How to design a heat pump system August 07, 2025 When it comes to installing air source heat pumps (ASHPs), proper sizing and specification are crucial for optimal performance and efficiency. As heating engineers, ensuring that the system is tailored to the building's specific needs will significantly improve its efficiency, provide reliable heating, and reduce energy consumption. In this blog, we'll guide you through the essential considerations for correctly sizing and designing a heat pump system. Proper Sizing & Design for Heat Pumps If the system is too small, it won’t meet the building’s heating demands, resulting in inadequate performance. On the other hand, an oversized unit will lead to unnecessary running costs and reduced efficiency. The key to finding the right balance lies in a thorough assessment of the property and its heating needs. 1. Understand the Heat Loss of the Building Before choosing a heat pump, understanding the property’s heat loss is crucial. The heat loss calculation accounts for factors such as: Room size Insulation quality of walls, windows, and doors Ceiling height Building materials Each room’s heat loss must be calculated to ensure that the heat pump selected can meet the building’s heating needs during the coldest periods. This can be done using tools like the Domestic Heating Design Guide or software provided by manufacturers like Grant UK. 2. Perform Full Heat Loss Calculations Heat loss calculations are essential to determine the amount of heating required for each room. Thermodynamics dictates that heat will always move from areas of high temperature to low temperature, so materials that conduct heat quickly will require more heating to maintain a comfortable indoor temperature. A proper heat loss calculation involves evaluating: U-values: This measures how well materials insulate a building and indicates how much heat is lost. Air change rates: The rate at which air in the building is replaced by outdoor air. Desired comfort levels: The ideal indoor temperature for different rooms in the home. Local climate conditions: The lowest temperatures are expected during winter in the area. Failure to carry out these calculations accurately can result in a system that is either overworked or underperforming, leading to discomfort and high energy bills. 3. Choosing the Right Heat Pump Capacity Once you’ve completed your heat loss calculations, selecting the appropriate heat pump capacity is the next step. It’s essential to consider the “drop-off” effect, which refers to the difference between a heat pump’s rated output and its actual output under lower external temperatures. For example, a 17kW heat pump may not always deliver the full 17kW when temperatures drop to -3°C (in southern regions) or -7°C (in colder areas). This performance variation must be factored into the system design to ensure reliable heating. A common mistake is to underestimate the required capacity or to oversize the unit. Both of these will affect the efficiency of the system and the homeowner’s comfort. 4. Selecting the Right Heat Emitters Air source heat pumps operate most efficiently with low-temperature heating systems. Therefore, selecting the right heat emitters is just as important as choosing the correct heat pump. Heat emitters like underfloor heating and larger radiators are ideal for heat pumps. Underfloor heating is especially beneficial in new builds and retrofits, as it allows the system to run at lower temperatures, maximising efficiency. In homes with existing radiators, installers may need to upgrade the radiator sizes to accommodate the lower flow temperatures required by the heat pump. As a general rule, radiators may need to be 2-2.5 times larger than those typically used in gas or oil systems to provide the same heat output. 5. Consider Additional Factors Air Change Rates Consider how often air is exchanged in the building In homes with poor ventilation, a higher heat pump capacity may be required. Winter Temperature Extremes The lowest temperature in your area will help you determine how much heating capacity is needed. Comfort Levels Some homeowners may prefer a higher temperature in certain areas, which will affect the overall sizing of the system. 6. Training and Resources For engineers looking to expand their knowledge, many manufacturers offer training programmes that cover heat pump system design. For example, Grant UK’s one-day Air Source Heat Pump System Design Course covers everything from heat loss calculations to emitter sizing and pressure loss calculation. Need assistance with your heat pump installation or sizing? Get in touch with our heating engineers across Hampshire, Portsmouth and Southampton by calling us on 07391473964 and we'll be happy to help.