Performance and Efficiency<\/h3>\n
Heat pump efficiency is not a fixed value. It varies with operating conditions. A correctly sized heat pump runs at lower flow temperatures, which increases COP.<\/p>\n
Example:<\/strong> An air source heat pump (ASHP) rated at COP 3.5 at 35\u00b0C flow temperature may drop to COP 2.4 at 55\u00b0C. System sizing identifies what flow temperature the building actually requires, enabling the installer to select the right unit and the right emitters.<\/p>\nComfort and Reliability<\/h3>\n
Rooms sized individually \u2014 not averaged \u2014 receive precisely the heat they need. Bathrooms require more heat per m\u00b2 than bedrooms. North-facing rooms lose more heat than south-facing ones. Room-by-room sizing eliminates cold spots and overheated zones.<\/p>\n
Grant Eligibility<\/h3>\n
Across the DACH region, funding programmes require documented system sizing:<\/p>\n
\n- Germany BEG:<\/strong> Heizlastberechnung nach EN 12831 mandatory for heat pump grants<\/li>\n
- Austria Raus aus \u00d6l und Gas:<\/strong> Certified energy consultant or installer must verify sizing<\/li>\n
- Switzerland cantonal programmes:<\/strong> Vary by canton, but professional load calculation is standard requirement<\/li>\n<\/ul>\n
<\/span>What are the Key Features of System Sizing for a Heat Pump Installation<\/span><\/h2>\nHeat Loss Calculation (Heizlastberechnung)<\/h3>\n
Definition:<\/strong> The calculation of heat energy lost through a building’s fabric and ventilation under design winter conditions.<\/p>\nPurpose:<\/strong> Establishes the maximum heating power the heat pump must deliver.<\/p>\nBenefits:<\/strong><\/p>\n\n- Prevents undersizing in extreme cold weather<\/li>\n
- Identifies the building envelope elements contributing most to heat loss<\/li>\n
- Provides the basis for room-by-room emitter design<\/li>\n<\/ul>\n
Practical application:<\/strong> A 150 m\u00b2 detached house in Vienna with 1990s-standard insulation may have a design heat load of 9\u201312 kW. The same house, retrofitted to current standards, may have a design heat load of 4\u20136 kW. System sizing reveals this difference \u2014 and therefore the correct pump capacity.<\/p>\nDesign Outdoor Temperature (Auslegungstemperatur)<\/h3>\n
Definition:<\/strong> The minimum outdoor temperature used as the basis for heat loss calculations \u2014 typically the lowest temperature that occurs for no more than 10 hours per year at a given location.<\/p>\nPurpose:<\/strong> Sets the worst-case thermal condition the heat pump must handle.<\/p>\nBenefits:<\/strong><\/p>\n\n- Prevents undersizing in rare but critical cold-weather events<\/li>\n
- Avoids oversizing based on extreme outlier temperatures<\/li>\n<\/ul>\n
Design outdoor temperatures by region (approximate):<\/strong><\/p>\n<\/div>\n\n
\n\n\n| Location<\/th>\n | Design Outdoor Temperature<\/th>\n<\/tr>\n<\/thead>\n |
\n\n| Berlin<\/td>\n | \u221212\u00b0C<\/td>\n<\/tr>\n |
\n| Munich<\/td>\n | \u221214\u00b0C<\/td>\n<\/tr>\n |
\n| Vienna<\/td>\n | \u221213\u00b0C<\/td>\n<\/tr>\n |
\n| Zurich<\/td>\n | \u221212\u00b0C<\/td>\n<\/tr>\n |
\n| Innsbruck<\/td>\n | \u221216\u00b0C<\/td>\n<\/tr>\n |
\n| Bozen \/ Bolzano<\/td>\n | \u221210\u00b0C<\/td>\n<\/tr>\n |
\n| Hamburg<\/td>\n | \u221210\u00b0C<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n These values come from national meteorological datasets and are defined in EN 12831 reference annexes for each country.<\/p>\n<\/div>\n Building Fabric Assessment (U-Wert-Analyse)<\/h3>\nDefinition:<\/strong> The measurement or calculation of heat transfer coefficients (U-values) for all external building elements \u2014 walls, roof, floor, windows, and doors.<\/p>\nPurpose:<\/strong> Quantifies how quickly heat escapes through each surface under a given temperature differential.<\/p>\nBenefits:<\/strong><\/p>\n\n- Identifies the weakest thermal links in the building envelope<\/li>\n
