<\/span><\/h2>\nBuilding energy demand is the total amount of thermal energy a building requires to maintain a defined indoor temperature over a specific period. It is expressed in kilowatt-hours per year (kWh\/a) or kilowatt-hours per square metre per year (kWh\/m\u00b2a). This value is the single most critical input for designing and sizing a heat pump system.<\/p>\n
Energy demand is not a fixed property. It changes with outdoor temperature, building age, insulation quality, occupant behaviour, and ventilation losses. A correctly determined building energy demand leads to a correctly sized heat pump \u2014 and a correctly sized heat pump delivers efficiency, comfort, and low operating costs.<\/p>\n
Primary Unit:<\/strong> kWh\/a (annual energy demand) and W or kW (peak heat load at design conditions)<\/p>\n<\/div><\/span>What is the Core Purpose of a Building Energy Demand for a Heat Pump Installation<\/span><\/h2>\nBuilding energy demand serves one fundamental purpose: it tells the heat pump installer exactly how much heating power the building needs under the worst-case outdoor conditions. This number governs every subsequent engineering decision \u2014 from selecting the heat pump model to sizing the distribution system, buffer storage, and domestic hot water preparation.<\/p>\n
Without an accurate energy demand calculation, a heat pump will be either undersized (unable to heat the building) or oversized (short-cycling, low efficiency, excessive wear). Both outcomes increase costs and reduce system lifespan.<\/p>\n<\/div>
<\/span>Why Building Energy Demand Is Needed<\/span><\/h2>\nCorrect Heat Pump Sizing<\/h3>\n
Heat pumps are rated by their heating output (kW). The rated output must match the building’s peak heat load. A mismatch of even 20\u201330% causes measurable efficiency loss. EN 12831-1 defines the calculation method for peak heat load across Europe.<\/p>\n
System Efficiency and COP<\/h3>\n
The Coefficient of Performance (COP) and Seasonal Coefficient of Performance (SCOP) of a heat pump depend directly on the temperature difference between the heat source and the distribution system. A building with high energy demand requires a higher flow temperature, which reduces COP. Reducing energy demand \u2014 through insulation improvements \u2014 raises COP and lowers operating costs.<\/p>\n
Regulatory Compliance<\/h3>\n
In Austria, Germany, and Switzerland, building energy performance is regulated by law:<\/p>\n
\n- Austria:<\/strong> OIB-Richtlinie 6 (Energieeinsparung und W\u00e4rmeschutz) and the Heizkostengesetz<\/li>\n
- Germany:<\/strong> Geb\u00e4udeenergiegesetz (GEG 2024) \u2014 replaces EnEV and EEW\u00e4rmeG<\/li>\n
- Switzerland:<\/strong> Mustervorschriften der Kantone im Energiebereich (MuKEn 2014)<\/li>\n
- EU:<\/strong> Energy Performance of Buildings Directive (EPBD, recast 2024)<\/li>\n<\/ul>\n
All these frameworks require an energy performance certificate (Energieausweis \/ Energiepass) that documents building energy demand. Heat pump system design must align with these certified values.<\/p>\n
Financial Subsidy Eligibility<\/h3>\n
Government subsidies for heat pump installation in Austria (Bundesf\u00f6rderung), Germany (BEG \u2014 Bundesf\u00f6rderung f\u00fcr effiziente Geb\u00e4ude), and Switzerland (kantonale F\u00f6rderprogramme) are tied to verified energy performance thresholds. A certified energy demand calculation is a mandatory document for most subsidy applications.<\/p>\n
<\/span>Key Features of Building Energy Demand<\/span><\/h2>\n<\/div>\n\n
\n\n\n| Feature<\/th>\n | Description<\/th>\n<\/tr>\n<\/thead>\n |
\n\n| Peak Heat Load (Normheizlast)<\/td>\n | Maximum heating power required at design outdoor temperature (W or kW)<\/td>\n<\/tr>\n |
