Is proportional balancing still the best approach to commissioning variable volume systems? Martin Lowe, Director at MLowe Consulting, suggests a faster and more practical method. iVariable volume fan coil and chilled beam systems continue to increase in popularity but, unfortunately, many of these systems are still commissioned using methods that were designed for constant volume systems. A single station balancing method, that takes full advantage of the new valve technologies used in variable volume systems, has the potential to balance the system more accurately, as well as saving a great deal of time.

Many variable volume systems combine 2-port valves on the fan coils with a balancing valve and 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 2-port valve and a flow limiting valve all in one assembly, reducing installation work and most importantly, giving a valve authority of almost 1.

However, to take full advantage of PI valves we also need to take a fresh look at commissioning them if we are to ensure they will perform efficiently 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.

Manifolds and SSB

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 SSB method, developed by our in-house engineers and used successfully in a number of projects, 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.

Crucially, as well as greatly reducing commissioning time, the SSB method is far more appropriate to the dynamic nature of a variable volume system as it replicates the day-to-day process of opening and closing valves in a variable volume system.