Technical Review: Determining the Minimum Water Volume for an 8kW Air-to-Water Heat Pump System
Mike Teahan
Expert in Heat Pump System Solutions - Hisense HVAC. Hitachi. LG. Swegon. e.VEN. e.Stór (Phasestor UK) (RBSi)
In the realm of heat pump installations, one critical aspect that demands meticulous attention is ensuring the proper system water volume, particularly in setups that lack buffer tanks. This technical review delves into the intricacies of determining the minimum water volume required for an 8kW air-to-water heat pump system. We cover key considerations, precise calculations, and the practical implications of ensuring your heat pump operates efficiently, avoiding issues like short cycling or system alarms.
Background: The Heat Pump and System Characteristics
The system under examination features an 8kW output air-to-water heat pump with a compressor that can modulate between 20 Hz and 90 Hz. The system is designed with an open circuit, without any buffer tank, making the determination of the correct system water volume even more essential. A minimum flow rate of 0.4 m3/h is necessary to prevent alarms.
The main challenge is ensuring that the system contains sufficient water volume to accommodate the heat output at the lowest compressor modulation (20 Hz) while maintaining stable operation.
Key System Specifications:
Calculating the Minimum Water Volume
To determine the minimum water volume, we first estimated the heat output at the minimum compressor modulation. Assuming linear modulation, the output at 20 Hz was estimated to be 1.78 kW, or approximately 20% of the maximum output.
The calculated minimum water volume required to ensure stable operation was approximately 51.1 liters. This volume is necessary to provide enough thermal mass to prevent short cycling and to maintain the required flow rate.
Accounting for Pipework Volume
Next, we considered the volume of water in the system's pipework. With an internal diameter of 19.2mm for the 22mm copper pipe and a total length of 10 meters, we calculated the pipe volume to be about 2.9 liters.
Final Minimum System Volume
By adding the pipework volume to the calculated minimum water volume, the total system volume required is approximately 54 liters. This is the minimum volume needed to avoid triggering low-flow alarms and ensure efficient system operation.
Expansion: Considering Underfloor Heating with 16mm x 2mm Pipework
In many modern heating systems, underfloor heating (UFH) is commonly used as an efficient means of distributing heat. Given that underfloor heating circuits often involve long lengths of pipework with smaller diameters, it's crucial to determine how much pipe is needed to meet the minimum system volume requirements in an open circuit.
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Key Assumptions and Specifications:
Calculation of Pipe Volume for 16mm Pipework
To determine the length of 16mm pipe needed, we first calculate the internal volume of the pipe per meter length.
Total Length of 16mm Pipe Required
To find the total length of pipe required to achieve the 54 liters of minimum volume:
Practical Considerations:
In an underfloor heating system, having nearly 479 meters of 16mm pipe in an open circuit is quite feasible, as underfloor heating circuits often extend over large areas. Depending on the design of the UFH system, multiple loops of piping could be employed to reach this total length, ensuring that the minimum water volume is achieved.
Conclusion and Practical Implications
For an 8kW air-to-water heat pump system, the minimum required water volume, including considerations for underfloor heating, is approximately 54 liters. This can be achieved either through a combination of primary circuit piping and underfloor heating circuits or solely through UFH, given the appropriate length of 16mm piping.
This calculation underscores the importance of system design in heat pump installations. Whether using standard piping or integrating underfloor heating, ensuring that the system has sufficient water volume is critical to avoiding short cycling, preventing alarms, and optimizing overall system efficiency.
By understanding these principles and applying them to your heat pump installations, you can ensure long-term reliability and efficiency in your heating systems.
Target Minimum System Volume: Approximately 54 liters
About the Author:
Mike Teahan is the Technical Director at RBSi, a leading provider of heat pump technology in Ireland and the UK. With over 25 years of experience in the industry, Mike is committed to advancing heating solutions through innovative design and expert knowledge.
Managing Director at GTEC Training and Heatly
2 个月Hi Mike Teahan, Is the Hisense unit you show the proportionate in its output ? I often see that although the compressor speeed reduces to say 20% that the thermal output is significantly higher say upwards of 40% even at 20% compressor frequency especially in warmer weather ?
Head of Heat Pump System Design & MCS Compliance at British Gas - Net Zero.
2 个月Hi Mike, good article ?? I have a few questions on this, such as why are we designing to the minimum, and also why are you looking at 6 stop starts per hour? Based on my estimate, that’s almost 15,000 compressor start stops a year, and with a standard compressor that has a life span of approximately 112,000 starts, it would kill the compressor in under 8 years. (Manufacturer warranty at 7 years is starting to make sense…) The majority of high level system designers and installers I speak to consider 3 per hour as an absolute worst case, but consistently aim for 2 and under, and this can be achieved through sufficient open system volume. Interested to get your view on this.
Plumbing Heating and Electrical Engineer
2 个月.
HVAC Consultant and Trainer | Consulting and educating busy HVAC professionals with limited resources to understand system design and control ?? Technical and Service Engineer at Panasonic ?? Ultramarathon Runner ????
2 个月Great article. However, volume below 54 L as calculated will not trigger flow errors if flow rate could be managed.
Head of Domestic Heat Pump Design Net Zero British Gas
2 个月Thanks Mike Teahan excellent content as usual.