Introduction

The quest for economical and low-carbon heating solutions in cold climates often leads designers to explore hybrid systems combining heat pumps with conventional heating methods. This article delves into the first year’s experience of implementing an air source chiller/heat pump in a low-temperature hydronic heating system at a public works garage in Montague, Massachusetts. The insights gained from this project are invaluable for general contractors and designers aiming to optimize such systems.

Project Overview

Location and Climate

The project is located in Turners Falls, Massachusetts, within ASHRAE Climate Zone 5A. The design outdoor temperature for winter heating is around 3°F (-16.1°C). The facility includes a new Department of Public Works building with a focus on reducing its carbon footprint through innovative heating solutions.

System Design

The heating system comprises a conventional air-cooled chiller/heat pump integrated into a low-temperature hydronic system. The office and administration areas are served by a Variable Refrigerant Flow (VRF) system, while the shop and vehicle storage areas utilize radiant floor heating. The hydronic system is designed to maximize the efficiency of the heat pump by maintaining low water temperatures.

Installation and Commissioning

Hybrid System Configuration

The hybrid system includes an air-cooled chiller/heat pump and oil-fired boilers. The chiller operates through a secondary loop with propylene glycol for freeze protection, connected via a plate and frame heat exchanger. The boilers provide backup heating, controlled to minimize electrical demand charges and ensure system redundancy.

Commissioning Challenges

During commissioning, several issues were addressed to optimize system performance:

• Minimizing hydronic water temperatures to enhance the heat pump’s Coefficient of Performance (COP).
• Sequencing the chiller/heat pump with the boilers.
• Implementing defrost cycle controls suitable for cold climates.
• Adjusting the system to avoid peak electrical demand charges.

First Year Performance

Efficiency and Performance

Despite some initial challenges with the heat pump's internal controls and defrost cycles, the system delivered reliable heating throughout the first year. Key performance metrics included:

• COP variations with ambient temperatures.
• Energy savings compared to conventional oil heating.
• Operational adjustments to maximize off-peak electricity usage.

Utility Cost and Carbon Emissions

The hybrid system achieved significant reductions in utility costs and carbon emissions:

• An 18% reduction in heating costs compared to oil heating.
• A 66% decrease in carbon emissions for the portion of heating displaced by the heat pump.
• Potential to displace up to 64% of oil-fired heating with optimized controls and defrost cycles.

Lessons Learned

Design and Implementation Insights

The project highlighted critical considerations for general contractors and system designers:

• Importance of precise control settings and system sequencing.
• Need for robust defrost cycle management in cold climates.
• Benefits of integrating hybrid systems to balance efficiency and redundancy.

Future Improvements

Advancements in heat pump technology and controls are expected to enhance the viability of such systems. Future designs will likely benefit from more realistic performance ratings and improved compressor cooling techniques.

Conclusion

The first year’s experience with the air source chiller/heat pump in Montague’s public works garage demonstrates the potential for hybrid systems to deliver low-carbon, cost-effective heating in cold climates. General contractors and designers should consider the lessons learned from this project to optimize the performance and reliability of similar systems.

Source: Burbank, Jason J., et al. "Air Source Chiller/Heat Pump in a Hybrid System for Cold Climate Heating, First Year's Experience." ASHRAE Transactions, vol. 128, no. 2, July 2022, pp. 107+