Air Source Heat Pump Ambient Loop
As the name suggests, the ambient loop operates at significantly lower temperatures than conventional public high-temperature heating systems. This means that a different approach is required when sizing an environmental loop distribution system. Calculations for such systems follow a procedure that uses diversity to determine the developed design flow. It also sized piping and circulation pumps for environmental utility network solutions. The required in-apartment flow for a heat pump of the chosen size can then be calculated.
The heat pump energy system uses an ambient public water circuit designed to 25°C and a water-to-water heat pump in the apartment to provide space heating, comfort cooling and hot water. The system has been successfully installed and started operating in numerous projects in the UK. These installations demonstrate that by reducing temperatures in the energy loop, heat pump energy systems can help address challenges associated with plant fuel sources, plant and riser spaces, and heat loss in common spaces. Heat distribution losses are one of the main reasons for the inefficiency of traditional public heating systems using centralized boiler systems, and the problem can be fundamentally solved by applying environmental distribution solutions.
"Guidelines for Designing Ambient Temperature Network Diversity Factors Using Heat Pump Systems" was created by TÜV SÜD, a global provider of technical testing and certification, in conjunction with Wallace Whittle, and provides guidelines for properly sizing environmental network factors using diversity factors.
How do we use the diversity factor to determine the design flow of development?
Once the correct sized air source heat pump is selected, the design flow of the system can be calculated as:
Mass Flow (kg/s) = Total Loop Load (kW) ÷ (ΔT x Cp)
Δt is the temperature difference between flow and return, which is 5k for the zeroth energy system. Cp refers to the specific heat capacity of water (4.18 kJ/(kg*K) at 25°C).
To complete the calculation, the total diversified peak load needs to be developed. This will be the sum of all air source heat pump duties:
Total Peak Load = (Space Heating Diversity x Total Apartment Heating Load) + (Hot Water Diversity x Sum of Zeroth Heat Pump Tasks.)
The total peak load calculation should include any systems connected to the environmental loop from any part of the development, such as apartments, common areas, commercial spaces. If cooling is provided by an Air Source Heat Pump Energy System, the total cooling load of the building, including any non-residential spaces, must be included.
The total loop load required for mass flow can now be calculated:
Total heat input to central equipment (total circuit load) = (space heating diversity x total energy extracted from the circuit in heating mode) + (hot water diversity x total extraction from the circuit in HWC* reheat mode) energy.)
The loop load is the energy required by each air source heat pump from the ambient loop. You can read more about how diversity calculations apply to HVAC systems and how it shapes HVAC specifications for residential developments here.
Selection of piping and environmental loop circulation pumps
As with traditional public heating system designs, the environmental loop of a heat pump energy system uses water as the distribution medium through pressurized plastic or steel pipe piping. The electrical energy required for pumping is the driving factor when sizing pipes for environmental loops. The size of the pipeline running away from the index may vary if desired. CIBSE Guide C should be consulted for guidance on speed and pressure drop limits.
The sizing of the ambient loop circulation pump will be determined by the flow rate of the index run and can be calculated using software or spreadsheets to achieve the minimum flow rate required. Correct calculations will help avoid oversizing the circulating pump, which can result in less efficient operation.
Heat pump energy systems are installed and ready to operate in many projects. For help with diversity calculations or system sizing, or to learn more about how Air Source Heat Pump Energy Systems can benefit your next project, get in touch with our team of HVAC experts.