  • Portable air-cooled units: Deploying independent portable HVAC units for critical local areas (such as server rooms, data centers, executive offices) as transitional emergency solutions
  • Phased zone construction: If the building has multiple chillers, a sequential replacement strategy can be adopted, utilizing remaining chillers to maintain partial cooling capacity, supplemented by demand management (such as adjusting thermostat setpoints, reducing HVAC operating hours)

Construction Sequencing and Interface Management

Construction sequencing is critical for non-disruptive work. Best practice schedules major demolition and installation during winter or shoulder seasons (November through March), when HVAC demand is lowest and construction impact on occupants is minimal. Construction sequence planning should include: schedules for old equipment removal and new equipment rigging (coordinating large crane access routes and timing), time windows for piping changeover (typically scheduled during nights or weekends), activation and decommissioning schedules for temporary cooling sources, and reserved time for system commissioning and balancing.

Risk Management Measures

Major risks in HVAC replacement projects include:

  • Unknown existing piping conditions: Piping in older buildings may have discrepancies between drawings and site conditions, wall thinning, and seized valves. Piping surveys should be conducted during the design phase, with reasonable contingency budgets (recommended 10% to 15% of construction costs)
  • Structural loading changes: New chillers may differ in weight and dimensions from old units, requiring structural engineer verification of mechanical room floor slab load capacity
  • Noise and vibration control: Construction noise and vibration must not affect normal building operations, particularly in noise-sensitive facilities such as courts and hospitals
  • Refrigerant leak prevention: Old equipment refrigerant recovery must comply with environmental regulations[3], with atmospheric release prohibited. Refrigerant recovery must be performed by qualified certified professionals

6. Energy Savings Verification and Carbon Reduction Reporting

The investment benefits of HVAC replacement ultimately need to be verified with objective data. For public works, energy savings verification is not only a technical management requirement but also a responsible attitude toward public fund utilization, as well as a concrete action toward implementing government net-zero carbon emission policies.

International Performance Measurement and Verification Protocol (IPMVP)

Energy savings verification should follow internationally recognized measurement and verification protocols. IPMVP (International Performance Measurement and Verification Protocol)[9] provides four verification methods: Option A (Partially Measured Retrofit Isolation), Option B (Retrofit Isolation), Option C (Whole Facility), and Option D (Calibrated Simulation). For chiller replacement projects, Option B or Option C is recommended: Option B precisely calculates actual chiller COP and annual energy consumption through long-term monitoring data from chiller power meters and chilled water flow meters; Option C uses utility bills for the entire building to compare energy use changes before and after replacement.

Verification Period and Data Collection

Energy savings verification should cover at least one full year of operating data to eliminate seasonal variation effects. Data collection during the verification period includes:

  • Chiller side: Chilled water supply/return temperatures, chilled water flow rate, chiller power consumption, condenser water supply/return temperatures
  • System side: Condenser water pump, chilled water pump, and cooling tower fan power consumption
  • Environmental side: Outdoor temperature and humidity, representative indoor zone temperature and humidity
  • Usage side: Building operating hours, occupancy density, special equipment load changes

Through comparison of baseline period (pre-improvement) and reporting period (post-improvement) data, with climate normalization using degree-day or regression analysis methods, actual HVAC system energy savings can be objectively calculated.

Carbon Reduction Reporting and ESG Integration

HVAC system carbon emissions include two aspects: indirect carbon emissions from electricity use (Scope 2) and direct carbon emissions from refrigerant leakage (Scope 1)[10]. Replacing with high-efficiency equipment reduces electricity-related emissions, while switching to low-GWP refrigerants simultaneously reduces refrigerant-related emissions. For example, a 300 RT chiller with COP improved from 4.5 to 6.5, operating 2,000 hours per year, yields annual electricity savings of approximately 145,000 kWh, corresponding to carbon reduction of approximately 72 metric tons CO2e (using 2025 grid emission factor of 0.495 kgCO2e/kWh).

Carbon reduction reporting provides multiple values for public agencies: serving as evidence for annual agency energy conservation and carbon reduction performance, responding to government policy requirements for comprehensive energy conservation measures, providing foundational data for future carbon fee calculations, and offering quantified environmental performance indicators for agency ESG sustainability reports.

Conclusion

Public building HVAC replacement is a comprehensive engineering project integrating technical judgment, financial planning, and operational management. From scientific assessment of replacement timing, comprehensive diagnosis of existing systems, careful comparison of upgrade options, forward-looking selection of chillers and refrigerants, non-disruptive construction strategies, to post-completion energy savings verification -- every phase requires professional engineer involvement and scientific methodology support.

Under the global net-zero carbon emission trend, public building HVAC replacement is no longer just an "old for new" equipment update but a strategic opportunity to redefine building energy efficiency and carbon emission trajectory. Under the hot and humid climate conditions of Kaohsiung and southern Taiwan, HVAC system performance has a particularly significant impact on building operating costs. Only by using life-cycle costs as the decision basis, energy audit data as the design foundation, and international standards as verification methods, can every investment of public resources be converted into long-term and quantifiable energy savings and carbon reduction benefits[11].