\n| Annual Heat Demand (Jahresheizw\u00e4rmebedarf)<\/td>\n | Total heating energy needed per year (kWh\/a)<\/td>\n<\/tr>\n |
\n| Specific Heat Demand<\/td>\n | Energy demand per square metre of heated floor area (kWh\/m\u00b2a)<\/td>\n<\/tr>\n |
\n| Transmission Heat Loss<\/td>\n | Heat lost through walls, roof, windows, floor, and doors<\/td>\n<\/tr>\n |
\n| Ventilation Heat Loss<\/td>\n | Heat lost through air exchange \u2014 controlled and infiltration<\/td>\n<\/tr>\n |
\n| Internal Gains<\/td>\n | Heat generated by occupants, appliances, and solar radiation<\/td>\n<\/tr>\n |
\n| Design Outdoor Temperature<\/td>\n | Lowest outdoor temperature used for worst-case calculation (varies by location)<\/td>\n<\/tr>\n |
\n| Climate Zone<\/td>\n | Geographic zone that defines outdoor temperature and solar radiation data<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/span>Detailed Explanation of Key Features<\/span><\/h2>\nPeak Heat Load (Normheizlast)<\/h3>\nDefinition:<\/strong> The peak heat load is the maximum thermal power a building needs when outdoor temperatures reach their lowest design value.<\/p>\nPurpose:<\/strong> It defines the minimum required output of the heat pump at design conditions.<\/p>\nBenefits:<\/strong> An accurate peak load prevents undersizing. It also reveals whether a monovalent or bivalent heat pump system is appropriate.<\/p>\nExample:<\/strong> A detached house in Vienna (design outdoor temperature: \u201313 \u00b0C) with a peak heat load of 9 kW requires a heat pump with a rated output of at least 9 kW at this temperature. A unit rated at 10 kW at A\u201313\/W35 covers the load with a small safety margin.<\/p>\nAnnual Heat Demand (Jahresheizw\u00e4rmebedarf)<\/h3>\nDefinition:<\/strong> The annual heat demand is the total thermal energy the building consumes for space heating over a full calendar year.<\/p>\nPurpose:<\/strong> It determines the expected energy consumption and annual operating cost of the heat pump.<\/p>\nBenefits:<\/strong> A lower annual demand means lower electricity bills. It also qualifies the building for better subsidy tiers and higher energy performance ratings.<\/p>\nExample:<\/strong> A 150 m\u00b2 house with an annual heat demand of 12,000 kWh\/a and a heat pump SCOP of 4.0 will consume approximately 3,000 kWh of electricity per year for space heating.<\/p>\nTransmission Heat Loss<\/h3>\nDefinition:<\/strong> Transmission loss is heat that flows from the warm interior to the cold exterior through the building envelope \u2014 walls, roof, windows, basement floor, and doors.<\/p>\nPurpose:<\/strong> It is the largest single component of heat demand in most existing buildings.<\/p>\nBenefits:<\/strong> Reducing transmission loss through insulation (increasing thermal resistance) directly lowers the required heat pump output and annual energy consumption.<\/p>\nCalculation basis:<\/strong> Transmission loss is calculated using:<\/p>\n\n- U-value (W\/m\u00b2K):<\/strong> The thermal transmittance of each building component<\/li>\n
- Area (m\u00b2):<\/strong> The surface area of each component<\/li>\n
- Temperature difference (K):<\/strong> Between indoor design temperature and outdoor design temperature<\/li>\n<\/ul>\n
