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Electrification and Sustainability Goals: Unveiling the Role of Heat Pumps

As we’ve all continually heard in recent years, electrification is a major sustainability goal for many municipalities, states, and even countries around the world¹. Along with electrification comes a plethora of buzzwords and phrases, one of the most familiar being heat pumps. Air-source heat pumps—the most common application due to the relatively low cost of such systems—can absorb heat from the ambient air and transfer that heat into an occupied building space. But how does that work when the outside ambient air is cold?

Air-Source Heat Pumps Demystified: Operation in Cold Climates

Heat pumps manipulate the chemical properties of refrigerants at different pressures to absorb and release heat energy, moving it from outside to inside to heat a building.

At the right pressure, the boiling point of the refrigerant will actually be lower than the cold outside air temperature—and since heat energy always moves from the higher temperature substance (in this case, the outside air) to the lower temperature substance (the refrigerant), heat is absorbed from the “cold” outside air and then cycled to the occupied space.

The Chilling Challenge: How Cold Can Air-Source Heat Pumps Go?

So, heat pumps can pull heat from relatively cold outside air—but how cold can that outside air be?

The short answer is that it depends on the type of heat pump you are working with. Most one-to-one split heat pump systems (i.e., a single outdoor unit connected to a single indoor unit) and packaged heat pump systems (e.g., packaged rooftop units) begin to significantly reduce their heating capacity around 40-45°F ambient temperature—which is when you really start needing the heat!

However, these systems are typically designed to accommodate this derated capacity as the temperature continues to drop. Regardless, one-to-one and packaged heat pump systems are limited in how much heat they can provide at very cold temperatures. Below the “balance point” of derated heat pump capacity and building heating load, supplemental heat becomes necessary, typically in the form of some electric resistance heaters.

Beyond Limits: Variable Refrigerant Flow (VRF) Systems Revolutionizing Cold Climate Heating

The most advanced heat pump technology available today takes the form of Variable Refrigerant Flow (VRF) systems. VRF systems have been around in East Asia and Europe for decades and have gained a foothold in the U.S. in the last 15-20 years. This technology has seen an explosion of progress in that time. A few decades ago, VRF systems were only rated for heating in the range of -5°F to -10°F, below which the systems were configured to shut down to protect their internal components from the “extreme” cold! However, VRF systems today have heating performance data for operation down to -22°F or less! Granted, at a significantly derated heating capacity, but pause and grasp that this technology can pull heat out of -22°F air and move that heat inside your building! The upshot is that the balance point of VRF systems is much lower than the one-to-one or packaged heat pump systems described above. 360 Engineering has designed systems for large buildings in Denver and other cold climates capable of meeting the full heating load at -5°F without any supplemental heat systems required.

Breaking the Cold Barrier: Technological Advances in VRF Systems

Multiple technological advances have allowed VRF to progress to such a viable heating system, even in cold climates. Physical accessories on the outside casing of the heat pump units, such as wind baffles and snow hoods, mitigate the effects of weather on the operation and efficiency of the VRF heat pumps. Inside the heat pumps, flash-injection technology—where the system introduces a modest quantity of mixed-phase refrigerant (i.e., a mixture of gas and liquid) to cool the compressor—allows the compressor to operate at higher speeds by mitigating friction and accumulation of internal heat². Higher compressor speed results in greater heating capacity for the system at lower ambient temperatures. On top of these technologies in the outdoor unit, VRF systems take advantage of heat recovery internally as well, moving energy from interior zones that have excess heat available to exterior zones that need that heat energy—bypassing the outdoor unit entirely through the use of an intermediate “mode control unit.”

Future Trends: The Journey of Heat Pump Technology in a World Moving Towards Electrification

Heat pump technologies have come a long way since they were first introduced in the HVAC industry. While limitations still exist for certain systems and applications, air-source heat pumps have become a viable and highly efficient option to provide the heat needed for buildings in cold climates. As the world continues toward electrification and the market demands better, higher-performing heat pumps, this technology will continue to progress toward greater heating capacities at colder ambient temperatures.

References

¹https://www.forbes.com/sites/energyinnovation/2022/11/15/the-worlds-three-largest-economies-go-all-in-on-heat-pumps-how-policy-can-cut-gas-use-and-energy-bills/?sh=532dbd9c564d

²https://www.achrnews.com/articles/145397-five-things-hvac-contractors-should-know-about-cold-climate-vrf

HVAC and plumbing systems are essential to creating a comfortable and productive environment in our buildings; however, nothing can bring down the look of a well-designed space more than a misplaced diffuser or unexpected thermostat. In most instances, concealing HVAC and plumbing designs into high-finish spaces can come with major cost impacts, but there are simple ways to elevate the integration of these systems into a space without major impacts on the budget. Let’s explore a few!

Alignment of Ceiling Devices

Aligning diffusers with other ceiling devices is a small task but can have a big impact on the uniformity of the ceiling to provide a clean and organized look. The goal of this approach would be to align diffusers and other ceiling devices to the centerline of the lights, along with creating uniform spacing between the diffuser and other ceiling devices. To ensure the engineer has time to coordinate the final diffuser and device locations, the final locations of lights and other ceiling components should be provided around 75-80% CDs for final alignment. If the aesthetics of a space is a priority, having an RCP coordination meeting between the architect, electrical engineer, and mechanical engineer can be very beneficial to talk through the priority of devices and ensure all parties are fully coordinated.

