Proportional balancing is still widely used for commissioning variable volume fan coil systems, but is it the best way? Martin Lowe, technical manager at Marflow Hydronics, suggests a faster and more practical method
OVER the last few years, variable volume fan coil systems have increased in popularity, but commissioning methods have not evolved at the same pace. As a result, many systems are commissioned in ways that are ideal for constant volume systems but not wholly appropriate for variable volume.

This article suggests that a single station balancing method, which takes full advantage of new valve technologies, is quicker, more practical and more cost-effective.

Many variable volume systems combine two-port valves on the fan coils with a differential pressure control valve (DPCV) serving each zone of fan coils. This method is effective but also expensive in terms of capital costs, and carries a heavy commissioning burden.

An increasingly popular alternative is the pressure independent (PI), dynamic flow control valve. PI valves combine a DPCV, a two-port valve and a balancing valve all in one assembly, reducing installation work and most importantly, giving a valve authority of 1.

However, to take full advantage of PI valves we also need to take a fresh look at commissioning them – particularly that they will perform in day-to-day operation with ports opening and closing continuously. In addition, the system employed should help to speed up the entire commissioning process. The combination of a manifold-based system and the single station balancing (SSB) technique described below delivers all of these benefits.

The practice of using a manifold system rather than installing individual valve assemblies at each terminal unit is now well established and offers a number of widely recognised benefits. Using prefabricated valve assemblies offers a cost-effective quality-assured solution whilst providing fewer measurement points for the commissioning engineer.

Even greater advantages are achieved when a manifold system fitted with PI valves is combined with the SSB technique that we are proposing as best practice for commissioning variable volume systems.

With conventional proportional balancing, the commissioning engineer needs to balance the entire system before problem circuits can be identified. Once those problems have been resolved the whole system then needs to be balanced again, leading to a lengthy process that can impact on the whole construction schedule.

The innovative SSB method, developed by our in-house engineers, is different because it uses a ‘subtraction’ technique to identify problem valves. This is based on knowing the design flow rates for each individual valve and, therefore, the total flow rate for that fan coil group.

Assuming all the valves are functioning correctly and bypasses are closed; isolating a group of valves will have a predictable effect on the total flow rate for the remaining valves. Where an unexpected result is observed it is a simple matter of elimination to quickly identify the valve that causes this, enabling the commissioning engineer to home in on the problem area.



The SSB technique has been fully tested within a live commissioning environment at a six-storey mixed-use development in Liverpool. The project used a total of 461 fan coils in 148 zones, each zone being controlled and commissioned through a single manifold that incorporates an automatic balancing valve for each terminal unit. The total commissioning time was significantly reduced compared to conventional methods.

Crucially, as well as greatly reducing commissioning time, the SSB method is far more appropriate to the dynamic nature of a variable volume system. This is because the very act of commissioning in this way replicates and checks the process of opening and closing valves that would occur during normal operation of a variable volume system.

Clearly, any such system will need to undergo the usual flow witnessing procedures and when dealing with variable volume we would strongly encourage engineers to adopt a slightly different and more flexible approach, for the following reasons.

In order to comply with the -0/+10 l/s tolerances stated in CIBSE Code W (which was written before PI valves were developed) it is common practice to play safe and design the system to run at 10% higher than should be required. However, most PI valves have tolerances of +/-10% so there is a slight chance, if all of the valves were +10%, that the system could end up running at 120% flow rate. In all likelihood, the pump would not cope efficiently with this oversizing.

Clearly this is a worst case scenario and the very nature of a variable volume system means the chances of all ports being fully open at the same time – except during conventional commissioning – are remote. Rather than sticking to the traditional oversizing philosophy, therefore, we would suggest that a more flexible and open-minded approach is the sensible solution for variable volume systems.

For instance, if a system were designed at 105% flow rate these potential problems would be eliminated. However, in the highly unlikely event of all of the manufacturers’ tolerances being –10%, flow rates could fall to 95% of design flow rate. In fact, this would not make any appreciable difference to the performance of the system – and there’s hardly any chance of this happening in real life situations. So, while there are very good reasons for variable volume systems being the first choice for the vast majority of fan coil and chilled beam systems, the full benefits of these systems will only be realised with a more open-minded approach. This requires a combination of innovative valves, specialist expertise and an innovative commissioning and witnessing philosophy.

Marflow Hydronics Ltd 0121 358 1555