You have completed your heating load calculation according to DIN EN 12831 – but what do all the numbers actually mean? This article explains all results in detail: from the room overview to the annual heat demand to specific renovation suggestions.

Our heating load calculator provides not only the standard-compliant heating load but also practical additional information for planning your heating system.

Results Overview

After the calculation, you will first see a compact summary of all key figures:

The results overview shows all rooms with their heating loads and radiator matching

The Most Important Key Figures at a Glance

Key Figure	Symbol	Meaning
Design outdoor temperature	θe	Coldest expected day at the location
Qtrans	Transmission heat loss	Heat escaping through building components
Qvent	Ventilation heat loss	Heat lost through air exchange
Qheat,R	Room heating load (sum)	For radiator sizing (100% ventilation)
Qheat,G	Building heating load	For heat generator sizing

Room Heating Load vs. Building Heating Load

An important difference that is often misunderstood:

Heating Load Type	Calculation	Use
Room heating load	Transmission + 100% ventilation	Sizing radiators per room
Building heating load	Transmission + 50% ventilation	Sizing the heat generator

Understanding the Room Table

For each room, the following values are displayed:

Column	Meaning
ts	Target indoor temperature (e.g. 20°C for living rooms)
ΔT	Temperature difference (indoor minus outdoor temperature)
Qtr	Transmission heat loss of the room
QV	Ventilation heat loss of the room
QR	Total room heating load
Required output	Required radiator output
Actual output	Installed radiator output
Difference	Over/under-provision in watts

The difference column shows at a glance whether your radiators are adequately sized:

• Green values (+): Radiator provides more than required
• Red values (-): Radiator is undersized

Detailed Results: Building Level

For deeper analysis, you can access the detailed results:

The building overview breaks down all heat losses by category

Building Data

Key Figure	Meaning
Net volume	Heated air volume in m³
Heated net floor area	Area of all heated rooms

Heat Losses Through Transmission

Transmission heat losses are broken down by destination:

Loss Path	Description	Typical Share
To outdoor air	Through external walls, windows, roof	60–80%
To ground	Through floor slab, basement walls	15–25%
To unheated rooms	To basement, attic, neighbours	5–15%

Heat Losses Through Ventilation

Value	Meaning
Sum (100%)	For room heating/radiator sizing
Sum (room-by-room, 50%)	For building heating load/heat generator

Detailed Results: Room Level

Each room can be analysed individually – with all building components and their heat losses:

Component-by-component breakdown of heat losses in the living room

The Component Table in Detail

For each component, the following is displayed:

Column	Explanation
Category	Wall, floor, ceiling, window, door
Component type	Specific construction from the catalogue
Orientation	Cardinal direction (N, E, S, W) or "-" for internal
Gross	Total area of the component
Deduction	Deduction areas (e.g. windows in wall)
Net	Effective area for calculation
U-value	Thermal transmittance in W/(m²·K)
Thermal bridge ΔU	Surcharge for thermal bridges
U-value corrected	U-value + ΔU
ΔT (K)	Temperature difference
Heat loss	Resulting loss in kW

Interpreting U-Values

The U-value is the most important indicator for the insulation quality of a component:

U-Value	Rating	Example
< 0.20	Very good	Passive house wall
0.20–0.30	Good	New build to GEG standard
0.30–0.50	Adequate	Renovated old building
0.50–1.00	Moderate	Unrenovated old building
> 1.00	Poor	Uninsulated external wall

Annual Heat Demand Progression

In addition to the design heating load (for the coldest day), our tool also calculates the annual heat demand – how much energy you actually need over the year:

Annual heat demand and heat pump electricity consumption based on PVGIS climate data

The Most Important Annual Key Figures

Key Figure	Meaning
Total heat demand	Annual sum in kWh/a
Heat pump electricity consumption	Estimate at typical SPF
Ø Daily demand	Average demand per day
Maximum hourly output	Peak load (corresponds approx. to design heating load)
Heating hours per year	Hours with heating demand
Ø Heating output	Average output during heating operation

Annual Progression Charts

Hourly heat demand throughout the year and monthly distribution

The upper chart shows:

• Green area: Heat demand in kW
• Blue line: Outdoor temperature in °C

The lower chart shows the monthly distribution of heat demand:

• January/February: Highest demand
• June–August: Almost no heating demand
• Transition months: Variable demand

Why Is the Annual Heat Demand Important?

