Heating Load & Heat Demand Calculator

Quick Start Guide

What does this calculator compute?

The heating load and heat demand calculator determines the standard heating load according to DIN EN 12831-1 as well as the annual heat demand for residential buildings. The heating load indicates the thermal power that the heating system must provide to bring your building to the desired room temperature even at the lowest outdoor temperatures. The heat demand additionally shows how much energy is actually needed throughout the year.

What do you need heating load and heat demand for?

• Heat generator sizing: Correctly size boilers, heat pumps or other heating systems
• Radiator sizing: Check whether existing radiators are sufficient or size new ones
• Optimize system temperatures: Determine low flow temperatures for efficient heat pump operation
• Renovation planning: Assess the impact of energy efficiency measures on heating load and heat demand
• Estimate operating costs: Calculate expected energy costs with the annual heat demand

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Two Calculation Methods

Option A: Simplified Input (Recommended for Single-Family Homes)

Ideal for single and two-family houses. You only enter the building volume (length, width, floors, roof shape) and some basic parameters. The calculator automatically generates realistic rooms with typical components.

Advantages:

• ✅ Fast: Only 5-10 minutes input time
• ✅ Simple: No detailed component knowledge required
• ✅ Automatic: Components selected from catalog based on construction year
• ✅ Realistic: Net internal dimensions, sloped ceilings and floor slabs calculated correctly

Result in 3 Steps:

1. Enter project basic data: Enter address → Climate data automatically loaded (standard outdoor temperature, air exchange)
2. Building geometry in wizard (5 steps):
   • Basics: Number of floors, basement, attic
   • Geometry: Length, width, heights, roof shape, roof pitch
   • Windows: Window area and distribution by orientation
   • Components: Check/adjust U-values (automatically from catalog)
   • Done: Summary and generate rooms
3. Calculate and receive result

What is generated automatically? A representative room with all components is created for each floor:

• External walls on all 4 orientations
• Windows according to your distribution
• Doors (ground floor only)
• Floors and ceilings (with correct correction factors!)
• Sloped ceilings (for attic, geometrically correctly calculated)

Option B: Detailed Room Input

For more complex buildings or when you want to define each room individually. Each room is entered separately with its components (walls, windows, doors, etc.).

Advantages:

• ✅ Precise: Each room with individual set temperature
• ✅ Flexible: Different components possible per room
• ✅ Detailed: Also suitable for complex building shapes

Important for manual input:

• Use net internal dimensions (without wall thicknesses)
• Subtract window and door areas from wall areas
• Set the correction factor to 0.0 for floor slabs between heated rooms!

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Important Input Parameters

Location and Climate Data

• Address: Enter the complete address. The calculator automatically determines:
  • Standard outdoor temperature (e.g. -14°C for Brandenburg, -12°C for Berlin, -16°C for Bavaria)
  • Recommended air exchange rate (typically 0.5 h⁻¹)

Building Data

• Year of construction: Crucial for the U-values of components

  • before 1980: High U-values (poor insulation), e.g. external wall 1.2 W/(m²·K)
  • 1995-2001: WSchVO 95, medium U-values
  • from 2021: GEG, low U-values, e.g. external wall 0.24 W/(m²·K)

• Building type: Influences the standard values (single-family house, multi-family house, etc.)

Heating System Settings

• Flow temperature (default: 55°C)

  • 35-45°C: Low temperature (underfloor heating, heat pump)
  • 45-65°C: Medium temperature (standard radiators)
  • 65-90°C: High temperature (old radiators)
  • Tip: Use low flow temperatures for better heat pump efficiency!

• Temperature spread (default: 10 K): Temperature difference between flow and return

  • 5 K: Large heating surfaces, high volume flow
  • 10 K: Standard for radiators
  • 15 K: Small heating surfaces, low volume flow

U-Values (Thermal Transmittance)

The U-value indicates how much heat is lost through a component. The lower, the better!

Typical U-values comparison:

Component	before 1980	1995-2001	from 2021 (GEG)
External wall	1.0-1.5	0.5-0.7	0.20-0.24
Window	2.5-3.5	1.5-2.0	0.90-1.1
Roof	0.8-1.2	0.3-0.4	0.14-0.18
Floor slab	0.8-1.2	0.5-0.7	0.25-0.35

For renovation: Use the U-values of the renovated components, e.g. 0.24 W/(m²·K) instead of 1.2 W/(m²·K) after facade insulation.

