Guide to Using the Heating Load Calculator

Table of Contents

1. Introduction
2. Calculation Principles
3. Step-by-Step Guide
4. Understanding Results
5. Radiator Sizing
6. Tips and Best Practices
7. Frequently Asked Questions
8. Background Information

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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:

• Sizing heat generation (boiler, heat pump, etc.)
• Designing heating surfaces (radiators, underfloor heating)
• Optimizing system temperatures (flow/return temperature)
• Efficiency considerations and economic calculations

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.

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Calculation Principles

2.1 Total Heating Load of a Room

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

Q̇HL,R = Q̇T + Q̇V + Q̇RH

Where:

• Q̇_T = Transmission heat loss (heat loss through components)
• Q̇_V = Ventilation heat loss (heat loss through air exchange)
• Q̇_RH = Reheat capacity (optional, for intermittent operation)

2.2 Transmission Heat Loss (Q̇_T)

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:

• A = Component area [m²]
• U = Thermal transmittance (U-value) [W/(m²·K)]
• f_T = Correction factor depending on adjacent space [-]
• θ_i = Room set temperature [°C]
• θ_e = Outdoor temperature / temperature of adjacent space [°C]

Correction Factors (f_T)

Adjacent Space	Correction Factor (fT)
Outdoor air	1.0
Ground	0.5 - 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 (f_T = 1.0)

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

2.3 Ventilation Heat Loss (Q̇_V)

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:

• V = Room volume [m³]
• n = Air change rate [1/h] (typical: 0.5 h⁻¹)
• 0.34 = Heat capacity of air [Wh/(m³·K)]

2.4 Building Heating Load (Q̇_HL,G)

According to DIN EN 12831-1:

Q̇HL,G = Σ Q̇T + 2 · Σ Q̇V

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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:

• Changes to project master data
• Adding/removing rooms
• Changes to components and radiators
• Calculation results

3.2 Enter Project Master Data

Location and Climate Data

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

Building Data

• Year of construction: Crucial for U-value selection
• Building type: Single-family house, multi-family house, etc.

Heating System Settings

• Flow temperature: Default 55°C

  • Low-temperature: 35-45°C
  • Medium-temperature: 55-70°C
  • High-temperature: 75-90°C

• Spread: Default 10 K

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 Concept	Description	Typical Air Change Rate
Window Ventilation	Manual ventilation through windows	0.5 h⁻¹
Mechanical without HR	Mechanical ventilation without heat recovery	0.4-0.6 h⁻¹
Mechanical with HR	Controlled ventilation with heat recovery	0.3-0.4 h⁻¹
Exhaust Air System	Exhaust air only (e.g., bathroom/WC)	0.5-1.0 h⁻¹

Adjustable Parameters:

• Air change rate [1/h]: How often the room volume is exchanged per hour (default: 0.5)
• HR efficiency [%]: Only for "Mechanical with HR" - heat recovery efficiency (typically 60-90%)

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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:

• Q̇_trans: Total transmission heat loss through components [kW]
• Q̇_vent: Total ventilation heat loss through air exchange [kW]
• Q̇_Heiz,R: Room heating load - sum of all rooms [kW]
• Q̇_Heiz,G: Building heating load according to DIN EN 12831-1 [kW]

Room breakdown: For each room you see:

• Transmission heat loss (Q̇_T)
• Ventilation heat loss (Q̇_V)
• Room heating load (Q̇_R)
• Required radiator output
• Actual radiator output (if radiators are defined)
• Coverage status (✅ Sufficient / ⌠Insufficient)

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:

   • θ_i = Weighted average room temperature
   • θ_e(h) = Outdoor temperature at hour h
   • θ_e,Norm = Standard outdoor temperature (design case)
   • Q̇_trans + Q̇_vent = 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):

• Total Annual Heat Demand: Annual energy requirement for heating [kWh/year]
• Heat Pump Electricity Consumption: Estimated electricity consumption with SPF 3.5 [kWh/year]
• Heating Hours per Year: Number of hours requiring heating
• Maximum Hourly Heat Demand: Maximum heating power [kW]

Monthly and Annual Charts:

• Monthly Heat Demand [kWh]: Bar chart showing distribution throughout the year
• Yearly Profile [kW]: Continuous profile of hourly heating power requirement

Detailed Monthly Table: For each month:

