Water Coils for Air-to-Water Heat Pump Systems

Choosing the Right Coil to Optimize Your System’s Performance

Air-to-water heat pump systems are highly efficient HVAC solutions, but optimizing their performance depends on addressing key factors – most notably, the hot water supply temperature (HWST). And when these heat pumps are paired with a VAV distribution system, it’s especially critical to select the correct coil configuration.

How Lower HWST Affects Heat Pump and VAV Systems

Lower HWST decreases the temperature difference between the heat source (ambient air) and heat sink (water loop) in the heat pump’s primary heat exchanger. This results in reduced compressor work, where smaller temperature lifts reduce energy consumption, as well as a higher coefficient of performance, where less compressor workload increases heating efficiency.

Challenges With Coil Designs in Low HWST Systems

In VAV systems, reheat coils face significant challenges in low HWST scenarios. To meet the building heating demands, higher fluid flow rates or larger coils – such as  three-row coils and high-capacity coils – have historically been used. These solutions, however, often come with the following drawbacks:

• Small Water ΔT: Using standard two-row coils can lead to insufficient temperature differences, or ΔT, which indicates excessive fluid velocity. This generates the need for significant pumping energy and larger distribution piping and can result in higher operational costs.
• Increased Air Pressure Drop: Standard three-row coils create higher resistance on the air side, leading to inefficiencies and greater fan energy use.
• Space Constraints: Larger components are less suitable for retrofits and confined spaces.

Proper coil selection and system design are essential for maintaining an adequate ΔT (10°F –20°F minimum) that balances heat transfer efficiency with hydronic system performance while avoiding these pitfalls.

Benefits of Integrating Low Water Temperature Coils

Low Water Temperature (LWT) coils, particularly in two-row configurations, provide efficient thermal performance without requiring three-row coils or oversized and duct-mounted alternatives. This makes for a cost-effective design, with a reduced first cost and improved heat transfer.

Further, compared  to a three-row coil configuration, two-row LWT coils trade significant air pressure drop for marginal fluid pressure drop. This is a good deal – slightly higher pump capacity is affordable, and frictional energy from pumping mostly adds heat to the water loop without significantly impacting system efficiency.

Applications for Lower HWST and LWT Coils

In new construction, LWT coils are ideal for modern building designs, enabling energy-efficient operation with a lower HWST. In retrofits, LWT coils provide a cost-effective solution for VAV system air-to-water heat pumps, allowing for a new hydronic system to be used without needing to completely redesign the air distribution system.

Lowering the HWST and integrating LWT coil designs improve air-to-water heat pump VAV system efficiency. These strategies boost a heat pump’s coefficient of performance, ensure an optimal ΔT in VAV systems and offer cost-effective solutions for both new and retrofit projects. With these methods, air-to-water heat pump VAV systems can achieve superior performance and sustainability in HVAC applications.

Price’s project and application engineers are ready to assist design engineers, building owners, architects and mechanical contractors with product choices that can optimize air distribution system efficiency.

Click here to find a sales representative or technical expert near you, or reach out to our team at airmovement@priceindustries.com.