Plastics

Plastics serve critical functions in HVAC systems ranging from piping and ductwork to insulation and structural components. Understanding their thermal properties, temperature limitations, and mechanical behavior under thermal stress is essential for proper material selection and system design.

Fundamental Thermal Properties

Plastics exhibit significantly different thermal characteristics compared to metals, requiring careful consideration in HVAC applications.

Thermal Conductivity Range

Most engineering plastics used in HVAC applications fall within a narrow thermal conductivity range:

These low conductivity values make plastics effective insulators but also create challenges for heat dissipation in certain applications.

Specific Heat Capacity

Plastic materials generally have higher specific heat capacities than metals:

Higher specific heat values result in slower temperature response times and greater thermal mass in plastic components.

Thermal Expansion Characteristics

Plastics exhibit thermal expansion coefficients 5 to 10 times greater than metals, creating significant design challenges.

Linear Thermal Expansion Coefficients

Design Implications:

For a 100 ft run of PVC piping experiencing a temperature change from 40°F to 140°F (ΔT = 100°F):

ΔL = α × L × ΔT = 30 × 10⁻⁶ /°F × 100 ft × 100°F = 0.30 ft = 3.6 inches

This substantial expansion requires proper accommodation through:

• Expansion loops or offsets
• Flexible couplings
• Expansion joints
• Anchoring and guiding strategies
• Allowance for movement at penetrations

Material-Specific Properties

Polyvinyl Chloride (PVC)

Thermal Properties:

• Thermal conductivity: 0.17 W/m·K (1.18 Btu·in/hr·ft²·°F)
• Maximum continuous service temperature: 60°C (140°F)
• Heat deflection temperature at 0.45 MPa: 65-75°C (149-167°F)
• Vicat softening point: 75-85°C (167-185°F)
• Glass transition temperature: 75-85°C (167-185°F)
• Coefficient of linear expansion: 50-54 × 10⁻⁶ /K

HVAC Applications:

• Drain, waste, and vent (DWV) piping
• Cold water distribution
• Condenser water piping (within temperature limits)
• Electrical conduit for control wiring
• Ductwork in low-temperature applications

Code References:

• ASTM D1784: Standard specification for rigid PVC compounds
• ASTM D2241: Standard specification for PVC pressure-rated pipe
• ASTM D2665: Standard specification for PVC DWV pipe and fittings
• NSF/ANSI 14: Plastic piping components and related materials
• UMC Chapter 7: Sanitary drainage

Limitations:

• Not suitable for hot water service above 140°F
• UV degradation without stabilizers
• Chemical incompatibility with certain solvents and oils
• Reduced impact resistance at low temperatures

Chlorinated Polyvinyl Chloride (CPVC)

Thermal Properties:

• Thermal conductivity: 0.14-0.16 W/m·K (0.97-1.11 Btu·in/hr·ft²·°F)
• Maximum continuous service temperature: 93°C (200°F)
• Short-term temperature tolerance: 107°C (225°F)
• Heat deflection temperature at 0.45 MPa: 100-107°C (212-225°F)
• Coefficient of linear expansion: 54-68 × 10⁻⁶ /K

HVAC Applications:

• Hot water distribution systems
• Hydronic heating piping (within limits)
• Hot condensate drain lines
• Makeup water lines for boilers and cooling towers
• Fire sprinkler systems (per ASTM F442)

Code References:

• ASTM D2846: Standard specification for CPVC hot and cold water distribution systems
• ASTM F441: Standard specification for CPVC Schedule 40 pipe
• ASTM F442: Standard specification for CPVC fire sprinkler systems
• NSF/ANSI 61: Drinking water system components

Design Considerations:

• Pressure derating required at elevated temperatures
• Greater thermal expansion than PVC requires closer hanger spacing
• System operating pressure must account for temperature effects
• Not recommended for compressed air due to rapid decompression concerns

Pressure-Temperature Relationship:

For a CPVC pipe rated at 400 psi at 73°F, the rating at 180°F is only 152 psi (400 × 0.38).

