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Guide to Using the Heating Load Calculator

Introduction

1.1 What is Heating Load?

The heating load is the thermal power (in watts or kilowatts) that a heating system must provide to bring a building to the desired room temperature and maintain it at the standard outdoor temperature (lowest expected outdoor temperature at the location).

Heating load calculation is fundamental for:

1.2 Standard Basis: DIN EN 12831-1

This calculator is based on DIN EN 12831-1 (Energy performance of buildings - Method for calculation of the design heat load). The standard defines a standardized procedure for calculating heating load for residential buildings under steady-state conditions (design case).

Important: Heating load is not annual heat demand. It describes the maximum power requirement at extreme outdoor temperatures, while annual heat demand indicates the energy required throughout the year.

Calculation Principles

2.1 Total Heating Load of a Room

The room heating load Q̇HL,R consists of several components:

Q̇HL,R = Q̇T + Q̇V + Q̇RH

Where:

2.2 Transmission Heat Loss (Q̇T)

Transmission heat loss describes the heat lost through walls, windows, doors, floors, and ceilings to the outside.

Formula for a single component:

Q̇T,Component = A · U · fT · (θi - θe)

Parameters:

Correction Factors (fT)

Adjacent SpaceCorrection Factor (fT)
Outdoor air1.0
Ground0.5 - 0.6
Unheated space (within thermal envelope)0.5
Heated space (same temperature)0.0

Thermal Bridge Surcharge (ΔU)

In addition to the component's U-value, a thermal bridge surcharge ΔU is considered:

Ueff = U + ΔU

Standard value: ΔU = 0.10 W/(m²·K) for exterior components (fT = 1.0)

Important: For components with fT = 0.0 (heated adjacent room), no thermal bridge surcharge is applied.

2.3 Ventilation Heat Loss (Q̇V)

Ventilation heat loss occurs through the exchange of warm room air with cold outdoor air.

Simplified formula:

Q̇V = V · n · 0.34 · (θi - θe)

Parameters:

2.4 Building Heating Load (Q̇HL,G)

According to DIN EN 12831-1:

Q̇HL,G = Σ Q̇T + 2 · Σ Q̇V

Step-by-Step Guide

3.1 Project Management

Create New Project

Click "Start Project" in the welcome screen or "New Project" in the action bar. The project wizard opens and guides you through all required inputs.

Load Existing Project 🆕

You can load an existing project anytime using the project key:

  1. Click "Load Project"
  2. Enter your 5-character project key (e.g., "ABC12")
  3. Click "Load"

The project key is displayed when creating a project and should be noted to access your project later.

Undo Changes 🆕

The calculator automatically saves a change history. Use the "↶ Undo" button in the project header to revert the last change.

Saved changes include:

Note: The change history is stored on the server. You can undo changes even after closing the browser, as long as you use the same project key.

3.2 Enter Project Master Data

Location and Climate Data

  1. Enter address: Street, house number, postal code, city
  2. Automatically load climate data

Important: For calculations compliant with standards, consult the BWP climate map.

Building Data

Heating System Settings

3.2 Option A: Simplified Input (Building Cubature)

The wizard guides you through 5 steps:

  1. Basic: Number of floors, basement, attic
  2. Geometry: Dimensions, roof shape
  3. Windows: Area and distribution
  4. Components: U-values
  5. Complete: Generate rooms

3.3 Option B: Detailed Room Input

Manually define each room with all components.

Per-Room Ventilation Concept 🆕

For each room, you can set an individual ventilation concept:

Ventilation Types:

Ventilation ConceptDescriptionTypical Air Change Rate
Window VentilationManual ventilation through windows0.5 h⁻¹
Mechanical without HRMechanical ventilation without heat recovery0.4-0.6 h⁻¹
Mechanical with HRControlled ventilation with heat recovery0.3-0.4 h⁻¹
Exhaust Air SystemExhaust air only (e.g., bathroom/WC)0.5-1.0 h⁻¹

Adjustable Parameters:

Tip: With heat recovery (HR) ventilation systems, ventilation heat loss is significantly reduced. An HR efficiency of 80% means 80% of the heat from exhaust air is recovered. This leads to considerable heating load reduction!

Important: Set correction factor to 0.0 for inter-floor ceilings between heated rooms!

Understanding Results

4.1 Results Overview

After calculation, you will receive comprehensive results in four tabs:

Tab 1: Heating Load (Main Results)

This tab shows the classic heating load calculation according to DIN EN 12831-1:

Summary values:

Q̇Heiz,G is decisive for sizing the heat generator (boiler, heat pump). It includes a surcharge (100% on ventilation losses) according to the standard for heating-up and system losses.