- Supports recommendations for fabric improvement before or alongside heat pump installation<\/li>\n
- Enables accurate calculation of transmission heat loss (HT)<\/li>\n<\/ul>\n
U-value reference benchmarks (EN ISO 6946):<\/strong><\/p>\n<\/div>\n\n \n\n\n| Building Element<\/th>\n | Pre-1980 U-value<\/th>\n | Current Standard (approx.)<\/th>\n<\/tr>\n<\/thead>\n | \n\n| External wall<\/td>\n | 1.0\u20131.5 W\/m\u00b2K<\/td>\n | 0.15\u20130.25 W\/m\u00b2K<\/td>\n<\/tr>\n | \n| Roof<\/td>\n | 0.8\u20131.2 W\/m\u00b2K<\/td>\n | 0.10\u20130.20 W\/m\u00b2K<\/td>\n<\/tr>\n | \n| Ground floor<\/td>\n | 0.8\u20131.0 W\/m\u00b2K<\/td>\n | 0.25\u20130.35 W\/m\u00b2K<\/td>\n<\/tr>\n | \n| Windows<\/td>\n | 2.5\u20134.0 W\/m\u00b2K<\/td>\n | 0.8\u20131.2 W\/m\u00b2K<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\nVentilation Heat Loss (L\u00fcftungsw\u00e4rmeverlust)<\/h3>\nDefinition:<\/strong> The heat energy lost through air exchange \u2014 both intentional ventilation and uncontrolled infiltration through gaps in the building fabric.<\/p>\nPurpose:<\/strong> Adds the ventilation component to the total heat loss figure, which can represent 20\u201340% of total heat demand in older buildings.<\/p>\nBenefits:<\/strong><\/p>\n\n- Ensures the heat load calculation is complete<\/li>\n
- Identifies buildings where airtightness improvements would significantly reduce heat demand before system sizing is finalised<\/li>\n<\/ul>\n
Key factors affecting ventilation heat loss:<\/strong><\/p>\n\n- Air change rate (n\u2085\u2080 value from blower door test, or assumed from building type)<\/li>\n
- Volume of each room or zone<\/li>\n
- Difference between indoor design temperature and outdoor design temperature<\/li>\n<\/ul>\n
Room-by-Room Load Distribution<\/h3>\nDefinition:<\/strong> The apportionment of the total building heat load across individual rooms or thermal zones.<\/p>\nPurpose:<\/strong> Enables accurate sizing of heat emitters \u2014 radiators or underfloor heating circuits \u2014 in each room.<\/p>\nBenefits:<\/strong><\/p>\n\n- Eliminates overheated and underheated rooms<\/li>\n
- Enables hydraulic balancing of the distribution system<\/li>\n
- Forms the basis for thermostatic control zone design<\/li>\n<\/ul>\n
Example room heat loads (150 m\u00b2 house, Vienna, refurbished):<\/strong><\/p>\n<\/div>\n\n \n\n\n| Room<\/th>\n | Area<\/th>\n | Heat Load<\/th>\n<\/tr>\n<\/thead>\n | \n\n| Living room<\/td>\n | 30 m\u00b2<\/td>\n | 900 W<\/td>\n<\/tr>\n | \n| Kitchen<\/td>\n | 15 m\u00b2<\/td>\n | 420 W<\/td>\n<\/tr>\n | \n| Master bedroom<\/td>\n | 14 m\u00b2<\/td>\n | 350 W<\/td>\n<\/tr>\n | \n| Bathroom<\/td>\n | 8 m\u00b2<\/td>\n | 400 W<\/td>\n<\/tr>\n | \n| Child’s room<\/td>\n | 12 m\u00b2<\/td>\n | 290 W<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n Note: Bathrooms carry high heat loads relative to their area due to elevated design temperatures (22\u201324\u00b0C) and frequent towel rail loads.<\/p>\n<\/div>\n Flow Temperature Determination (Vorlauftemperatur)<\/h3>\nDefinition:<\/strong> The target water temperature the heat pump must supply to the heat distribution system to meet the design heat load.<\/p>\nPurpose:<\/strong> Determines whether existing emitters can work with the heat pump, and at what efficiency the heat pump will operate.<\/p>\nBenefits:<\/strong><\/p>\n\n- Lower flow temperatures = higher COP = lower electricity costs<\/li>\n
