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To determine how much energy is needed annually to heat a building to a desired temperature, a heat demand calculation is required. Ideally, this should be performed by a professional company. If you’re curious about what this calculation entails, this article explains the key factors and metrics involved.
What is a Heat Demand Calculation?
A heat demand calculation accounts for all energy losses and gains in a building. The heat demand is typically expressed in kilowatt-hours (kWh) per year.
How to Perform a Heat Demand Calculation?
An accurate heat demand calculation in accordance with DIN 4108-6 requires the determination of the following values:
- Degree Days (GTZ) and Degree Day Factor (GTZ Factor):
Degree days represent the difference between the outside temperature and the desired indoor temperature over a given period. They are used to estimate heating needs based on weather conditions. The GTZ factor accounts for regional climatic variations and is specified in standards like DIN 4108-6. - Correction Factor for Nighttime Temperature Reduction:
The correction factor reduces heat loss due to lower heating settings at night, typically between 0.8 and 0.9. The exact factor depends on the degree of nighttime temperature reduction and the building’s insulation. - U-Value:
The U-value indicates how much heat is lost through a component (e.g., a wall or window). It’s measured in W/(m²K), with lower values signifying better insulation. - Surface Areas of Components:
Includes walls, floors, and doors exposed to the ground, outside air, or unheated spaces. - Thermal Bridge Allowance:
This accounts for heat loss through thermal bridges (e.g., windows, corners). A standard factor (e.g., 0.05–0.10) is added to the building’s total surface area or calculated for each bridge individually. - Building Envelope Surface Area:
This encompasses all external surfaces enclosing the heated interior space. - Air Exchange Rate: Represents the rate at which indoor air is replaced with outdoor air per hour. Standard values for living spaces range from 0.5 to 1.0 air exchanges per hour.
- Heated Air Volume:
The volume of air requiring heating, calculated by multiplying the floor area of heated spaces by their height, expressed in cubic meters (m³). - Solar Radiation:
Describes the energy from sunlight that enters a building. Factors like orientation, tilt, and shading are considered. - Window Solar Energy Transmittance:
The g-value indicates the fraction of solar energy passing through a window, ranging from 0 to 1. - Correction Factors:
These adjust for specific conditions, such as partial shading of windows. - Building Volume and Usable Area:
Building volume is the total internal space, while usable area is the portion actively heated. - Usable floor area: The usable floor area is the portion of the building volume that is actually heated. A simple rule of thumb is that the usable floor area is approximately 0.32 times the building volume.
- Heating Period Duration:
Usually ranges from 180 to 220 days annually, depending on regional climate.
Based on these values, you can now determine the heat losses and gains needed to calculate the heat demand:
Losses through the building envelope and ventilation:
- Transmission loss:
(Correction factor x U-value x component area) + (thermal bridge allowance x building envelope area) - Ventilation heat loss:
0.34 x air exchange rate x heated air volume
Solar and internal gains:
- Solar gains:
Solar radiation x energy transmittance factor of the windows x area x correction factors - Internal heat gains:
5 x usable floor area x length of the heating period x 0.024
Heat Demand Calculation Formula
The annual heating energy demand (kWh) can be calculated using the formula:
Annual Heating Demand (kWh) = [Degree Day Factor × (Transmission Heat Loss + Ventilation Heat Loss)] – [Utilization Factor × (Solar Gains + Internal Gains)]
How Much Heat Demand per m²?
To determine the heat demand per square meter, divide the annual heating demand by the usable area. The result represents kilowatt-hours per square meter per year.
Guidelines:
- New Buildings (well-insulated): 50–70 kWh/m² annually.
- Older Buildings (moderately insulated): 100–150 kWh/m² annually.
- Older Buildings (uninsulated): 150–300 kWh/m² annually.
Difference Between Heat Demand, Heating Load, and Heating Capacity
Heat demand, heating load, and heating capacity are distinct concepts:
Heat Demand:
Total energy required to maintain a comfortable indoor environment over a period, measured in kWh.
Heating Load:
Maximum heat output needed to keep a building warm on the coldest days, measured in kW.
Heating Capacity:
The actual heat output that a heating system can provide, also measured in kW.
Do you need help selecting the right mobile heating system for your building? We are, of course, here to advise you.
Sources:
www.heizung.de (as of October 10, 2024)
Image by TarikVision – stock.adobe.com
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