Design Development Phase

The Design Development (DD) phase represents the critical transition from conceptual design to a fully coordinated, detailed engineering solution. During this phase, HVAC systems evolve from schematic representations into precisely defined systems with equipment selections, distribution layouts, control strategies, and coordination with all building disciplines.

DD Phase Objectives

The primary objectives of the Design Development phase include:

System Refinement

Finalize HVAC system configurations and zoning strategies
Complete equipment selection with manufacturer-specific data
Size all major distribution components (ductwork, piping, terminals)
Develop comprehensive control sequences and strategies

Interdisciplinary Coordination

Coordinate mechanical systems with architectural, structural, electrical, and plumbing designs
Resolve spatial conflicts and establish routing corridors
Coordinate ceiling heights, shaft locations, and equipment room layouts
Integrate mechanical systems with building envelope performance

Documentation Development

Produce DD drawings at 60-75% completion level
Develop outline specifications for major equipment and systems
Prepare cost estimates within ±10-15% accuracy
Document design decisions and basis of design updates

Owner Review and Approval

Present detailed design solutions to owner for review
Obtain approvals on equipment selections and system configurations
Confirm compliance with owner’s project requirements (OPR)
Address value engineering opportunities

Design Development requires refinement of preliminary load calculations to reflect actual building design conditions.

Updated Input Parameters

Finalized building envelope specifications (wall assemblies, glazing performance, roof construction)
Actual window-to-wall ratios and orientation-specific glazing properties
Interior finish schedules affecting thermal mass and surface reflectance
Confirmed occupancy densities and operational schedules
Lighting power densities from electrical design
Process load verification from owner and equipment vendors

Calculation Methodology

Perform room-by-room load calculations using ASHRAE transfer function method or radiant time series
Account for diversity factors in simultaneous cooling loads
Calculate heating loads at design outdoor conditions with appropriate safety factors
Determine ventilation airflow requirements per ASHRAE 62.1 using actual occupancy and space classifications
Calculate latent loads from occupants, outdoor air, and internal moisture sources

Load Documentation

Summarize peak heating and cooling loads by zone and system
Document critical assumptions and design conditions
Identify zones with special requirements (high internal gains, humidity control, temperature precision)
Provide load breakdown showing envelope, ventilation, lighting, equipment, and occupancy components

Equipment Selection and Specification

Equipment selection during DD phase transitions from budgetary sizing to manufacturer-specific selections with performance data.

Central Plant Equipment

Chillers require detailed selection based on:

Cooling capacity at design entering and leaving water temperatures
Part-load performance characteristics (IPLV, NPLV per AHRI 550/590)
Refrigerant type and compliance with environmental regulations
Physical dimensions, rigging weights, and clearance requirements
Sound power levels and vibration characteristics
Control interfaces and communication protocols
Efficiency compliance with ASHRAE 90.1 or local energy codes

Boilers must be specified with:

Heating capacity at design supply and return water temperatures
Turndown ratio and modulation capabilities
Thermal efficiency at full and part-load conditions (AFUE, combustion efficiency)
Fuel type, supply pressure requirements, and consumption rates
Venting requirements (flue diameter, materials, termination clearances)
Condensing or non-condensing operation characteristics
Integrated controls and safety devices

Air Handling Equipment

Air handling units require complete schedules specifying:

Airflow capacity at design external static pressure
Coil selections with entering/leaving conditions, rows, fins per inch, face velocity
Filter specifications (MERV rating, pressure drop, dimensions, quantity)
Fan type (plenum, housed, class), motor horsepower, drive arrangement
Heat recovery device performance (effectiveness, pressure drop, bypass dampers)
Economizer dampers, actuators, and minimum outdoor air provisions
Access sections, insulation specifications, casing construction (single/double wall)
Drain pan materials, slope, and connections

Terminal devices must be scheduled with:

VAV box airflow range (minimum and maximum CFM)
Inlet size and connection type (round, rectangular, flexible)
Control type (pressure independent, pressure dependent)
Reheat coil capacity and configurations (hot water, electric)
Actuator specifications (spring return, electronic, control signal type)
Sound ratings (NC levels at minimum and maximum flow)

Pumps and Hydronic Components

Pump selections require:

Flow rate (GPM) at design head (feet of water)
Impeller diameter, speed, and trim
Motor horsepower, voltage, and efficiency class
Pump curve with operating point clearly marked
NPSH requirements and verification of available NPSH
Seal type, materials of construction, and coupling type
Variable speed drive requirements and control integration

Ductwork Design and Sizing

Ductwork distribution systems must be sized to deliver required airflow while maintaining acceptable velocities, static pressures, and acoustic performance.

