Two-pipe systems with forced circulation deliver water at a sufficiently even temperature to the individual heat emitters, so these emitters are not exposed to the continual temperature drops typical of one-pipe systems. The condition, of course, is that the system is correctly sized both thermally and hydraulically.
Figure 1.1 shows an illustrative layout of a two-pipe system with direct return.
In general, circuits installed in this way are not able to ensure optimum control of differential pressures (these increase in proportion to the distance from the last branch served - the radiator).
Example: To ensure the nominal flow rate in the most distant radiator, far more of the medium than necessary has to be delivered to the nearest radiator (where radiators are spread over several floors, this problem is multiplied). A flow rate that does not match the required value can lead to a heat output that differs from the design value, to noise, erosion and so on.
The degree of imbalance in the pipe network depends essentially on the number of emitters served. If their number is limited, the differences between the required and the achievable flow rate may be acceptable. With a large number, on the other hand, the deviations can prove quite significant. In such cases further measures need to be taken.
A variant of the two-pipe system with direct return described above is the two-pipe system with reverse return (see Figure 1.2). This solution ensures balance through the layout of the system itself (i.e. roughly equal differential pressures can be ensured for all the emitters served, given equal pipe lengths), regardless of the control components used.
Basic summary:
• In small and medium-sized systems with constant flow, well-sized pipework is enough to ensure the hydraulic balance of the circuits (i.e. to supply the heat emitters with the amount of heat transfer medium that ensures the required level of heating).
• In large systems or systems with variable flow, certain devices for regulating the water flow have to be used to achieve hydraulically balanced circuits. Despite the better performance of two-pipe systems compared with one-pipe systems, the geometry and layout of the circuit sometimes cannot guarantee pressure balance at the inlets and outlets of the heat emitters. The above parameters therefore have to be “balanced” by means of appropriate balancing valves or by using suitable stabilising devices (regulators or two-way/three-way control valves).
Examples of circuits with balancing valves
These are circuits in which balancing valves can be used to set the flow rate of the heat transfer medium passing through the heat emitters in the circuit. In this way an even distribution of the medium is achieved even in very complex systems and extensive pipe networks (see Figure 1.3).
For these circuits to work correctly, the following are needed:
• Suitable design work, including calculation of the exact orifice setting for each valve.
• Thorough calibration.
• Regular, ongoing maintenance and checking of the settings.
An alternative to balancing valves is circuits using “autoflow” devices, which automatically regulate the flow of the medium through the relevant heat emitters. The advantage of these regulating devices is that they simplify the calculation of the hydraulic parameters of the circuits and do not require calibration.
Examples of circuits with a three-way control valve
These are circuits in which suitable three-way valves can be used to vary the quantity or temperature of the water flowing through the heat emitters. These circuits may have a simple structure, a compensated return, balancing valves, autoflow devices, etc. (see Figure 1.4).