Application	Benefit
Economic viability	Calculation of annual heating costs
Heat pump planning	Sizing and SPF estimation
Solar coupling	Determining the solar coverage rate
Comparison	Before/after comparison for renovations

Renovation Suggestions

Based on your building components, our calculator automatically analyses the optimisation potential according to GEG 2024:

Automatic analysis of savings potential according to GEG standards

Overall Potential

Key Figure	Meaning
Total energy savings	Possible annual savings in kWh
Total heating load reduction	Possible reduction of heating load in kW
Reference temperature	Design outdoor temperature at location

Potential by Component Group

For each component group (external wall, roof, windows, floor slab), the analysis shows:

Value	Description
Area	Total area of the component group
U-value ACTUAL (Ø)	Current average U-value
U-value TARGET (GEG)	GEG requirement for renovation
Energy savings	Annual savings with renovation
Heating load reduction	Reduction of design heating load

Typical Savings Potentials

Measure	U-value before	U-value after	Savings
External wall insulation	1.0 W/(m²·K)	0.24 W/(m²·K)	60–70%
Roof insulation	0.8 W/(m²·K)	0.20 W/(m²·K)	70–75%
Window replacement	2.8 W/(m²·K)	1.10 W/(m²·K)	55–65%
Basement ceiling insulation	0.8 W/(m²·K)	0.25 W/(m²·K)	65–70%

Radiator Optimisation

A particularly practical feature is the automatic radiator analysis:

Intelligent radiator optimisation with specific replacement suggestions

The 2-Stage Analysis

Our algorithm checks two optimisation strategies:

1. Upgrade to maximum output: Same dimensions, higher radiator type
2. Downsizing where possible: Smaller radiator with over-provision

System-Wide Effects

Key Figure	Meaning
Current flow temperature	Previous system temperature
Possible new flow temperature	Achievable after optimisation
Energy savings	Percentage savings
Annual heat demand current	Before optimisation
Annual heat demand optimised	After optimisation

Room-by-Room Analysis

For each room, the analysis shows:

CURRENT STATE	OPTIMISED
Current radiator type	Recommended radiator type
Current dimensions	New dimensions (if changed)
Heat output at system temperature	New heat output
Coverage rate (< 100% = undersupplied)	New coverage rate (≥ 100%)

The replacement costs provide a rough guide for the investment.

Understanding Coverage Rate

Coverage Rate	Rating	Recommendation
< 80%	Critically undersupplied	Radiator replacement urgent
80–99%	Slightly undersupplied	Replacement recommended
100–120%	Optimal	No change needed
> 120%	Oversized	Downsizing possible

What to Do with the Results?

For New Build or Heating System Replacement

1. Size the heat generator: Use building heating load Q_heat,G
2. Size radiators: Room heating loads Q_R per room
3. Plan buffer tank: Consider potential oversizing for heat pumps

For Renovation Planning

1. Identify weak points: Components with high U-values
2. Prioritise measures: Sort by savings potential
3. Check economic viability: Savings vs. investment costs
4. Use subsidies: KfW/BAFA programmes for energy renovation

For Radiator Problems

1. Undersupplied rooms: Radiator replacement according to optimisation suggestion
2. Lower flow temperature: If all rooms are oversupplied
3. Hydraulic balancing: Carry out after optimisation

Conclusion

Try it now: Go to the Heating Load Calculator

Further Reading

• Guide to Heating Load Calculation
• Heat Pump Key Figures: COP and SPF

Sources

• DIN EN 12831-1: Energy performance of buildings – Method for calculation of the design heat load
• GEG 2024: Building Energy Act
• VDI 6030: Designing room heating surfaces