Sizing Methodology

Equal friction method provides balanced pressure drop:

Select friction rate (typically 0.08-0.15 in. w.g. per 100 ft)
Calculate duct diameter or dimensions for each section based on airflow
Verify velocity limits (main ducts: 1,200-2,000 FPM; branches: 800-1,200 FPM; final runouts: 600-800 FPM)
Check total system pressure drop against fan capacity

Static regain method for constant static pressure at takeoffs:

Size ducts to maintain velocity pressure reduction equal to friction losses
Downstream ducts increase in size to reduce velocity
Provides uniform static pressure for branch takeoffs
Preferred for systems with numerous branches or long runs

Duct Construction Standards

Specify duct construction per SMACNA standards:

Pressure class (0.5", 1", 2", 3", 4", 6", 10" w.g.)
Reinforcement requirements (maximum unsupported spans)
Sealing class (Seal Class A, B, or C per SMACNA)
Insulation specifications (internal or external, thickness, facing)
Joint types (TDC, TDF, flanged, welded)

Acoustic Considerations

Control noise generation and transmission:

Limit duct velocities to maintain acceptable NC levels
Specify sound attenuators at air handler discharge and zone entry points
Calculate required attenuation by octave band
Avoid abrupt transitions causing turbulence and regenerated noise
Specify acoustic duct lining where required (1" or 2" thickness)

Piping Design and Sizing

Hydronic piping systems require proper sizing to deliver design flow rates while maintaining acceptable pressure drops and flow velocities.

Sizing Criteria

Pipe sizing based on friction loss and velocity limits:

Friction rate: 1-4 feet of head per 100 feet of pipe (typical: 2-2.5 ft/100 ft)
Velocity limits for noise control: 4-8 FPS for pipe sizes under 2", up to 10 FPS for larger pipes
Return piping typically one to two sizes larger than supply
Critical circuits (long runs, high flow) require detailed pressure drop analysis

System Types and Configurations

Primary-secondary pumping systems:

Primary loop serves production equipment (chillers, boilers) at constant flow
Secondary loop serves building loads with variable flow
Decoupler piping section connects loops with minimal pressure drop
Provides hydraulic separation allowing independent flow control

Variable primary flow systems:

Single loop serving both production and distribution
Chillers or boilers arranged in parallel with individual isolation valves
Minimum flow bypass ensures equipment minimum flow requirements
Requires careful control sequence to avoid low-flow conditions

Pipe Material Selection

Specify appropriate materials based on service:

Chilled water: Steel (Schedule 40, black), copper (Type L), CPVC for smaller sizes
Hot water heating: Steel (Schedule 40, black), copper (Type L)
Condenser water: Steel (Schedule 40, black), 304/316 stainless for corrosive conditions
Steam: Steel (Schedule 40, black) with appropriate pressure ratings
Condensate return: Steel (Schedule 80 for vented, Schedule 40 for pressurized)

Control System Design

Control sequences developed during DD phase provide the framework for system operation and energy management.

Control Architecture

Define control system hierarchy:

Building automation system (BAS) architecture and communication protocols (BACnet, Modbus, LON)
Network topology (distributed controllers, field panels, workstation locations)
Integration with existing BAS or standalone operation
Data point requirements (physical points, virtual points, trending capacity)

Sequence Development

Air handling unit control sequences must address:

Supply air temperature reset based on zone demand or outdoor air temperature
Economizer operation with differential enthalpy or dry-bulb comparison
Minimum outdoor air control with CO2 or occupancy-based demand control ventilation
Morning warm-up and night setback strategies
Freeze protection for cooling coils (discharge air low-limit, mixed air low-limit)
Filter status monitoring and alarm generation
Smoke detection response and fan shutdown/override

VAV system sequences include:

Zone temperature control with airflow modulation (cooling mode)
Minimum airflow setpoints (ventilation requirement or 30% of maximum, whichever is greater)
Reheat coil staging or modulation when airflow reaches minimum
Static pressure reset based on zone damper positions (trim and respond logic)
Supply air temperature reset based on zone with highest cooling demand
Occupied/unoccupied setpoint changes and scheduling

Chilled water plant sequences specify:

Chiller staging based on load (lead-lag-lag configuration typical)
Condenser water temperature optimization and tower fan staging
Chilled water supply temperature reset based on outdoor air or return temperature
Primary/secondary pump control and flow verification
Free cooling operation if applicable (waterside economizer)
Equipment rotation for equal run-time distribution
Optimal start algorithms for equipment pre-cooling

Control Valve and Damper Sizing

Size control devices for proper authority and rangeability:

Control valves: Select Cv based on pressure drop across valve equal to 25-50% of total circuit pressure drop
Valve authority (N) = ΔP_valve / (ΔP_valve + ΔP_system) should be 0.25-0.50
Damper sizing: Face velocity typically 1,000-1,500 FPM through wide-open damper
Specify actuator torque requirements accounting for startup and airstream forces

Coordination with Other Disciplines

Interdisciplinary coordination intensifies during DD phase as systems become spatially defined.

