Embodied and operational carbon in HVAC systems

The carbon footprint of an HVAC system has two parts. Operational carbon comes from the energy the system uses every year: gas burned on site and electricity drawn from the grid. Embodied carbon comes from making, transporting, installing, maintaining and eventually disposing of the equipment, ductwork and piping. For HVAC there is a third component that is easy to overlook: refrigerant that leaks out over the system's life, much of which has a global warming potential (GWP) hundreds or thousands of times that of carbon dioxide.

As buildings become more efficient and grids cleaner, embodied and refrigerant emissions become a larger share of the total. Owners pursuing net-zero carbon, the CAGBC Zero Carbon Building Standard or municipal requirements in cities such as Toronto and Vancouver increasingly need to understand all three.

The whole-life carbon framework

Whole-life carbon assessment commonly follows the life cycle stages defined in EN 15978 and ISO 21930, which group emissions as follows:

Embodied carbon usually refers to everything except operational energy (B6) and operational water (B7). Refrigerant leakage is reported in the use stage but is often discussed separately because of its size.

Operational carbon

Operational carbon is energy use multiplied by an emission factor for each fuel.

• Natural gas has a relatively fixed emission factor per unit of energy burned.
• Electricity depends on how the provincial grid generates power. Provinces with mostly hydroelectric or nuclear generation, such as Quebec, British Columbia, Manitoba and Ontario, have low grid intensity. Alberta, Saskatchewan and Nova Scotia have historically relied more on fossil fuels and have much higher intensity, although their grids are changing.

Emission factors for each province are published in Canada's National Inventory Report, and many programs specify which factors to use. Because grid intensity varies so much, the same HVAC design can have very different operational carbon in Calgary and in Vancouver. A gas boiler and an electric heat pump may be close in carbon terms on a coal-heavy grid today, while a heat pump is far lower on a hydro grid. Long-life decisions should also consider projected grid intensity, which is falling in most provinces.

Hourly analysis improves accuracy where marginal emissions vary by time of day or season, though most current programs use annual average factors.

Refrigerant emissions

Refrigerant can be the largest single source of embodied emissions in HVAC systems that use large charges, such as VRF and split systems with long piping runs. Emissions arise from:

• Leaks during normal operation.
• Losses during service and repairs.
• Catastrophic leaks from damaged piping or failed components.
• Incomplete recovery at end of life.

Approximate 100-year GWP values (IPCC Fourth Assessment Report basis, as used in the Kigali Amendment) illustrate the range:

Canada is phasing down HFC consumption under the Kigali Amendment through the Ozone-depleting Substances and Halocarbon Alternatives Regulations, which also restrict high-GWP refrigerants in certain new equipment. Lower-GWP alternatives often have different safety classifications under CSA B52 and ASHRAE 34, which affect charge limits and equipment location. See refrigerant regulations and low-GWP options in Canada.

CIBSE TM65 provides a method for estimating the embodied carbon of building services equipment when manufacturer environmental product declarations (EPDs) are not available, and it includes refrigerant leakage assumptions.

Embodied carbon of equipment and distribution

Beyond refrigerant, embodied carbon in HVAC comes mainly from metals: steel in chillers, AHU casings and pipe, copper in coils and refrigerant piping, and aluminium in fins and ductwork. Practical points:

• Mechanical systems are replaced several times over a building's life, so their embodied carbon recurs. A 60-year building may see three generations of equipment.
• EPDs for HVAC equipment are becoming more common but are not yet available for many products.
• Oversizing adds embodied carbon as well as cost. Right-sizing based on accurate loads reduces both.
• Large central plants can have lower embodied carbon per unit capacity than many distributed units, but distribution adds piping or ductwork.

Design strategies to reduce whole-life carbon

1. Reduce loads first: envelope performance and heat recovery reduce both operational energy and equipment size.
2. Electrify heating with heat pumps where the grid is low carbon or decarbonizing. See electrification of commercial heating.
3. Choose low-GWP refrigerants and minimize charge: hydronic distribution with refrigerant confined to a plant room or packaged outdoor unit typically has a much smaller charge than extensive refrigerant piping.
4. Design for leak detection and tightness: brazed joints, fewer fittings, leak detection and regular inspection.
5. Right-size equipment: avoid stacking safety factors.
6. Plan for end of life: specify refrigerant recovery and recycling, and design for component replacement rather than whole-unit replacement.
7. Compare options on a whole-life basis, using a consistent study period and emission factors.

Comparing options

A useful comparison for an HVAC decision includes for each option:

• Annual operational energy by fuel, from an energy model or calculation.
• Operational carbon using current and projected provincial emission factors.
• Refrigerant type, charge, assumed annual leakage rate and end-of-life recovery rate.
• Embodied carbon of major equipment from EPDs or TM65 estimates.
• Replacement cycles over the study period.

Pair the carbon comparison with a life-cycle cost analysis so trade-offs between cost and carbon are visible.

Frequently asked questions

Is embodied carbon or operational carbon bigger for HVAC?

It depends on the grid, the building's efficiency and the refrigerant. In efficient buildings on low-carbon grids, embodied and refrigerant emissions can rival or exceed operational emissions. On high-carbon grids, operational carbon usually dominates.

Do I need an EPD for every piece of equipment?

Not usually. Most assessments focus on the largest items and use generic data or TM65 estimates for the rest. Requirements depend on the program or municipal standard involved.

Does switching to a low-GWP refrigerant change maintenance?

Sometimes. Mildly flammable (A2L) and flammable (A3) refrigerants require trained technicians, specific tools and attention to CSA B52 requirements.

Key takeaways

• HVAC carbon includes operational energy, equipment manufacture and refrigerant leakage.
• Provincial grid intensity strongly affects operational carbon and the case for electrification.
• Refrigerant type, charge and leakage can dominate embodied emissions in refrigerant-based systems.
• Equipment is replaced several times over a building's life, so embodied carbon recurs.
• Compare options on a whole-life basis alongside life-cycle cost.