Acrylonitrile Butadiene Styrene (ABS)

Thermal Properties:

• Thermal conductivity: 0.18-0.22 W/m·K (1.25-1.53 Btu·in/hr·ft²·°F)
• Maximum continuous service temperature: 80-90°C (176-194°F)
• Heat deflection temperature at 0.45 MPa: 93-102°C (199-216°F)
• Coefficient of linear expansion: 72-108 × 10⁻⁶ /K
• Specific heat: 1.30-1.47 kJ/kg·K

HVAC Applications:

• Drain, waste, and vent piping
• Condensate drain lines
• Vent termination components
• Control valve bodies and actuators
• Refrigerant sight glasses and accessories

Code References:

• ASTM D1527: Standard specification for ABS plastic pipe
• ASTM D2661: Standard specification for ABS Schedule 40 plastic drain pipe
• ASTM D2751: Standard specification for ABS sewer pipe and fittings
• UPC Table 702.1: Plastic pipe materials

Advantages over PVC:

• Superior low-temperature impact resistance
• Better dimensional stability
• Easier solvent cementing at low temperatures
• Less brittle in cold environments

Limitations:

• UV degradation without stabilizers
• Not approved for pressure water distribution in all jurisdictions
• Higher thermal expansion than PVC

Polycarbonate (PC)

Thermal Properties:

• Thermal conductivity: 0.19-0.22 W/m·K (1.32-1.53 Btu·in/hr·ft²·°F)
• Maximum continuous service temperature: 115-130°C (239-266°F)
• Heat deflection temperature at 0.45 MPa: 132-138°C (270-280°F)
• Glass transition temperature: 145-150°C (293-302°F)
• Coefficient of linear expansion: 65-70 × 10⁻⁶ /K

HVAC Applications:

• Sight glasses for refrigerant systems
• Level indicators for tanks and receivers
• Protective covers for sensors and gauges
• Transparent portions of air handling unit access doors
• Machine guards for rotating equipment

Advantages:

• Excellent impact resistance across temperature range
• High optical clarity
• Good dimensional stability
• Flame retardant grades available (UL 94 V-0, V-2)

Limitations:

• Susceptible to chemical attack by many solvents
• UV degradation requires protective coatings
• Stress cracking with certain cleaners and lubricants
• Higher cost than commodity plastics

Polyethylene (PE) and High-Density Polyethylene (HDPE)

Thermal Properties:

• Thermal conductivity: 0.38-0.51 W/m·K (2.64-3.54 Btu·in/hr·ft²·°F)
• Maximum continuous service temperature: 60-80°C (140-176°F) depending on density
• Melting point: 120-135°C (248-275°F)
• Coefficient of linear expansion: 100-200 × 10⁻⁶ /K
• Specific heat: 1.90-2.30 kJ/kg·K

HVAC Applications:

• Ground-source heat pump earth loop piping
• Chilled water and condenser water distribution
• Condensate drain piping
• Geothermal well piping
• Underground service water lines

Code References:

• ASTM D2447: Standard specification for PE based plastic pipe
• ASTM D3035: Standard specification for PE plastic pipe (SDR-PR)
• ASTM F714: Standard specification for PE plastic pipe based on outside diameter
• IGSHPA: International Ground Source Heat Pump Association standards

Advantages:

• Excellent chemical resistance
• Flexibility allows for coiled installation
• Heat fusion joining creates monolithic systems
• Long service life in ground applications (50+ years projected)

Heat Fusion Joining:

HDPE piping requires thermal fusion at controlled temperatures:

• Butt fusion temperature: 400-450°F (204-232°C)
• Heat soak time: Based on wall thickness (typically 5-30 seconds per mm)
• Cooling time under pressure: Minimum 1 minute per mm of wall thickness

Polypropylene (PP)

Thermal Properties:

• Thermal conductivity: 0.22-0.24 W/m·K (1.53-1.67 Btu·in/hr·ft²·°F)
• Maximum continuous service temperature: 90-100°C (194-212°F)
• Melting point: 160-165°C (320-329°F)
• Coefficient of linear expansion: 90-150 × 10⁻⁶ /K
• Specific heat: 1.88-1.92 kJ/kg·K

HVAC Applications:

• Laboratory exhaust systems handling corrosive fumes
• Chemical drain lines
• Neutralization tank construction
• Acid-resistant pump housings
• Fume hood ductwork

Advantages:

• Excellent chemical resistance including strong acids and bases
• Good heat resistance
• Low moisture absorption
• Weldable using thermal methods

Limitations:

• High thermal expansion coefficient requires careful piping design
• UV degradation without stabilizers
• Limited impact resistance at low temperatures
• Not suitable for solvent cementing (requires thermal fusion or mechanical joints)

Fiberglass Reinforced Plastic (FRP)

Thermal Properties:

• Thermal conductivity: 0.23-0.35 W/m·K (1.60-2.43 Btu·in/hr·ft²·°F)
• Maximum continuous service temperature: 93-149°C (200-300°F) depending on resin
• Coefficient of linear expansion: 10-30 × 10⁻⁶ /K
• Specific heat: 1.05-1.26 kJ/kg·K

Resin Systems:

HVAC Applications:

• Corrosive exhaust ductwork
• Chemical fume hood systems
• Cooling tower basins and fill
• Scrubber housings
• Underground piping in aggressive soils
• Acid waste neutralization systems

Code References:

• ASTM D2996: Standard specification for FRP pipe
• ASTM D3917: Standard specification for FRP duct
• SMACNA FRP Duct Construction Manual
• NFPA 90A: Installation of air-conditioning and ventilating systems (flame spread/smoke developed limits)

Flame Spread and Smoke Development:

For air-moving applications, FRP must meet:

• Flame spread index ≤ 25 (Class I or A)
• Smoke developed index ≤ 50

Per ASTM E84 (Steiner Tunnel Test) as referenced in NFPA 90A and IMC Section 603.

Design Considerations:

• Lower thermal expansion than unreinforced plastics
• Anisotropic properties depend on fiber orientation
• Requires different joining methods (adhesive bonding, mechanical fastening)
• Thermal conductivity varies with fiber content (30-70% by weight)
• Custom fabrication allows for complex geometries

Temperature Limitations and Derating

Continuous vs. Intermittent Service

Maximum service temperatures are typically specified for continuous operation (24/7 exposure). Short-term or intermittent exposure may allow higher temperatures:

Repeated thermal cycling to peak temperatures accelerates degradation and reduces service life.

Pressure Derating

Plastic pipe pressure ratings decrease with temperature. The Hydrostatic Design Stress (HDS) decreases as molecular mobility increases at elevated temperatures.

Example Calculation for CPVC:

Base pressure rating at 73°F: 400 psi Operating temperature: 180°F Temperature multiplier: 0.38

Working pressure = 400 psi × 0.38 = 152 psi

Additional safety factor for system design: 0.5 Design pressure = 152 psi × 0.5 = 76 psi

Design Considerations for HVAC Systems

Piping Support and Anchoring

Higher thermal expansion coefficients require closer hanger spacing than metal piping.

Recommended Maximum Hanger Spacing:

For heated piping systems (CPVC hot water), reduce spacing by 25-33% to account for thermal expansion.

Expansion Accommodation

Required expansion loop dimensions can be calculated using:

L = √(2.5 × d × ΔL)

Where:

• L = Loop leg length (inches)
• d = Pipe outside diameter (inches)
• ΔL = Thermal expansion (inches)

For a 2-inch CPVC line with 3.6 inches of expansion: L = √(2.5 × 2.375 × 3.6) = √21.4 = 4.6 feet minimum leg length

Alternative expansion accommodation methods:

• Flexible couplings (limited to small movements)
• Expansion joints (bellows-type for larger movements)
• Direction changes and offsets in piping layout
• Flexible connections at equipment

Combustibility and Flame Spread

Plastics used in air distribution systems must meet flame spread and smoke development criteria per NFPA 90A, NFPA 90B, and IMC Section 603.

ASTM E84 Requirements for Ducts:

Many unmodified plastics exceed these limits and require flame retardant additives or alternative materials for air-moving applications.

Chemical Compatibility

Plastics exhibit varying resistance to chemicals encountered in HVAC systems:

Common Chemical Exposures:

E = Excellent, G = Good, NR = Not Recommended, V = Varies by resin type

Ultraviolet (UV) Resistance

Most plastics degrade under prolonged UV exposure unless stabilized:

• PVC: Requires UV stabilizers for outdoor use; becomes brittle and discolored
• CPVC: Similar to PVC; UV inhibitors necessary for exterior applications
• ABS: Particularly susceptible to UV degradation; not recommended for outdoor use without protection
• Polypropylene: Fair UV resistance; stabilized grades available
• HDPE: Good UV resistance when properly stabilized; often contains 2-3% carbon black
• FRP: Gel coat or surface veil provides UV protection; periodic maintenance required

For outdoor HVAC applications, specify UV-stabilized grades or provide physical protection (wraps, enclosures, coatings).