Architectural Coordination

Coordinate mechanical systems with architectural design:

Confirm equipment room sizes, door widths, and ceiling heights accommodate equipment and maintenance clearances
Verify shaft dimensions and locations for vertical distribution
Coordinate ceiling heights with ductwork, piping, and lighting
Establish locations for air terminals, grilles, registers, and thermostats
Confirm exterior louver locations, sizes, and architectural finishes
Coordinate mechanical equipment visibility and screening requirements

Structural Coordination

Coordinate with structural engineer:

Provide equipment weights and seismic loads for structural analysis
Identify required roof openings for equipment rigging
Coordinate hanger and support locations with structural framing
Identify locations requiring supplemental steel for heavy equipment
Provide vibration isolation requirements for rotating equipment
Confirm slab depressions or pits for equipment

Electrical Coordination

Coordinate power requirements with electrical engineer:

Provide complete equipment electrical schedules (voltage, phase, FLA, MCA, MOCP)
Identify VFD locations and harmonic mitigation requirements
Coordinate control power requirements (120V, 24V transformers)
Provide motor starter locations and control wiring requirements
Coordinate emergency power requirements for life safety systems
Establish BAS network power and communication infrastructure

Plumbing Coordination

Coordinate with plumbing engineer:

Heating and cooling coil drain connections to sanitary or storm systems
Humidifier water supply and drain requirements
Equipment and floor drain locations in mechanical rooms
Makeup water connections for cooling towers and closed-loop systems
Steam and condensate piping coordination if applicable
Domestic water heating integration with HVAC systems

Fire Protection Coordination

Coordinate with fire protection engineer:

Sprinkler head locations to avoid air diffusers and ductwork
Smoke dampers in duct penetrations through fire-rated assemblies
Fire dampers in air transfer openings
Smoke detection integration with HVAC controls
Stair pressurization system coordination with building pressurization

Spatial Coordination and Routing

Develop major distribution routing during DD phase to establish feasibility and spatial requirements.

Horizontal Distribution

Establish primary distribution paths:

Main duct and pipe runs in corridors or above circulation spaces
Coordinate with structural beam depths and lighting layouts
Maintain minimum clearances (typically 12-18" above finished floor for access)
Identify areas requiring bulkheads or soffits
Plan for future access to valves, dampers, and control devices

Vertical Distribution

Design shaft systems for efficient vertical transport:

Size shafts for ductwork and piping with 20-25% additional space for future needs
Provide access doors at each floor for installation and maintenance
Coordinate shaft locations for minimal impact on floor plans
Establish fire-rated shaft construction requirements
Plan for seismic separation at floor penetrations

Equipment Room Layouts

Develop mechanical room arrangements:

Equipment placement with code-required clearances (typically 36" access, 48" for maintenance)
Establish maintenance clearances for tube bundle removal, filter access, motor replacement
Provide clear path for equipment replacement and rigging
Locate isolation valves and service disconnects for accessibility
Plan floor drains, housekeeping pads, and equipment bases

Energy Modeling and Analysis

Energy analysis during DD phase provides accurate performance predictions and identifies optimization opportunities.

Model Development

Develop detailed energy model reflecting DD design:

Model geometry matching architectural floor plans and elevations
HVAC systems as designed (equipment types, efficiencies, control strategies)
Actual equipment performance data from manufacturer selections
Lighting power densities from electrical design
Plug load assumptions confirmed with owner
Operational schedules by space type and day of week

Analysis Output

Energy model provides critical design information:

Annual energy consumption by end use (heating, cooling, fans, pumps, lighting, plug loads)
Peak demand analysis for utility connection sizing
Comparison to baseline performance (ASHRAE 90.1 Appendix G)
Energy cost analysis with local utility rates
Identification of high-energy systems for targeted efficiency improvements

Optimization Studies

Evaluate design alternatives during DD phase:

Equipment efficiency impacts (premium efficiency vs. standard)
Heat recovery options (energy recovery ventilators, waterside economizers)
Control strategy refinements (optimal start/stop, demand-based ventilation)
System configuration alternatives (VAV vs. fan coil, chilled beams)
Renewable energy integration (solar thermal, ground-source heat pumps)

Cost Estimating

DD phase cost estimates provide refined budget validation and value engineering opportunities.