Room breakdown: For each room you see:

Tab 2: Annual Heat Demand Profile 🆕

Detailed analysis of annual heat demand based on real weather data (PVGIS TMY - Typical Meteorological Year):

Calculation methodology:

  1. For each hour of the year (8760 hours), outdoor temperature is read from PVGIS data

  2. If outdoor temperature < heating limit temperature:

    Q̇(h) = (Q̇trans + Q̇vent) · (θi - θe(h)) / (θi - θe,Norm)

    Where:

    • θi = Weighted average room temperature
    • θe(h) = Outdoor temperature at hour h
    • θe,Norm = Standard outdoor temperature (design case)
    • Q̇trans + Q̇vent = Calculated heating load

  3. Hourly heat demand [Wh] = Q̇(h) · 1 hour

  4. Annual heat demand = Sum of all hourly values

Data displayed:

Main KPIs (top area, blue box):

Monthly and Annual Charts:

Detailed Monthly Table: For each month:

Difference from heating load: Heating load (Tab 1) is the maximum power at standard outdoor temperature for extreme conditions. Annual heat demand (Tab 2) is based on real weather data and shows typical operation. Heating load is typically higher as it is designed for worst-case conditions.

Use cases:

Tab 3: Building Envelope Optimization Suggestions 🆕

Automatic analysis of optimization potential for the building envelope based on current GEG 2024 standards.

Calculation methodology:

1. Degree-Day Method for Energy Savings:

Energy savings [kWh/a] = A · (U_CURRENT - U_TARGET) · GTZ · 0.024

Where:

Formula for degree days:

GTZ = (20°C - θe,Norm) · Heating days

Example: θe,Norm = -12°C → GTZ = (20 - (-12)) · 200 = 6400 Kd/a

2. Heating Load Reduction:

Heating load reduction [kW] = A · (U_CURRENT - U_TARGET) · Î"T_Norm

Where:

GEG 2024 Target U-values (§ 48 GEG):

Component GroupTarget U-value [W/(m²·K)]
Exterior walls0.24
Roof / Top floor ceiling0.14
Floor to ground0.25
Windows0.95
Doors1.80

Data displayed:

For each component group with savings potential:

Prioritization: Component groups are sorted by energy savings (descending). Components with the greatest savings potential are listed first and highlighted.

Important: Optimization suggestions are indicative values for rough orientation. For binding planning, consult an energy consultant. Investment costs and payback periods are not included.

Calculation basis:

Practical Example:

Exterior wall 100 m², U_CURRENT = 1.2 W/(m²·K), U_TARGET = 0.24 W/(m²·K), GTZ = 6400 Kd/a

Energy savings = 100 · (1.2 - 0.24) · 6400 · 0.024
                = 100 · 0.96 · 153.6
                = 14,746 kWh/a

Tab 4: Smart Radiator Optimization 🆕

This tab provides an intelligent analysis of your radiators and shows optimization potential for heat pump operation.

Main Functions:

  1. Flow Temperature Optimization

    • Calculation of the minimum possible flow temperature
    • Display of potential temperature reduction (in Kelvin)
    • Energy savings in percent and kWh/year

  2. Fan Convector Option

    • Activatable via checkbox "Allow fan convectors"
    • Enables lower flow temperatures through active convection
    • Particularly useful for undersized existing radiators

  3. Room-by-Room Analysis

    • Overview of all rooms with target/actual comparison
    • Color coding: 🟢 Sufficient, 🟡 Marginal, 🔴 Insufficient
    • Specific recommendations per room:
      • Enlarge radiator
      • Additional heating surface (underfloor heating)
      • Use fan convector

Displayed Values:

ValueDescription
Current flow temp.Your configured flow temperature
Possible flow temp.Lowest achievable flow temperature
Energy savingsPercentage savings with lower flow temp.
Annual heat demand currentEnergy demand with current flow temperature
Annual heat demand optimizedEnergy demand after optimization

Tip for heat pumps: Reducing the flow temperature by 5K increases the seasonal performance factor (SPF) by approx. 10-15%. With a reduction from 55°C to 45°C, you can save up to 25% electricity!

4.2 PDF Export 📄

Click "Export Complete PDF Report" to receive a comprehensive report including:

Section 1: Heating Load Summary

Section 2: Annual Heat Demand Profile 🆕

Section 3: Building Envelope Optimization Suggestions 🆕

Section 4: Disclaimer

The PDF is optimized for printing (A4 format) and can be used for documentation, offers, or subsidy applications.

4.3 Optimal Flow Temperature

Calculate the lowest flow temperature at which all rooms achieve their target output.

Interpretation:

Radiator Sizing

5.1 Calculate Radiator Output

Φ = Φn · (Δθm / Δθn)^n

Important: At lower flow temperatures, radiator output decreases significantly!

Tips and Best Practices

Frequently Asked Questions

Why is building heating load higher than sum of room loads?

It includes a 100% surcharge on ventilation losses according to DIN EN 12831-1.

How do I size radiators for heat pumps?

Use factor 1.5-2.0 compared to standard output, or add underfloor heating.

Background Information

8.1 Difference: Heating Load vs. Heat Demand

Heating LoadHeat Demand
Power [kW]Energy [kWh/a]
Maximum at design temperatureAnnual requirement
For sizingFor energy balance

8.2 Historical U-values

PeriodExterior Wall [W/(m²·K)]Window [W/(m²·K)]
before 19801.0 - 1.52.5 - 3.5
1995-20010.5 - 0.71.5 - 2.0
from 2021 (GEG)0.20 - 0.240.90 - 1.1

Last updated: December 2025