Joining Methods

Different plastic materials require specific joining techniques:

Solvent Cementing:

• Applicable to: PVC, CPVC, ABS
• Creates chemical bond through partial dissolution
• Temperature-dependent cure time (faster in warmer conditions)
• Not reversible
• Requires primer for certain materials (PVC, CPVC)

Thermal Fusion:

• Applicable to: HDPE, PP, PVDF
• Methods: Butt fusion, socket fusion, electrofusion
• Creates monolithic joint
• Requires specialized equipment and training
• Temperature and pressure critical to joint quality

Mechanical Joining:

• Applicable to: All plastics
• Methods: Threaded, flanged, compression fittings, victaulic-style couplings
• Allows disassembly
• Suitable for dissimilar material connections
• May require gaskets and proper torque values

Adhesive Bonding:

• Applicable to: FRP, dissimilar plastics
• Structural epoxies or specialized adhesives
• Surface preparation critical
• Cure time and temperature dependent
• Used for custom fabrications

ASHRAE References and Standards

ASHRAE Handbook - Fundamentals (2021):

• Chapter 33: Physical Properties of Materials (thermal conductivity data)
• Chapter 23: Thermal and Water Vapor Transmission Data
• Chapter 27: Energy Resources (material embodied energy)

ASHRAE Handbook - HVAC Systems and Equipment (2020):

• Chapter 45: Building Air Intake and Exhaust Design (plastic exhaust materials)
• Chapter 49: Corrosion and Fouling (plastic pipe corrosion resistance)

ASHRAE Standards:

• ANSI/ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality (material emissions)
• ANSI/ASHRAE Standard 90.1: Energy Standard for Buildings (insulation requirements)

Code and Standard References

Plumbing Codes:

• International Plumbing Code (IPC)
• Uniform Plumbing Code (UPC)
• National Standard Plumbing Code (NSPC)

Mechanical Codes:

• International Mechanical Code (IMC) Section 603: Duct construction and installation
• NFPA 90A: Installation of Air-Conditioning and Ventilating Systems
• NFPA 90B: Installation of Warm Air Heating and Air-Conditioning Systems

Material Standards:

• ASTM International standards (as listed under specific materials)
• NSF/ANSI standards for potable water contact
• UL standards for flame ratings

Industry Standards:

• SMACNA: Thermoplastic Duct Construction Manual
• SMACNA: FRP Duct Construction Manual
• PPI: Plastic Pipe Institute technical reports
• Uni-Bell PVC Pipe Association guidelines

Service Life Considerations

Expected service life of plastic materials in HVAC applications depends on multiple factors:

Life-Reducing Factors:

• Elevated operating temperatures
• Thermal cycling frequency and magnitude
• UV exposure
• Chemical exposure
• Sustained stress levels
• Impact and mechanical abuse
• Installation quality

Typical Design Life Expectations:

These values assume proper installation, operation within design parameters, and normal maintenance.

Thermal Analysis Applications

Heat Loss Through Plastic Piping

For uninsulated plastic pipe carrying hot water:

Q = 2πkL(T₁ - T₂) / ln(r₂/r₁)

Where:

• Q = Heat transfer rate (W)
• k = Thermal conductivity of pipe wall (W/m·K)
• L = Pipe length (m)
• T₁ = Inner surface temperature (K)
• T₂ = Outer surface temperature (K)
• r₁ = Inner radius (m)
• r₂ = Outer radius (m)

The low thermal conductivity of plastics reduces heat loss compared to metal piping, but insulation is still required for energy efficiency in hot water and heating applications.

Condensation Prevention

Plastic pipe’s lower thermal conductivity reduces condensation risk compared to metal piping in cold water applications:

For a 2-inch copper pipe (k = 400 W/m·K) vs. PVC pipe (k = 0.17 W/m·K) in identical conditions:

The outer surface temperature of plastic pipe will be closer to ambient, reducing the likelihood of reaching dew point temperature and forming condensation.

Despite this advantage, insulation is still recommended for chilled water systems below 55°F to prevent condensation and energy waste.

Summary

Plastics offer unique advantages in HVAC systems including corrosion resistance, low thermal conductivity, light weight, and ease of installation. However, their thermal properties—particularly high thermal expansion coefficients and temperature limitations—require careful consideration during design. Proper material selection based on operating temperature, pressure, chemical exposure, and code requirements ensures reliable long-term performance. Understanding thermal behavior allows engineers to design appropriate support systems, expansion accommodation, and insulation strategies for plastic components in HVAC applications.