Estimate Accuracy

DD estimates target ±10-15% accuracy through:

Quantity takeoffs from DD drawings (major equipment, duct/pipe lengths by size)
Manufacturer budget pricing for specified equipment
Unit pricing for distribution systems ($/lb ductwork, $/LF piping by size)
Regional labor rates and productivity factors
Material and equipment escalation to bid date

Estimate Components

Comprehensive mechanical estimate includes:

Equipment costs (chillers, boilers, air handlers, terminal units, pumps, specialties)
Distribution costs (ductwork, piping, fittings, hangers, insulation)
Controls and automation systems
Testing, adjusting, and balancing (TAB)
Equipment start-up and commissioning support
General conditions and contractor overhead/profit

Value Engineering

DD phase provides optimal timing for value engineering:

System alternative comparisons with life-cycle cost analysis
Equipment efficiency trade-offs balancing first cost and operating cost
Distribution system optimization (centralized vs. distributed equipment)
Control system sophistication relative to operational capabilities
Phasing opportunities to spread capital expenditure
Energy incentive and rebate opportunities

DD Drawings and Documentation

Design Development drawings communicate system design to owner, contractors, and coordination team.

Drawing Content and Detail Level

Mechanical plans (scale 1/8" = 1’-0" typical):

Equipment locations with dimensions to building grid
Major ductwork and piping distribution with sizes indicated
Air terminals, grilles, registers, and diffusers located
Thermostats and control devices shown
Fire dampers, smoke dampers, and volume dampers indicated
Coordination with architectural elements (ceiling heights, door swings, casework)

Mechanical sections and details:

Equipment room sections showing equipment arrangement and clearances
Shaft sections illustrating vertical distribution and penetrations
Typical connection details for common conditions
Critical coordination sections showing interferences and resolutions

Schedules:

Equipment schedules with capacities, sizes, electrical characteristics, and performance data
Ductwork schedules summarizing sizes, pressure classes, and insulation requirements
Piping schedules listing sizes, materials, insulation, and heat tracing
Air device schedules with types, sizes, and airflow quantities

Control diagrams:

System riser diagrams showing control architecture
Sequence flow diagrams illustrating operating modes
Control point summaries listing all monitored and controlled points

Specification Development

DD specifications transition from outline format to technical sections:

Division 23 - HVAC sections organized by CSI MasterFormat
Equipment specifications with detailed performance requirements
Material specifications for ductwork, piping, insulation, supports
Execution requirements for installation, testing, and commissioning
Submittal requirements and approval procedures
Warranty and maintenance requirements

Basis of Design Updates

Update basis of design report to document DD decisions:

Design criteria refinement (loads, flow rates, temperatures)
Equipment selection rationale and alternatives considered
Control philosophy and sequence narrative descriptions
Energy analysis results and compliance documentation
Code compliance summary and applicable standards
Outstanding design issues requiring resolution in CD phase

Owner Review and Approval Process

DD phase concludes with formal owner review and approval to authorize Construction Documents phase.

DD Submission Package

Prepare comprehensive submission:

Complete DD drawing set (mechanical, electrical, plumbing, fire protection)
Updated basis of design report
Equipment specifications and performance data
Cost estimate with line-item breakdown
Energy model results and analysis
Value engineering recommendations if applicable

Design Review Meeting

Present design to owner and stakeholders:

Walkthrough of mechanical systems and equipment selections
Discussion of spatial impacts and coordination challenges
Review of cost estimate relative to budget
Energy performance and sustainability features
Operation and maintenance considerations
Schedule impacts and long-lead equipment items

Approval and Authorization

Obtain formal approval to proceed:

Owner approval of equipment selections and system configurations
Budget confirmation or value engineering direction
Resolution of outstanding design issues
Authorization to proceed to Construction Documents phase
Updated project schedule and milestone dates

DD Phase Deliverables Checklist

Verify completion of all DD requirements:

Drawings

Mechanical floor plans at 1/8" scale showing equipment, distribution, and devices
Equipment room plans and sections with dimensions and clearances
Mechanical schedules (equipment, ductwork, piping, air devices)
Control diagrams and riser diagrams
Coordination sections resolving critical interferences

Calculations

Room-by-room load calculations
Duct and pipe sizing calculations
Pump and fan head calculations
Diversity and load profile analysis
Energy model and annual consumption analysis

Specifications

Outline specifications for all Division 23 sections
Equipment performance specifications
Material and installation requirements
Testing and commissioning requirements

Reports

Updated basis of design report
Cost estimate with quantity takeoffs
Energy analysis and code compliance documentation
Value engineering study results if performed

Approvals

Owner approval of equipment selections
Design review meeting minutes
Budget approval and authorization to proceed to CD

The Design Development phase establishes the complete framework for construction documentation. All major design decisions are finalized, equipment is selected with performance data, distribution systems are sized and routed, and coordination with other disciplines resolves spatial conflicts. The DD deliverables provide the foundation for preparing complete and coordinated construction documents that contractors can bid and build from with confidence.

HVAC Systems Encyclopedia