For a room to be brought and kept at the desired temperature, it is crucial that the thermal end units (radiators, convectors, etc.) are provided with the correct flow rates. An excessive flow rate would lead to too high a temperature, while an insufficient flow rate would result in a cold room. Unfortunate for our sector, water always follows the path of least resistance and hence the hydraulic balance is not guaranteed as such. Below we answer seven questions about the hydraulic balancing of installations.
1. Why is hydraulic balancing that important?
Water always follows the path of least resistance. Radiators close to the central pump often have a higher flow rate while distant radiators on the top floor have inadequate flow rate, simply because the water experiences significantly more resistance to reach those distant radiators.
The resulting comfort complaints lead trial and error during which the pump speeds are unnecessarily increased down or the boiler temperature is increased. Useless interventions that make the problem even worse and increase energy consumption.
The only correct solution is the hydraulic balancing of the installation. Because hydraulic balancing increases comfort and saves energy.
2. What happens during the hydraulic balancing process?
To balance an installation hydraulically, extra hydraulic resistance is created through balancing valves so that the water always experiences the same hydraulic resistance, regardless of the path followed.
Distance to central pump
For example, the regulating valves in circuits near the central pump will have to create more resistance. As the circuits are further away, the balancing valves will have to create less resistance. For example, the balancing valves of radiators 1, 3, 4 and 5 in Figure 1 will have to be set with a decreasing hydraulic resistance.
Not only the distance to the central pump plays a role. The thermal end units also typically have a different delivery capacity and therefore assume a different flow rate
In figure 1 radiators 1 and 2 may be equally far away from the central pump, but the greater design power of radiator 2 compared to radiator 1 also presupposes a higher flow rate. If we assume that both radiators have the same hydraulic resistance, they will both allow the same flow. To balance the radiators, extra hydraulic resistance must be provided in the other radiators (1, 3, 4, 5 in Figure 1).
3. What is the energy saving potential?
A frequently asked and justified question which unfortunately cannot be answered with a single prefix. We note that the saving potential depends greatly on the control concept of the installation, or in other words how the heat loads are controlled in the installation.
Without additional ‘individual control valves’
Figure 2a, on the left shows a common control concept in which the heat loads are only controlled by means of a so-called ‘weather compensation’. The supply temperature is compensated to the outside temperature via a heating curve as a rough estimation of the heating need.
Clearly, this is only a very rough arrangement and in the absence of additional ‘individual control valves’, this control concept is extremely sensitive to a hydraulic imbalance in the system. After all, comfort issues due to hydraulic imbalance remain unnoticed for the building management system and therefore cannot be rectified.
Hydraulic balancing can realise energy savings of up to 25% in such installations.
With additional ‘individual control valves’
Increasingly, installations are also provided with additional ‘individual control valves‘ whereby the heat loads in the rooms are aligned with the individual need by intervening on the flow rate through the end units (Figure 2, on the right). Any comfort issues caused by an hydraulic imbalance are therefore noticed by the building management system and hence can be partially adjusted. But often not entirely.
The energy savings from hydraulic balancing in such installations remain rather limited to 5%.
Less pump energy
Note that a hydraulically balanced installation also consumes less pump energy. In an imbalanced installation, too much water is pumped around. On the other hand, In a balanced installation, the pumping flow will be significantly lower which provides savings on pump energy up to 55%.
In some rather rare occasions, we have noticed an increased consumption after the installation is balanced. If, for example, a hospital wing was previously too cold because there was hardly any flow, this wing will be able to be brought to the desired temperature after the hydraulic balancing. As a result, this requires additional heat supply and hence increased energy consumption. First of all, hydraulic balancing is still about realizing the desired comfort.
4. I have control valves, why do I still have to balance the system?
Or in other words: “do I still need a balancing valve if I already have a control valve for each room?” The answer is: yes! There is an important difference between a control valve and a balancing valve:
- A control valve continuously adjusts the flow according to requirements.
- A balancing valve must ensure that the design flow (at maximum load, extreme weather conditions) can be achieved. In principle, a balancing valve is only set once to the desired design flow rate.
