Equipment and control

The minimum requirements for the operation and safety equipment of hot water boilers are set out in DIN EN 12953-6. This includes all main shut-off valves on the pipes that are connected to the boiler, safety devices to prevent excessive pressure, temperature and water shortages, the heating equipment, and all the valves/fittings and measuring devices required for operation and control. All such equipment must be approved in accordance with the Pressure Equipment Directive.

Information about heat output

In principle, it can be deduced from this that the boiler spread, in conjunction with the boiler capacity, determines the required volume flow. In principle, there are two types of circulation pumps.

Variable-speed pumps

If the circulation flow in the consumer network drops, the boiler pumps reduce the flow rate at the heat generator and, at the same time, lower their power consumption. By regulating the boiler water flows, unnecessary burner modulation is avoided. The reference peaks have been smoothed accordingly.

Fixed pumps

The circulation pumps deliver a constant volume flow or operate at 100 % output in accordance with the pump characteristic. Depending on the boiler system’s application and the heat demand on the consumer side, the burner’s modulation is less than optimal when compared to a variable-speed pump.

Information about circulation pumps

Return temperature safeguard

To reliably prevent excessive thermal stresses in conventionally fired boilers, care must be taken to ensure that the maximum temperature spread between the flow and return temperatures is not exceeded. It is also important that the temperature of the flue gas does not fall below the condensation temperature inside the boiler, in order to prevent water forming in the flue gas and, consequently, corrosion.

To this end, the permissible return temperature in the boiler is limited to a minimum value. The minimum return temperatures are 50 °C for UT-L, UT-M and UT-H boilers, and 60 °C for UT-HZ boilers.

However, as lower temperatures may occur in the heating distribution network or during start-up, the return flow temperature may need to be raised to these minimum values.

The maximum temperature spread between the flow and return pipes at the boiler is also limited. For UT-L and UT-M boilers, the maximum spread is 50 K; for UT-H and UT-HZ boilers, it is a maximum of 40 K. If there are significant temperature spreads coming from the heat consumers, the return temperature to the boiler must also be increased in these systems.

Depending on the boiler’s heat output and the water volume flow within the boiler, various systems are available for this purpose.

Return flow temperature boosting with a pump

This type of wiring is appropriate where on-site network pumps are already in place and are also responsible for dissipating heat from the boiler, e.g. where only supply and return manifolds are present on the network side, or in district heating systems with central network circulation pump stations. The lifting pump is designed according to the network return temperature or the minimum boiler water flow rate. The pump’s flow rate is designed to ensure that enough water is added from the supply line to achieve the boiler return temperature specified for the boiler in the project. The lifting pump always runs when the measured temperature at the return (before mixing) is less than the specified setpoint.

The variable-speed lifting pump increases or decreases its speed when the boiler return temperature falls below or rises above the setpoint. This keeps the return temperature at the set minimum value. This means that, compared to an on/off pump, electrical energy can be saved.

Return temperature maintenance using a three-way mixing valve and a boiler circuit pump

This type of circuit is appropriate for conventionally fired boilers where a low loss header or a manifold without differential pressure is installed. The boiler circuit pump ensures that the entire volume of water is circulated on the generator side. When designing the boiler circuit pump, water-side resistance to the low loss header must be taken into account.

The three-way valve regulates the boiler return temperature by mixing in hot flow water while reducing the water volume supplied to the system.

The variable-speed boiler circuit pump ensures that the quantity of heat delivered from the boiler to the network is regulated according to demand. The return flow temperature in the network (directly upstream of the three-way valve) can be used as a proxy for the quantity of heat extracted from the system by the consumers. If the return temperature in the network drops, the consumers draw more energy from the system than is being replenished by the boilers. The speed of the boiler circuit pump is then increased, which also increases the heat output into the network (as a direct consequence, the boiler increases its output to maintain the flow setpoint temperature). If, on the other hand, the network return temperature increases, less heat is extracted from the network. Reducing the pump speed can save electricity.

Flow temperature protection with three-way control valve (ELHB)

The flow temperature protection with three-way control valve serves the following purpose:

• Ensuring the required flow temperature at the boiler outlet
• Ensuring adequate flow through the heating bundle

Unlike conventionally fired boilers, return temperature protection is not required for electrically powered heating and hot water boilers, as there is no risk of condensation forming due to the absence of flue gases. Consequently, the minimum water inlet temperature for electric boilers is not 50 °C or 60 °C, as is the case with conventional boilers, but just 5 °C.

Nevertheless, even for electrically powered boilers, there are limits on the temperature spread between the boiler flow and return. These are defined as a maximum of 40 K and a minimum of 20 K, in order to, on the one hand, ensure that the minimum water volume in the heating bundle required for heat dissipation is not undershot and, on the other hand, prevent the flow speed from becoming too high when the temperature spread is lower.