Standard circuits

QM for Biomass DH Plants recommends using standard circuits wherever possible. If a standard circuit is chosen, the design and description of the system's operation are particularly simple: calculations are performed using pre-established tables, and questions relating to the system's design can be answered simply by ticking the corresponding boxes. This ensures effective quality assurance and minimises planning errors.

For further details, see the publications Standard Circuits Part I (Volume 2) and Standard Circuits Part II (Volume 5) – German version.

The selection and description of standard circuits follow the eight principles below:

1. A hydraulic circuit proven by application in the field of heat production.
    
2. Heat generation that can be expanded as desired in terms of hydraulics and control technology. An exception has been made only for monovalent wood-fired heating systems without storage tanks, where a minimal solution is permitted in addition to the standard solution, but this cannot be expanded.
    
3. The main boiler and secondary boiler are not defined hydraulically. This means that only parallel circuits (no series circuits) are used for heat generation.
    
4. The control variable of the main controller it
   • for systems with storage tanks, the charge status of the storage tank
   • for systems without storage tanks, the main flow temperature.
5. The setting for the main controller is normally the setpoint for the combustion power of the internal controller of the wood-fired boiler, for example in the sequence boiler 1 two points – boiler 1 continuously – boiler 2 two points – boiler 2 continuously.
    
6. Strictly low pressure difference coupling of the hydraulic circuits. This means that between two hydraulic circuits (each with its own pump), there is always a generously dimensioned bypass (‘hydraulic diverter’).
    
7. All heat consumer circuits for the lowest possible return temperature
   • in the power plant with low pressure difference connection
   • on the remote line with pressure difference connection.
8. Compliance with minimum valve authorities:
   • Three-way valves greater than 0.5
   • Direct passage valves greater than 0.3

The eight standard circuits WE1 to WE8 described in Part I are proven solutions for monovalent or bivalent heat production systems for one or two wood-fired boilers, with or without a storage tank. Numerous solutions for space heating and domestic hot water production are also described for the heat consumer side.

The standard circuits in Part I comply with Principle 5 (see above). Consequently, only wood boilers capable of processing an external setpoint signal for combustion power (from the higher-level control system) are suitable.

Part II presents six other proven solutions, WE11 to WE16. It describes control strategies that do not require any external setpoint signal to regulate the combustion power, but use the boiler outlet temperature as the control variable.

In addition to parallel connections of multi-boiler systems, series connections are also defined here as standard connections.

Below you will find detailed information on the standard circuits from Part I that have a heat accumulator:

• Circuit standard WE 2 - A wood-fired boiler with heat accumulator
• Circuit standard WE 4 - A wood-fired boiler with oil/gas boiler and thermal storage tank
• Circuit standard WE 6 - Two wood-fired boilers with heat accumulators
• Circuit standard WE 8 - Two wood-fired boilers with oil/gas boiler and thermal accumulator