Heat Pumps and VRF Systems: Transforming Domestic Hot Water Production

The landscape of domestic hot water (DHW) production is evolving rapidly, driven by a combination of technological advancements, energy efficiency mandates, and sustainability goals. Traditional methods of heating water, including natural gas burners and electric resistance heating, are being replaced by heat pumps and variable refrigerant flow (VRF) systems. These innovations offer increased efficiency, reduced carbon emissions, and lower operating costs. This article explores the benefits, sizing methodologies, and technological advancements shaping the future of DHW production.

The Need for Change in DHW Production

Domestic hot water production is one of the most energy-intensive processes in buildings, making it a key area for energy optimization. Regulatory requirements at federal, provincial, and municipal levels increasingly mandate reductions in energy demand intensity (TEDi & TEUi) and carbon emissions. Additionally, DHW consumption is highly user-driven and unpredictable, requiring efficient system designs to balance demand fluctuations while minimizing energy waste.

Historically, gas-fired water heaters and electric resistance elements dominated the market. However, with rising energy costs and the push toward decarbonization, these solutions are becoming less viable. Heat pumps, particularly air-source and water-source varieties, offer significant advantages by leveraging an energy source to produce hot water more efficiently.

Shifting the Sizing Approach

Traditional DHW system designs often prioritize high-capacity, low-storage configurations. While effective in a world of cheap natural gas, this approach is less suitable for heat pumps due to their higher upfront cost per unit of heating capacity and greater sensitivity to peak demand fluctuations.

A more effective strategy is to prioritize storage-based sizing over instantaneous demand sizing. This involves:

• Upsizing storage tanks to handle peak demand while allowing heat pumps to operate at a steady, optimized rate.
• Sizing heat pumps based on daily recovery capacity rather than peak demand, ensuring they run efficiently for longer periods.
• Leveraging heat pump efficiency to reduce electrical infrastructure requirements compared to electric resistance heating.

Understanding Single-Pass vs. Multi-Pass Systems

Two main piping configurations exist for integrating heat pumps into DHW production: single-pass and multi-pass systems.

Single-Pass Arrangement

Single-pass systems are generally more efficient as they maximize the temperature differential between the incoming water and the heat pump’s condensing temperature and pressure. Key characteristics include:

• A higher delta T (temperature difference) for improved efficiency.
• A fixed outlet temperature, with incoming water passing through the heat pump only once.
• A swing tank to manage recirculation losses, often supplemented with an electric or gas backup.
• Greater reliance on thermal stratification in storage tanks to optimize performance.

Single-pass systems are particularly well-suited for CO2-based heat pumps, which excel in heating water from lower inlet temperatures with minimal efficiency penalties. These systems enable higher storage temperatures (e.g., 160-180°F), reducing the total storage volume required.

Multi-Pass Arrangement

Multi-pass systems, while slightly less efficient, offer greater flexibility and ease of control. Features include:

• Water recirculates multiple times through the heat pump until it reaches the desired temperature.
• No need for a swing tank, simplifying piping layouts.
• Higher-capacity heat pumps to compensate for lower efficiency compared to single-pass setups.

Multi-pass systems are often preferred in scenarios where space constraints or existing infrastructure limit the ability to implement a highly stratified storage tank design, or if the incoming domestic water is pre-heated.

Air-Source vs. Water-Source Heat Pumps

Selecting the right heat pump technology depends on the available energy sources and building conditions.

Air-Source Heat Pumps (ASHPs)

ASHPs extract heat from ambient air and transfer it to water. These systems are highly adaptable but must account for:

• Defrost cycles: In cold climates, frost buildup on outdoor coils requires periodic defrosting, momentarily reducing heat output.
• Variable refrigerant options: Modern ASHPs use refrigerants such as CO2, R513a, and R454b, offering improved efficiency and reduced environmental impact.
• Secondary benefits: ASHPs installed indoors can provide useful cooling, particularly in high-ceiling mechanical rooms or spaces with waste heat.

A notable innovation is the integration of Phase Change Material (PCM), which enables thermal storage in a compact footprint. PCM systems store heat at a predetermined temperature, reducing the need for large water storage tanks—an excellent option for retrofit projects where space is limited.

Water-Source Heat Pumps (WSHPs)

WSHPs use water as a heat exchange medium, offering several advantages:

• Potentially more stable source temperatures compared to air-source systems, improving efficiency.
• Ability to integrate with geothermal or wastewater recovery systems, further enhancing sustainability.
• Potential for secondary cooling benefits in buildings with high internal heat loads.

A particularly promising application is using wastewater as a heat source, capturing and repurposing heat from drainage systems to preheat incoming water. Studies indicate that such systems can recover 70-80% of otherwise wasted heat, significantly improving overall system efficiency.

VRF Driven Domestic Hot Water Production

Variable Refrigerant Flow (VRF) technology, widely used for space and ventilation heating and cooling, is now being adapted for DHW production. The VRF solution exemplifies this approach by leveraging:

• A two-stage refrigeration process to handle both peak and recirculation loads efficiently.
• Inverter-driven compressors and pumps for precise load matching and energy optimization under various loading scenarios.
• A split installation option, allowing all water-handling components to remain indoors, eliminating the need for glycol or heat tracing in cold climates.

The VRF driven solution can heat water up to 194°F and operates effectively in ambient conditions as low as -4°F, with no low-ambient cut-off, making it ideal for colder regions. 

Example of split installation: 

Key Takeaways and Implementation Considerations

As the industry shifts toward heat pump and VRF-driven DHW production, several key considerations emerge:

1. Sizing Methodology Matters: Transitioning from instantaneous to storage-based sizing is crucial for maximizing efficiency and cost-effectiveness.
2. Technology Selection is Context-Dependent: Air-source, water-source, and VRF systems each have unique benefits, with CO2 heat pumps excelling in single-pass applications.
3. Backup Systems Remain Important: In cold climates, supplemental electric or gas backup may be required to ensure reliability during extreme conditions.
4. Integration with Existing Infrastructure is Possible: Retrofits can benefit from technologies like PCM-based thermal storage and wastewater heat recovery.
5. Expert Guidance is Essential: Given the complexity of system selection, sizing, and integration, consulting with specialists can help optimize project outcomes.

Conclusion

The move toward heat pumps and VRF-driven DHW production represents a major leap in energy efficiency, sustainability, and long-term cost savings. By adopting a storage-based sizing approach, leveraging advanced refrigerants like CO2, and integrating innovative technologies like VRF and PCM storage, buildings can achieve superior performance while reducing their carbon footprint.

With the right design strategies and expert support, the future of domestic hot water production is not only smarter but also greener—paving the way for a more sustainable built environment.