A control valve is selected based on the minimum required valve authority, selected from a discrete set of Kvs values (hydraulic conductivity of the valve). The valve selection therefore has nothing to do with guaranteeing the design flow.
To obtain an accurate and stable flow control and to bring the control valve within its operating range, a control valve must always be accompanied by a balancing valve.
5. Is my installation hydraulically balanced?
In principle, it should be. However, reality usually shows the opposite.
Radiators staying cold, rooms becoming too warm, flucuating room temperatures or noisy flow sounds are typical symptoms of an hydraulic imbalance. Also the delta-T over the circuits gives a quick and easy indication of the quality of the hydraulic balance. When visiting a boiler room, we always look directly at the temperature difference between the departure and return lines.
If, for example, the radiators are dimensioned at a 70/50/20°C regime, you expect a difference between the supply and return temperatures of about 20°C (can be a little lower at partial load). If we determine a temperature difference of, for example, 1°C or 2°C, this means that the installation has not been commissioned properly. Too much water is pumped around so that the water has ‘less time’ to dissipate its heat, hence the small delta-T. Note, however, that the delta-T and the partial load behaviour are also strongly dependent on the hydraulic circuits used (mixing circuit, dividing circuit, throttle circuit, low loss header) and the control strategy applied.
6. What is the difference between static and dynamic balancing?
In the case of static balancing, so-called balancing valves are used which are commissioned to create a fixed hydraulic resistance and hence balance the full load flows. If during the actual operation of the installation, control valves start to close, the hydraulic balance is no longer guaranteed. As illustrated in Figure 3a, the flow rate in the lower and upper radiators increases by 18% and 37% respectively when the two middle radiators are closed. By closing the middle radiators, the other radiators have to endure a larger share of the pump pressure, which increases the flow rate.
In case of dynamic balancing, this flow increase is compensated by the dynamic balancing valve itself. Via a diaphragm spring system, the pressure increase which comes with the flow increase is compensated by the dynamic balancing valve, such that the flow rate remains constant as a net result. A dynamic balancing valve therefore continuously always adjusts to guarantee the hydraulic balance. In this example, the flow rate changes in the lower and upper radiators are therefore limited to -3 and -4%.
We distinguish dynamic balancing valves in the DPCV (Differential Pressure Control Valve), the PICV (Pressure Independent Control Valve) and the flow limiter
- The DPCV is typically used to keep the pressure drop over a part of the installation or a branch constant to protect the downstream (static) regulating valves and control valves from pressure variations caused elsewhere in the installation.
- A PICV is a combination of a control valve, a balancing valve and a DPCV combined in a single component. The built-in DPCV maintains a constant pressure over the internal control and balancing valve to guarantee the hydraulic balance at all times. An even more important advantage is that the desired flow rate can be set directly on the PICV, whereas in the case of a static balancing valve this can only be set by measuring.
- Note that the PICV principle is now also available in miniature scale and can be integrated into thermostatic radiator valves. Note that a PICV requires a minimum working pressure of approx. 10kPa to 40kPa and should therefore be included in the calculation of the pump head.
- The flow limiter is used to limit the flow to a certain value. Below this maximum flow limit, the component does not act. Above this value, the autoflow will induce and increased pressure drop to limit the flow. Flow limiters cannot be set and are selected from a discrete set of flow limits.
7. How are installations commissioned?
After finishing the installation, the system needs to be hydraulically balanced, which is part of the so-called ‘commissioning’.
For dynamic balancing, it is important to set the desired flow rates directly on the dynamic balancing valves and nothing else. The pump head must be calculated in such a way that the minimum working pressure of the PICV’s, DPCV’s and flow limiters is respected.
In the case of static balancing, however, the commissioning procedure is much more cumbersome because the desired flow rate can only be set by means of measurement via a pressure drop measurement over the balancing valve. In addition, static balancing is an iterative process in which the adjustment actions on a balancing valve disrupt the hydraulic balance of already adjusted valves. These iterative adjustment procedures are therefore very labour-intensive and practice shows that the final result of the hydraulic balance is commensurate. Unless this can be calculated with dedicated hydronic software.