Heating and Ventilating

There are significant benefits to be gained from incorporating commissioning modules into a fan coil system. Lars Fabricius explains how to exploit them
Commissioning modules are able to deliver significant benefits to the design, installation and commissioning of fan coil systems, as well as chilled beams, radiators, air handling units and radiant panels - compared to installing commissioning valves on each terminal unit. These include easier compliance with CIBSE Code W, particularly for ultra-low flow rates (<0.015 l/s), as well as greater flexibility, improved cost control, fewer operatives onsite and a better solution for the end client.

Plus there are significant time savings. Indeed, a BSRIA study has shown that commissioning modules save 28 per cent of installation time, 43 per cent of the time required to flush horizontal mains pipework, 23 per cent of time spent flushing fan coil unit circuits and 44 per cent on commissioning time - compared to traditional systems with commissioning valves on individual terminal units.

The purpose of this article is to highlight the main factors that will ensure all of these benefits are achieved.

Acting as distribution 'hub'
Usually pre-fabricated and pre-tested offsite, a commissioning module essentially acts as a distribution 'hub' for a group of up to seven fan coils or other terminal units. Each module incorporates all of the valves required for commissioning, including two-port actuated control valves and a differential pressure control valve to ensure that all two-port control valves operate with excellent authority.

Consequently, the commissioning module enables the units to be commissioned from the module.

The fact that all of the control valves for a group of terminal units are in one location also brings major benefits to the commissioning process. For example, rather than taking measurements at each terminal unit, generally working at height and perhaps having to remove ceiling tiles, all of the measurements for that group of terminal units can be taken at the commissioning module. Not only does this make commissioning simpler, it also saves time and can make all the difference to the project being handed over on schedule.

From the end client's point of view, commissioning modules make it easier to re-commission the system at a future date, should building layout or usage change.
Commissioning modules also enable use of the subtraction method of measuring flow rates, one of the methods recommended in the latest CIBSE Code W and BSRIA Commissioning Guide for dealing with ultra-low flows (<0.015 l/s). Ultra-low flows are a common problem in variable volume hydronic heating systems, where the flow through a single terminal unit may be too low to measure accurately with standard flow measurement devices.

The subtraction method, which can be used with all flow rates, measures the flow through all of the terminal units connected to the commissioning module to obtain a combined flow rate. Then, when one terminal unit is isolated, the flow rate through the remaining terminal units is still high enough to measure and the difference is equivalent to the flow rate of the unit that was isolated.

Commissioning modules can be used in conjunction with 'remote sensor' pump control, delivering higher efficiencies than those achieved with constant flow systems. Using remote sensor control, the pump speed is controlled such that the pressure differential across the pump reduces towards the design pressure differential across the most remote pressure controlled sub-branches (or modules). Differential pressure sensors, wired back to the BMS or pump, are required across the selected sub-branches/modules.

Used in conjunction with properly designed variable speed pump control, modules can achieve energy savings in the range of between 65-70 per cent relative to traditional constant flow systems (source BSRIA Guide BG12/2011).

Early planning is an important aspect of a project using commissioning modules, so that module locations are optimised for the terminal units they are serving. It also makes sense to involve the supplier of the commissioning modules at an early stage as their specialist knowledge will help to spot potential problems at design stage and nip them in the bud before work starts on site.

Given an appropriate level of planning, specifiers and installers can utilise commissioning modules to full effect so that all parties - and not least the end client - enjoy the benefits of a successful project.

Saving time and money
Use of pre-fabricated commissioning modules, manufactured under controlled factory conditions and pre-tested before delivery to site results in a significant cost and time savings, including:

Reduced onsite installation work

Optimum use of skilled operatives

Fewer delays with trades waiting for each other

Fewer connections minimises risk of leaks

Less risk of material price fluctuations during the project

Pre-insulated box with vapour-sealed penetrations ensures that commissioning does not compromise thermal insulation of system

Facilitates use of flexible, pre-insulated composite plastic/aluminium pipe and pressfit connections

Easier re-location of terminal units if design changes during the project

End client has ability to easily move terminal units if layouts change

// The author is managing director of
SAV Systems //