Frequently, the perception in the industry is that these priorities — especially cost — are incompatible with designs and technologies that help decarbonize buildings.    

However, our experience with heating, ventilation and air-conditioning (HVAC) systems has shown that cutting emissions and optimizing costs are not mutually exclusive.  

Decarbonization isn't a one-time event. Reducing emissions is a continuous process with many paths. Organizations can take an incremental approach without compromising on cost. Finding the right balance requires weighing project priorities, the building’s environment, short- and long-range facility plans and available technology solutions. To achieve this balance, energy modeling is an essential tool. 

WEIGHING UP DESIGN GOALS  

When it comes to sustainability and decarbonization in HVAC systems, there are three primary levers: electrification of heating using heat pumps, improving energy efficiency and using lower global warming potential (GWP) refrigerants. Each project's physical environment and framework conditions will determine the mix of all three levers. Cost priorities can often be aligned more easily with sustainability goals than you might think.   

For example, the local climate substantially impacts electrification. In hotter climates, electrification is far easier than in cold ones, where heat pumps require much more electricity when heating, and, in extreme temperatures, may not be able to stay online. This is because heat pumps that use outside air as a heat source must work harder as the air temperature drops, and their efficiency, capacity and emission benefits decline substantially. They eventually require a back-up like natural gas (NG), which can deliver heat more efficiently, cheaply and with lower emissions in extremely cold conditions.  

A hybrid system combining NG for supplemental heat and an electric heat pump can ensure that the best possible fuel source is used for different climate conditions, addressing cost and decarbonization as well as flexibility and redundancy.  

Using NG as a back-up to the heat pump also avoids oversizing the heat pump to cope with the most extreme conditions – which only occur infrequently.  

HYBRID HEATING SYSTEMS COME INTO THEIR OWN 

Hybrid systems can significantly reduce emissions compared to fossil-fuel-only heating and save costs, especially in areas with access to cheap renewable electricity from solar PV plants, wind farms and hydro or geothermal sources. By switching over to NG when temperatures get too low or when insufficient renewable electricity is available, building owners can not only save costs but also hedge against grid outages in the coldest months of the year.  

It’s critical to pick the right point for your hybrid system to switch from one fuel source to another. By changing to the lowest rate — electricity or gas prices — at any particular time, you can save money while making significant CO2 savings. Alternatively, the time to switch could be based not only on energy costs but also on emissions reductions, allowing the designer to optimize around different design goals.  

Indeed, milder and sunnier winter days, when heat pumps still work efficiently, also tend to have a higher percentage of solar electricity generation, improving the real-time grid emissions factor and giving a double benefit of improved emissions and lower cost. 

SUSTAINABILITY DOESN’T NECESSARILY COMMAND A PREMIUM  

This example underlines that decarbonization and broader sustainability goals can be achieved alongside other design goals. A study from the National Renewable Energy Laboratory bears this out. When comparing conventional school buildings with zero-energy schools — buildings that produce as much renewable energy as they need to operate — the NREL found that the latter offers a number of benefits. Zero-energy schools require less energy and offset demand using on-site renewable energy.  

The study indicates that zero-energy school buildings can be designed and built within conventional budgets — and they can cost less. By applying an integrated approach to design and construction, the extra cost of the zero-energy systems can be offset by the greater efficiencies they facilitate. The study lists downsizing heating, ventilation and air conditioning among the benefits that help reduce capital and operational expenditures.   

However, as we have seen, balancing each building's inherent complexities is the key to achieving these benefits. This is where energy modeling comes into play.   

BALANCING SUSTAINABILITY AND DESIGN GOALS 

Energy models tie together all factors affecting HVAC design and installation. They are a tool for comparing many design alternatives quickly and cost-effectively to find the most appropriate solution to the project’s design goals.   

Energy modeling can simulate various system configurations to show the effect of using different types of equipment, such as an electric heat pump compared to a conventional HVAC system. Modeling can also factor in decarbonization policy incentives that may affect the economics and viability of a proposed system.  

In addition, life-cycle analyses of different solutions can provide a clearer long-term financial picture. Models can forecast long-term costs, performance and return on investment of various system-design options, which can make it easier to get sign-off projects from investors.  

With its data-driven approach, energy modeling can also prevent overengineering that might otherwise impact a project's costs. A US Department of Energy study has shown that projects using energy modeling perform better and can have shorter returns on investment than those that don’t.  

As sustainability codes, standards and regulations evolve throughout the US, energy modeling emerges as a vital tool for harmonizing environmental priorities with core design objectives — enabling balanced, effective solutions that drive greener, more resilient outcomes for buildings, businesses and the planet. 

EXAMPLE ANALYSIS  

A 54,000 ft² office building prototype, compliant with ASHRAE 90.1 standards, was modeled in Denver, Colorado, using Daikin Applied EA Pro software, powered by the US DOE’s EnergyPlus simulation engine. Rebel® heat pump rooftop units were assigned to serve the building. Two HVAC alternatives were analyzed for the heat pumps: (1) electric back-up and (2) gas back-up. 

The Environmental Protection Agency’s eGRID emission factors, representing the greater Denver regional grid, were used to calculate emissions, along with average electricity and gas utility rates for the region. 

The results indicate that while total energy usage is lower with the electric back-up scenario—due to the higher efficiency of electric resistance heating compared to gas combustion—the annual utility costs and equivalent carbon emissions are lower with the gas back-up scenario. This is primarily because of higher electric utility rates and the electric emissions profile of the regional electric grid. 

This example represents a specific scenario for demonstration purposes but highlights the importance of holistic, project-specific considerations when evaluating design alternatives to meet customer goals.