Heat Load and Heat Pump Sizing Calculator

This is an explicitly simplified steady-state application of the transmission-plus-ventilation approach associated with DIN EN 12831-1:2017-09 and German DIN/TS 12831-1:2020-04. It is not an implementation of every normative provision. Check against the standard in force.

For each room, add $H_T=\sum (U_j+\Delta U)A_j$; the floor term alone also receives the entered reduction factor $b_f$. Ventilation is $H_V=0.33V[n_{inf}+n_{vent}(1-\eta_v)]$ in W/K. Multiply by indoor minus outdoor temperature and sum the counted rooms. The 0.33 Wh/(m³·K) air value follows the independently worked Reina report, pp.12–13. Thermal bridging is an explicit user allowance, not a copied DIN value.

For a gas-bill period, subtract the daily non-heating fuel allowance multiplied by the number of days, then multiply by seasonal efficiency. Divide useful heat by the degree-hour integral to infer $H$ in W/K. Extrapolate $Q=H(T_i-T_e)$ with no solar or internal gain credit. This is a one-period signature with known baseload, corresponding to $E=b\,days+h\,HDD$; it does not estimate both coefficients from a single observation. Degree Days.net explains day normalization for monthly regression. Different HDD bases require new HDD data, not just a changed temperature label.

Capacity is a straight interpolation between two thermal-output points at one water flow temperature. The historical WPL 15 AS example uses 6.16 kW at A−15/W35 and the 7.40 kW maximum at A7/W35, STIEBEL guide p.6. The result is a chord approximation, not the full manufacturer envelope. Intersect the straight building load and capacity lines for bivalence with either sign of their slope difference. State which side of an intersection has sufficient capacity. If there is no crossing in the supplied range, use the endpoint surpluses to report coverage or supplementary heat throughout that range. Parallel lines are valid; coincident lines match throughout and have no unique bivalence temperature. Supplementary heat is max(0, load − capacity). Ideal water flow is $\dot V=Q/(\rho c_p\Delta T_w)$, assuming water at 1000 kg/m³ and 4180 J/(kg·K).

Small outdoor-temperature examples

These are examples of the input, not city design values. Obtain the actual German postcode value from the BWP climate lookup (https://www.waermepumpe.de/werkzeuge/klimakarte/); no DIN climate tables are reproduced.

• Four room types or zones; counts multiply identical rooms. Default groups represent six rooms in a roughly 106 m² heated dwelling (265 m³ of air). Geometry, U-values, 0.05 W/(m²·K) bridge allowance and −12°C design temperature are author-selected scenarios, not survey data.
• Common fabric U-values, ventilation rates and floor factor apply to all groups. Window and door area is removed from opaque areas. Heated internal boundaries are omitted; the resulting group load is an external-loss contribution, not a full emitter load for rooms at different setpoints.
• Room losses exclude internal and solar gains and reheating allowances. Ground / basement effects use only the entered reduction factor; no detailed ground model is claimed.
• Heat recovery applies only to intentional ventilation; infiltration remains unrecovered. Enter operating air change, not n50. Avoid counting the same outdoor airflow twice.
• Bill records are actual readings for unchanged occupancy, heating schedules and fabric. Non-heating gas is a constant daily fuel allowance; efficiency must match the billed heating-value convention. HDD is the daily positive base-minus-outdoor sum, not an incompatible degree-day convention. Changing the base field alone does not change HDD.
• The bill method is a diagnostic cross-check. Underheating, fireplaces, vacant rooms, distribution losses and uncertain efficiency may bias it. It cannot reveal individual room loads.
• Performance data must be maximum available thermal output at the required water flow temperature and operating mode. The published example is historical, uses a two-point approximation and does not constitute a current product selection. Custom capacity may increase or decrease with outdoor temperature. Parallel or coincident demand and capacity lines remain valid comparisons.
• No automatic oversizing margin, hot-water allowance or diversity factor is hidden. When no bivalence point lies in the supplied range, report coverage or shortfall throughout it. Parallel slopes and coincidence are detected using relative numerical tolerances of 1e-10; these handle roundoff rather than physical measurement uncertainty. Water-flow screening excludes glycol.
• The hydraulic screening assumes passive heat emitters with no additional colder heat sink. Each circuit return must exceed its own room temperature. A mixed return is a flow-weighted average, so in room mode it must exceed the coldest active room. It may lie below the warmest room when parallel circuits mix; this prompts individual circuit and emitter checks. Supply water must exceed every active room setpoint.

Non-binding estimate. This calculator does not replace a room-by-room DIN EN 12831 calculation, subsidy evidence, hydraulic balancing method B, radiator sizing or a qualified designer's assessment.

Method context: DIN EN 12831-1:2017-09 and DIN/TS 12831-1:2020-04; regulatory context checked 25 October 2026 using the official BEG technical FAQ and government guidance. For funding and method B obtain the required room-by-room calculation and documentation. Recognised equivalent balancing procedures have separate approval requirements; this screening tool is not one. Check current programme conditions and the standard in force.

Use positive heat loss with outdoor temperatures below indoor temperatures. Supply capacity points bracketing the design condition and below room setpoints; extrapolation is blocked. Supply must exceed the mixed return and every active room temperature (or the representative indoor temperature in bill mode). In room mode, a mixed return at or below the coldest active room blocks results: passive emitters alone cannot produce it. Raise the return temperature or assess any additional colder heat sink outside this model. Returns above the coldest room but at or below the warmest room remain allowed with a warning to verify individual circuit temperatures and emitter outputs. The published heat-pump example is restricted to 35°C flow. A short bill or a period with few heating degree days is weak evidence.

The straight capacity segment omits local defrost dips, compressor limits, weather compensation, minimum modulation, silent-mode derating and control behaviour. A two-point comparison cannot certify year-round coverage or annual auxiliary energy. Design-point supplementary heat is a thermal shortfall, not installed electrical demand. DHW cylinder recovery and heating interruptions require separate design.

Installation, electrical connection, refrigeration and hydraulic adjustment belong to qualified persons.