Formula:<\/strong> Q_T = \u03a3 (U \u00d7 A \u00d7 \u0394T) [in Watts]<\/p>\nVentilation Heat Loss<\/h3>\nDefinition:<\/strong> Ventilation heat loss is thermal energy lost through air exchange \u2014 both controlled mechanical ventilation and uncontrolled air infiltration through gaps in the building envelope.<\/p>\nPurpose:<\/strong> It is the second major component of heat demand and grows in importance as insulation improves.<\/p>\nBenefits:<\/strong> A mechanical ventilation system with heat recovery (MVHR \/ Wohnrauml\u00fcftung mit W\u00e4rmer\u00fcckgewinnung) can recover 80\u201390% of this heat, significantly reducing demand.<\/p>\nKey value:<\/strong> Air change rate (ACH \u2014 Air Changes per Hour) defines how often the total air volume is replaced. Passive House standard targets \u2264 0.6 ACH at 50 Pa (n50 test value).<\/p>\nInternal and Solar Gains<\/h3>\nDefinition:<\/strong> Internal gains are heat produced inside the building by occupants, lighting, appliances, and computers. Solar gains are heat entering through glazed surfaces from solar radiation.<\/p>\nPurpose:<\/strong> These gains reduce the net heating energy the heat pump must supply.<\/p>\nBenefits:<\/strong> A building optimised for passive solar gains through south-facing glazing (in the northern hemisphere) reduces annual heating demand without additional system cost.<\/p>\nLimitation:<\/strong> Gains must be modelled carefully. Overestimating gains leads to an undersized heat pump that cannot cover demand during cloudy cold periods.<\/p>\nDesign Outdoor Temperature<\/h3>\nDefinition:<\/strong> The design outdoor temperature (Auslegungsau\u00dfentemperatur) is the lowest air temperature used in the peak heat load calculation. It represents an extreme cold event, not the average winter temperature.<\/p>\nPurpose:<\/strong> It defines the worst-case operating conditions for the heat pump.<\/p>\nWhy it matters:<\/strong> Heat pump output decreases as outdoor temperature drops. The heat pump must still cover the building’s full heat load at the design outdoor temperature.<\/p>\nExample values by location:<\/strong><\/p>\n<\/div>\n\n \n\n\n| City<\/th>\n | Design Outdoor Temperature (EN 12831)<\/th>\n<\/tr>\n<\/thead>\n | \n\n| Vienna (Wien)<\/td>\n | \u201313 \u00b0C<\/td>\n<\/tr>\n | \n| Munich (M\u00fcnchen)<\/td>\n | \u201314 \u00b0C<\/td>\n<\/tr>\n | \n| Zurich (Z\u00fcrich)<\/td>\n | \u201310 \u00b0C<\/td>\n<\/tr>\n | \n| Innsbruck<\/td>\n | \u201317 \u00b0C<\/td>\n<\/tr>\n | \n| Bolzano \/ Bozen<\/td>\n | \u20138 \u00b0C<\/td>\n<\/tr>\n | \n| Berlin<\/td>\n | \u201314 \u00b0C<\/td>\n<\/tr>\n | \n| Hamburg<\/td>\n | \u201312 \u00b0C<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/span>Types of Building Energy Demand Calculations<\/span><\/h2>\nSimplified Estimation (\u00dcberschlagsrechnung)<\/h3>\nWhat it is:<\/strong> A rough calculation based on building size, construction year, and building type. Uses standardised benchmarks (e.g., 100 kWh\/m\u00b2a for an uninsulated 1970s building).<\/p>\nWhen used:<\/strong> Pre-feasibility assessment. Initial heat pump model selection. Rough subsidy eligibility check.<\/p>\nAccuracy:<\/strong> \u00b130\u201350%. Not suitable for final system design.<\/p>\nLimitation:<\/strong> Does not account for individual building characteristics. Leads to significant over- or undersizing.<\/p>\nEnergy Performance Certificate Calculation (Energieausweis)<\/h3>\nWhat it is:<\/strong> A standardised calculation of building energy demand performed by a certified energy consultant. Required for building permits, property sales, and rental listings across the EU.<\/p>\nStandard:<\/strong> OIB-Richtlinie 6 (Austria), DIN V 18599 (Germany), SIA 380\/1 (Switzerland)<\/p>\nOutput:<\/strong> Heating energy demand class (A++ to G), specific energy demand (kWh\/m\u00b2a), CO\u2082 emissions, and primary energy factor.<\/p>\nWhen used:<\/strong> Mandatory for subsidy applications. Used as input for heat pump pre-sizing.<\/p>\nAccuracy:<\/strong> \u00b115\u201325%. Suitable for subsidy applications and general system planning.<\/p>\nDetailed Heat Load Calculation per EN 12831<\/h3>\nWhat it is:<\/strong> A room-by-room thermal calculation of peak heat load according to European Standard EN 12831-1. Considers every building component, orientation, thermal bridges, climate data, and ventilation system.