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Understanding Results

After calculation you receive comprehensive results in three tabs:

Tab 1: Heating Load (Main Results)

• Q̇_trans: Total transmission heat loss through walls, windows, doors, floors, ceilings [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] → Decisive for heat generator sizing!

Tab 2: Annual Heat Demand Profile 🆕

Detailed analysis of annual heat demand based on real weather data (PVGIS):

• Total Annual Heat Demand: Annual energy requirement for heating [kWh/year]
• Heat Pump Electricity Consumption: Estimated electricity consumption at SPF 3.5 [kWh/year]
• Heating Hours per Year: Number of hours requiring heating
• Maximum Hourly Heat Demand: Highest occurring heating power [kW]
• Monthly Breakdown: Heat demand, heating hours and average temperatures per month
• Annual Profile Diagram: Visualization of hourly/monthly heat demand

Tab 3: Building Envelope Optimization Suggestions 🆕

Automatic analysis of optimization potentials:

• Savings Potential: Energy savings and heating load reduction per building component group
• U-Value Comparison: CURRENT state vs. TARGET according to GEG 2024
• Prioritization: Components with highest savings potential are highlighted
• Calculation Basis: Based on degree-day method and current GEG 2024 standards

Displayed for each component group:

• Total area [m²]
• Average CURRENT U-value [W/(m²·K)]
• TARGET U-value according to GEG 2024 [W/(m²·K)]
• Annual energy savings [kWh/a]
• Heating load reduction [kW]

PDF Export 📄

Click "Export Complete PDF Report" for a comprehensive report including:

• Summary of all building data and heating loads
• Detailed breakdown per room with all components
• Annual heat demand profile with monthly table
• Optimization suggestions with GEG 2024 target values
• Disclaimer and calculation basis

Per Room

• Q̇_R: Room heating load [kW]
• Target performance: Required radiator performance [kW]
• Actual performance: Actually available radiator performance [kW] (if radiators defined)
• Status:
  • 🟢 Sufficient: Radiators are correctly sized
  • 🔴 Too low: Radiators are too small → Room will not get warm enough!

Calculate Optimal Flow Temperature

Click "Calculate optimal flow temperature" to determine the lowest system temperature at which all rooms are adequately heated.

Interpretation:

• 35-55°C: ✅ Ideal for heat pumps (high COP = low operating costs)
• 55-65°C: ⚠️ Standard for radiators, still acceptable for heat pumps but borderline
• >65°C: ❌ Radiators too small, heat pump operation inefficient → Enlarge radiators or add underfloor heating

The "critical room" is the room with the worst coverage - here a radiator that is too small limits the entire system temperature!

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Frequently Asked Questions

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

The building heating load includes a surcharge (100% on ventilation losses) according to DIN EN 12831-1 to account for heating-up processes and system losses. This is standard-compliant and important for correct sizing!

My building has high heating loads - is this normal?

This depends heavily on the construction year and insulation standard:

• Old building (before 1980, unrenovated): 80-150 W/m² living space is normal
• Standard (EnEV 2014): 40-70 W/m²
• Passive house: <15 W/m²

Can I add radiators later?

Yes! Click "Add radiator" in the room view and select type and size. The calculator automatically calculates the actual performance at your flow temperature.

What does "correction factor" mean?

The correction factor (f_T) accounts for the temperature difference to the adjacent room:

• f_T = 1.0: Outside air (full heat loss)
• f_T = 0.5: Unheated basement, attic (half heat loss)
• f_T = 0.0: Heated adjacent room (no heat loss!)

How accurate is the automatic climate data?

The automatically determined values are based on geographical data and are good reference values. For official, standard-compliant calculations you should compare the values with the BWP climate map or the DIN tables.

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Tips for Best Results

✅ Use realistic inputs: Measure building dimensions or use construction plans

✅ Check construction year: Do the automatically selected U-values match your building? Adjust for renovations!

✅ Note correction factors: Always use f_T = 0.0 for floor slabs between heated rooms!

✅ For heat pumps: Use "Optimal flow temperature" - if >55°C, you should check larger radiators

✅ Compare: Calculate current state first, then after renovation → see the savings directly!

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To heating load guide with formulas and calculation fundamentals →