Heat demand
The energy-efficient and economically optimal operation of hot water systems requires the heat energy demand to be determined precisely. The precise determination of the required heat output is therefore a central component of the system design and crucial for efficient energy use.
The heat energy demand depends on numerous parameters that influence the planning, e.g.:
- Maximum heating capacity of individual consumers
- Simultaneous heating capacity of multiple consumers
- Temperature levels of the consumers
- Heating-up times and peak consumption
- Daily load profile, weekly load profile
- Annual load profile, especially for heating systems and district heating systems
- Thermal losses in pipework
The basis of every project should be a needs analysis that is as detailed as possible and takes into account the specific characteristics of the project in question.
Needs analysis for residential construction and public buildings
When using hot water or heating boilers for residential construction, e.g. one or more residential buildings or for a housing estate, the heat energy demand can be calculated from the built-up area of the buildings and the thermal insulation regulations that have been in force for years. The relevant values can be found on the energy performance certificate.
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Single-family homes
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Terraced houses/ |
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Poor thermal insulation |
155 |
125 |
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Medium thermal insulation |
120 |
100 |
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Good thermal insulation |
100 |
80 |
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Very good thermal insulation |
80 |
65 |
In addition to the planned construction volume, the development plan provides information on the number of floors, type of building and use, e.g. as a commercial building, nursery or for retail space. For domestic hot water generation, a 10 % surcharge should be added to the calculated heat energy demand for full supply.
When installing ventilation and air-conditioning systems, the heat energy demand must be determined based on the area and the required air exchange rate.
Needs analysis for hospitals and clinics
In the case of heating systems for hospitals and clinics, determining the total heat output required is significantly more complex. For a rough estimate, you can base your calculations on empirical values, which are around 20 to 30 kW per bed. This covers the heat energy demand for building heating, ventilation and air conditioning as well as for heating potable water. Steam consumption for laundry and thermal disinfection is also taken into account. The proportion of steam consumption accounts for approximately one third of the estimated total heat demand.
This estimate can be used for hospitals and clinics with approximately 500 to 2,000 beds.
Needs analysis for industrial or district heating systems
In the case of industrial or district heating systems, energy consumption values are normally available from existing production facilities or facilities with similar production conditions.
The values are usually given as energy consumption per unit of time, e.g. for heat in kW/h or kW/h per day, steam in t/h or t/day, and for electrical current in kW/h or kW/h per day.
Since production volume is usually also known in terms of units per hour or working day or in other quantities, the specific energy consumption for production can be determined and converted to newly planned production quantities. If consumption meters are used in heating plants that are already in operation, reference heating plants or, in general, in process heat generators, the daily cycle and the load peaks that occur are also known.
If no consumption curves or consumption values are available, energy consumption can also be determined from the purchase values provided by the commercial department, such as invoices for fuel deliveries or electricity bills. These values can then be converted to the planned production volumes.
The energy consumption for heating, ventilation and domestic hot water generation in buildings can be determined based on the planned floor space, the type of use and the planned number of employees.
Most district heating grids are designed for a maximum flow temperature of 130 °C. This is due to the maximum temperature that polyurethane thermal insulation, currently the most commonly used plastic sheath pipe, can withstand in continuous operation (max. 145 °C in short-term operation). However, the 130 °C temperature is only required when there is a high heat energy demand in winter. As a rule, the flow temperature is adjusted between 70 °C and 130 °C depending on the outside temperature. The lowest flow temperature of 70 °C is necessary to ensure that domestic hot water is heated to 60 °C in the domestic hot water stations. The return temperature is set at 70 °C during the design, and at < 50 °C in modern networks.
Low water temperatures in both the supply and return are beneficial in terms of energy efficiency, as they result in higher efficiency. This also means that the heat generation costs are lower. Furthermore, lower water temperatures also reduce heat losses in the network. For this reason, modern networks aim to achieve a constant return temperature of ≤ 50 °C in addition to variable flow temperature control.
Safety measures during the design
Of course, it makes sense to take appropriate safety measures into account when operating at maximum heat output.
In order to avoid incorrect dimensioning, the following aspects should be taken into account when determining the collateral.
- Coordination with the system operator
- Incorporate future developments of the operation
- Explicit indication of the safeguards in the design of the system
- Avoid multiplying collateral in different places
- Safety devices are usually already included in the design of heat exchangers and consumers
Problems caused by oversizing
- Higher investment costs
- Increased energy consumption due to partial load operation
- Cyclical operation of the burners and increased wear
- Reduced service life of boilers and components
Problems of undersizing
- Failure to meet performance data
- Insufficient heat supply
- No fail-safety