<\/p>\nStandard:<\/strong> EN 12831-1:2017 (Heat load calculation) and EN 12831-3 (Pipe system design)<\/p>\nOutput:<\/strong> Peak heat load for the entire building (kW) and for each individual room (W). Basis for radiator and underfloor heating design.<\/p>\nWhen used:<\/strong> Required for correct heat pump sizing. Mandatory input for underfloor heating and radiator design. Required for many subsidy programmes.<\/p>\nAccuracy:<\/strong> \u00b15\u201310%. Industry standard for professional heat pump installation.<\/p>\nDynamic Building Simulation<\/h3>\nWhat it is:<\/strong> A time-resolved simulation of building thermal performance using hourly climate data. Tools include EnergyPlus, TRNSYS, IDA ICE, and DesignBuilder.<\/p>\nWhen used:<\/strong> Complex commercial buildings. Passive House certification (PHPP \u2014 Passive House Planning Package). Research and high-performance building design.<\/p>\nAccuracy:<\/strong> \u00b15% when properly calibrated.<\/p>\nLimitation:<\/strong> Time-intensive and requires specialist expertise. Rarely used for standard residential heat pump installation.<\/p>\n<\/span>What are the Use Cases of a Building Energy Demand in a Heat Pump Installation<\/span><\/h2>\nExisting Building Renovation (Bestandsgeb\u00e4ude)<\/h3>\nScenario:<\/strong> A homeowner in Graz replaces a gas boiler with a heat pump.<\/p>\nChallenge:<\/strong> The existing building has unknown thermal properties. Radiators are sized for high-flow temperatures (70\u201380 \u00b0C). The heat pump operates most efficiently at low flow temperatures (35\u201345 \u00b0C).<\/p>\nSolution process:<\/strong><\/p>\n\n- Conduct EN 12831 heat load calculation<\/li>\n
- Assess existing radiators \u2014 can they deliver sufficient heat at 45 \u00b0C?<\/li>\n
- Identify insulation upgrades that reduce peak load to enable monovalent operation<\/li>\n
- Select heat pump with output matching verified peak load at the local design outdoor temperature<\/li>\n<\/ol>\n
Key output:<\/strong> The heat load calculation reveals which rooms require radiator replacement and what flow temperature the system needs.<\/p>\nNew Construction (Neubau)<\/h3>\nScenario:<\/strong> A developer builds a multi-family building in Zurich to Minergie-P standard.<\/p>\nChallenge:<\/strong> Subsidy eligibility and building permit require verified energy demand below 30 kWh\/m\u00b2a.<\/p>\nSolution process:<\/strong><\/p>\n\n- Calculate energy demand during design phase using SIA 380\/1 \/ PHPP<\/li>\n
- Optimise building envelope to meet target demand<\/li>\n
- Size heat pump based on EN 12831 peak load<\/li>\n
- Document all values for Minergie certification and subsidy application<\/li>\n<\/ol>\n
Key output:<\/strong> Certified energy demand enables subsidy approval and heat pump selection.<\/p>\nRural Alpine Location (Alpines Gebiet)<\/h3>\nScenario:<\/strong> A farmhouse in Tyrol at 900 m elevation needs a heat pump.<\/p>\nChallenge:<\/strong> Design outdoor temperature reaches \u201320 \u00b0C or lower. Standard air-source heat pumps lose capacity at extreme cold.<\/p>\nSolution process:<\/strong><\/p>\n\n- Use EN 12831 with local climate data (\u00d6NORM B 8135 for Austria)<\/li>\n
- Determine peak load at \u201320 \u00b0C<\/li>\n
- Evaluate air-source heat pump performance curves at \u201320 \u00b0C<\/li>\n
- Consider bivalent system (heat pump + backup heater) or ground-source heat pump (GSHP) which maintains stable output<\/li>\n<\/ol>\n
Key output:<\/strong> The energy demand calculation determines whether monovalent or bivalent operation is technically and economically appropriate.<\/p>\n<\/span>What are the Benefits of Accurate Building Energy Demand Assessment | |