Elevate the Diffuser Specifications

Standard cone, louvered, or perforated diffusers can be swapped out to a square plaque diffuser for both the supply and return in the ceiling for a small cost increase ($20-$30/diffuser*). If both the supply and return diffusers are revised to this specification, this can create a clean, uniform look in the ceiling. Square plaque diffusers have a very efficient supply air distribution, providing low-pressure drop and low sound levels with efficient mixing. However, it should be considered when using this specification for return air that this diffuser has less return capacity than other return diffusers with more free area, so additional return grilles may need to be added with this specification.

In addition to revising the specifications for square ceiling diffusers, there are many aesthetically pleasing grilles and slots that can be incorporated into the design for supply and return air. The architect should coordinate with the mechanical engineer early in the design process to discuss linear grilles or slot diffuser options to ensure these are properly coordinated into the ceiling and wall details. Although linear grilles and slot diffusers can add significant cost compared to standard louvered grilles or diffusers, applying slots to strategic spaces like lobbies or conference rooms can elevate these common spaces and set the tone for the rest of the building.

Coordinating Thermostat Locations and Specifications

When placing the thermostats on the plans, the engineer must be strategic with the placement to ensure optimal control of the mechanical system. If this is not properly coordinated during design, this can lead to unexpected locations on the wall come time for the final punch. When we are placing thermostats in the design, we are considering the following: direct sunlight, exterior walls, the path of supply airflow, proximity to major heat-producing equipment, location within the occupied/breathing zone, and many more. In some spaces, these considerations can make thermostat placement challenging.

For proper placement coordination, the architect should review the proposed thermostat locations on the drawings once the engineer has laid them out to ensure the locations do not conflict with their design.  For more complex spaces, deeper coordination may be required to shift around the thermostats or even shift wall finishes around to accommodate the ideal thermostat location. A coordination meeting between the architect and mechanical engineer may be ideal for these more complex applications.

If local control of the temperature setpoint in the space is not required, remote temperature sensors can be considered for minimal added cost. This would locate a thermostat in a concealed or central location for temperature adjustment (If DDC controls are provided, this could be provided via a web-based controller) with a remote temperature sensor placed in the space being measured. There are several types of remote temperature sensors available, including button-style sensors that are about 1” in diameter and can be stainless steel, brass, or paintable and can be easily integrated into the design.

Although these recommendations are simple, they can be effective if proactive coordination is achieved. Having upfront discussions on the design intent of the mechanical and plumbing system integration throughout the project will help the engineer provide solutions early to achieve the aesthetic goals of the space. Additionally, if the aesthetics of these systems are a priority in a space, but the budget is tight, these simple solutions and several others can be applied without busting the budget.

Last month, we summarized some of the why and how of electrification related to mechanical and plumbing systems. When it comes to all-electric HVAC, the common denominator is refrigerant. Just about any mechanical system providing heating and/or cooling without using fossil fuels will include refrigerant at some level—heat pumps, chillers, geothermal they all include refrigerant compressor circuits. So, on top of local, state, and federal regulations pushing the industry toward building systems electrification, Congress has kept things interesting by passing the AIM Act in 2020.

The AIM Act and Refrigerant Regulation

The American Innovation and Manufacturing (AIM) Act grants the Environmental Protection Agency (EPA) the authority to regulate and phase out hydrofluorocarbon (HFC) refrigerants in the coming years. HFCs are potent greenhouse gases with a global warming potential (GWP) thousands of times higher than carbon dioxide (GWP = 1). To combat this, the EPA has set GWP limits for refrigerants manufactured or imported in the U.S.

Impact on the HVAC Industry

Current Refrigerant Landscape: Most heat pumps (including water-source heat pumps, variable refrigerant flow systems, etc.) and DX cooling units currently use R-410a refrigerant, boasting a GWP rating of around 3,000.

EPA’s GWP Limits: Starting in 2025-2026, the EPA mandates a GWP limit of 700 for refrigerants in these systems. Other types of equipment (such as supermarket refrigerated displays) have even lower GWP limits.

Industry Response: HVAC equipment manufacturers are investing significant time and effort in redesigning equipment to accommodate alternative refrigerants that comply with EPA requirements.

Choosing the Right Refrigerant

• Various refrigerant options are available, but only a few are practical replacements for HFCs in HVAC systems.
• Selection involves economic considerations and evaluation of potential life-safety risks.
• Codes and standards such as the International Mechanical Code and ASHRAE Standard 15 guide allowable refrigerant volumes based on health hazards, flammability, and reactivity.
• Our job as engineers is to calculate and confirm that a leak in the piping or equipment we’ve specified would not result in a concentration above this limit inside the smallest enclosed space served by our system.

Understanding Allowable Concentrations

• For instance, R-410a has an allowable concentration of 26 lbs. per 1,000 cubic feet, with a health hazard rating of 2 (coupled with flammability and reactivity hazard ratings of 0).
• R-32, a potential R-410a replacement, has a lower health hazard rating of 1 but a higher flammability hazard rating of 4, resulting in an allowable concentration of only 4.8 lbs. per 1,000 cubic feet.

Implications for HVAC Systems

• The AIM Act’s impact extends to architectural, electrical, and structural designs.
• Uncertainty remains about manufacturers’ refrigerant choices and their effects on system designs.
• Potential changes could lead to larger equipment and piping, challenging installation, maintenance, and clearances.
• Increased electrical loads may strain infrastructure, especially in buildings aiming for full electrification.

In summary, while the HVAC industry faces uncertainty due to the AIM Act, it is actively preparing for the changes. As engineers, we anticipate and embrace these challenges and are excited about the innovations driving our industry toward a more efficient and sustainable future.

For further details, visit the International Code Council website.