- Reveals whether radiator upgrades are needed<\/li>\n
- Informs the choice between monovalent and bivalent system design<\/li>\n<\/ul>\n
Flow temperature impact on COP (typical ASHP, -7\u00b0C outdoor):<\/strong><\/p>\n<\/div>\n\n \n\n\n| Flow Temperature<\/th>\n | Approximate COP<\/th>\n<\/tr>\n<\/thead>\n | \n\n| 35\u00b0C<\/td>\n | 3.2\u20134.0<\/td>\n<\/tr>\n | \n| 45\u00b0C<\/td>\n | 2.5\u20133.2<\/td>\n<\/tr>\n | \n| 55\u00b0C<\/td>\n | 2.0\u20132.5<\/td>\n<\/tr>\n | \n| 65\u00b0C<\/td>\n | 1.6\u20132.0<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n The heat loss calculation directly determines what flow temperature is required. That flow temperature directly sets the system’s annual efficiency. System sizing is therefore the primary lever for minimising heat pump running costs.<\/p>\n<\/div>\n Domestic Hot Water (DHW) Load Assessment<\/h3>\nDefinition:<\/strong> The calculation of hot water demand for sanitary use, expressed as daily volume (litres) and peak power demand (kW).<\/p>\nPurpose:<\/strong> Determines whether the heat pump must cover DHW production, and if a separate DHW heat pump or immersion heater top-up is required.<\/p>\nBenefits:<\/strong><\/p>\n\n- Correctly sizes the hot water storage cylinder<\/li>\n
- Prevents DHW production from competing with space heating during peak demand<\/li>\n
- Informs legionella protection planning (60\u00b0C pasteurisation cycles)<\/li>\n<\/ul>\n
DHW load estimation guidance:<\/strong><\/p>\n<\/div>\n\n \n\n\n| Household Size<\/th>\n | Estimated Daily DHW<\/th>\n | Storage Cylinder<\/th>\n<\/tr>\n<\/thead>\n | \n\n| 1\u20132 persons<\/td>\n | 80\u2013120 litres at 55\u00b0C<\/td>\n | 150\u2013200 litres<\/td>\n<\/tr>\n | \n| 3\u20134 persons<\/td>\n | 150\u2013200 litres at 55\u00b0C<\/td>\n | 200\u2013300 litres<\/td>\n<\/tr>\n | \n| 5+ persons<\/td>\n | 200\u2013300 litres at 55\u00b0C<\/td>\n | 300\u2013500 litres<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/span>Types of System Sizing Methods<\/span><\/h2>\nFull EN 12831 Heat Loss Calculation<\/h3>\nThe gold-standard method. Performed by a certified engineer or qualified installer using room-by-room calculations based on actual U-values, dimensions, ventilation rates, and location-specific design temperatures.<\/p>\n Used for:<\/strong> All grant-funded installations, new builds, major retrofits.<\/p>\nOutput:<\/strong> Detailed heat load report, room-by-room breakdown, system design specification.<\/p>\nAccuracy:<\/strong> \u00b15\u201310% of actual heat demand when building data is correct.<\/p>\nSoftware-Assisted Calculation<\/h3>\nSpecialist software tools \u2014 such as TRNSYS<\/strong>, DesignBuilder<\/strong>, CalHeat<\/strong> (Austria), or proprietary tools from manufacturers like Vaillant, Viessmann, and Daikin \u2014 perform EN 12831 calculations with guided data input.<\/p>\nUsed for:<\/strong> Standard residential and light commercial installations.<\/p>\nOutput:<\/strong> Automated load report, equipment recommendation, system schematic.<\/p>\nAccuracy:<\/strong> Equivalent to manual EN 12831 when input data is accurate.<\/p>\nRule-of-Thumb Estimation (Faustformel)<\/h3>\nA simplified method using estimated W\/m\u00b2 values based on building type and age. Not suitable for grant applications or new builds.<\/p>\n Common W\/m\u00b2 reference values (approximate, uninsulated to well-insulated):<\/strong><\/p>\n<\/div>\n\n | | | | |