• Month name
• Heating hours
• Average outdoor temperature [°C]
• Heat demand [kWh]
• Maximum hourly demand [kW]

Use cases:

• Estimate annual heating costs
• Size heat pump (bivalence point, JAZ estimation)
• Evaluate renovation measures (before/after comparison)
• Calculate payback period for efficiency improvements

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:

• A = Component area [m²]
• U_CURRENT = Current average U-value [W/(m²·K)]
• U_TARGET = Target U-value according to GEG 2024 [W/(m²·K)]
• GTZ = Degree days [Kd/a] (automatically calculated from standard outdoor temperature)
• 0.024 = Conversion factor (kWh/Wh · 24h/d)

Formula for degree days:

GTZ = (20°C - θe,Norm) · Heating days

Example: θ_e,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:

• ÃŽ"T_Norm = θ_i - θ_e,Norm (e.g. 20°C - (-12°C) = 32 K)

GEG 2024 Target U-values (§ 48 GEG):

Component Group	Target U-value [W/(m²·K)]
Exterior walls	0.24
Roof / Top floor ceiling	0.14
Floor to ground	0.25
Windows	0.95
Doors	1.80

Data displayed:

For each component group with savings potential:

• Component group (e.g., "Exterior walls", "Windows")
• Total area [m²]
• CURRENT U-value (average) [W/(m²·K)]
• TARGET U-value according to GEG 2024 [W/(m²·K)]
• Annual energy savings [kWh/a] - Degree-day method
• Heating load reduction [kW] - Difference at standard outdoor temperature

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

Calculation basis:

• Standard outdoor temperature: θ_e,Norm [°C] from project
• Degree days: GTZ [Kd/a] - calculated from standard temperature
• Component areas: From your input (manually or wizard-generated)
• Current U-values: From your component definitions
• Target U-values: According to GEG 2024 § 48

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:

Value	Description
Current flow temp.	Your configured flow temperature
Possible flow temp.	Lowest achievable flow temperature
Energy savings	Percentage savings with lower flow temp.
Annual heat demand current	Energy demand with current flow temperature
Annual heat demand optimized	Energy demand after optimization

4.2 PDF Export 📄

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

Section 1: Heating Load Summary

• All project master data (address, climate data, building data)
• Building heating load (Q̇_Heiz,G) and all components
• Detailed room breakdown with all components
• Radiator overview (if defined)
• Optimal flow temperature (if calculated)

Section 2: Annual Heat Demand Profile 🆕

• Total annual heat demand [kWh/year]
• Heat pump electricity consumption estimate [kWh/year]
• Maximum hourly heat demand [kW]
• Heating hours per year
• Detailed monthly table with:
  • Month name
  • Heating hours
  • Average temperature
  • Heat demand [kWh]
  • Maximum hourly demand [kW]

Section 3: Building Envelope Optimization Suggestions 🆕

• Overview of calculation parameters (standard outdoor temperature, degree days)
• Total annual energy savings [kWh/year]
• Total heating load reduction [kW]
• Detailed table by component group:
  • Area, current U-value, target U-value (GEG 2024)
  • Annual energy savings [kWh/a]
  • Heating load reduction [kW]

Section 4: Disclaimer

• Indication of calculation basis according to DIN EN 12831-1
• Data sources (PVGIS, component catalog)
• Notes on indicative nature of optimization suggestions
• Calculation timestamp

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:

• 35-55°C: ✅ Ideal for heat pumps
• 55-65°C: ⚠️ Standard, acceptable
• >65°C: ❌ Radiators too small

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Radiator Sizing

5.1 Calculate Radiator Output

Φ = Φn · (Δθm / Δθn)^n

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Tips and Best Practices

• ✅ Use net interior dimensions
• ✅ Choose realistic U-values
• ✅ Set correction factors correctly
• ✅ For heat pumps: Aim for low flow temperatures

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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.

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Background Information

8.1 Difference: Heating Load vs. Heat Demand

Heating Load	Heat Demand
Power [kW]	Energy [kWh/a]
Maximum at design temperature	Annual requirement
For sizing	For energy balance

8.2 Historical U-values

Period	Exterior Wall [W/(m²·K)]	Window [W/(m²·K)]
before 1980	1.0 - 1.5	2.5 - 3.5
1995-2001	0.5 - 0.7	1.5 - 2.0
from 2021 (GEG)	0.20 - 0.24	0.90 - 1.1

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Last updated: December 2025