AC CL 1.5 - Sub System Hieracrchy
Posted by Mike Gordon on 22 July 2015 11:54 AM



NOS level: 3 & 4

Scope: England & Wales and Northern Ireland


Clarification of an air conditioning sub system and the air conditioning sub system hierarchy to all parties. Any existing asset register id or existing client numbering can be included for each component within an air conditioning sub system.

An air conditioning sub system is a group of components whose combined performance delivers cooling and/or ventilation to a space. For example, a chiller(s), air handling unit(s) and the terminal units delivering the cooling (via whatever medium) and the ventilation air (where provided) comprise a sub system.

An understanding of all the sub systems on a site will aid the Energy Assessor (EA) to give relevant and specific advice to the client.

A clear description of the sub systems on a site will assist Accreditation schemes to quality assure reports.


The following methodology is to define an identification reference of an air conditioning system that is standard, consistent, and easily recognisable for all inspections.


The client is often not technical and identifies departments by function. This is replicated in the Volume Identifier (VOL ID). For example, in many old hospitals the Outpatients department is spread over a number of areas in a number of buildings. An office block may have different departments on different floors.

The VOL ID enables Energy Assessors (EAs) to replicate this logical group of air conditioning equipment, by function, or location within a building, or a group of equipment spread across multiple buildings.

Each volume will be assigned a VOL ID in the form VOL001, VOL002, ...VOLnnn


VOL001 = Outpatients

VOL002 = Dermatology

VOL003 = Optics

VOL001 = Gresham Building

VOL002 = Turner Building

VOL003 = Eastern Building


The System Identifier (SYS ID) identifies an air conditioning sub system This SYS id will contain all the components that are within the air conditioning sub system. This will assists the EA to identify components that may be affected by suggested changes to components within a sub system.

For example if two packaged split units are serving the same room, the performance of one will affect the performance of the other in that if one was switched off, the remaining unit would have to serve a higher load. In this case the two independent split systems would be described as being in the same air conditioning sub system and would both have the same SYS ID, even if their controls are not linked.

If the same two packaged split units are serving two separate rooms, the performance of one will not affect the performance of the other. In this case the two independent split units would be described as being in different air conditioning sub systems and would have a different SYS ID.

A common arrangement is to have a selection of packaged split units and a centralised air conditioning system. The splits could be grouped as one system or itemised individually and the centralised system would be one system – the EA can decide what an appropriate representation would be.

Each System will be assigned a SYS ID in the form SYS001, SYS002, ...SYSnnn


SYS001 = Two Split Packaged Units in Canteen – openable windows (no AHU)

SYS002 = One Split Packaged Unit in the Ground Floor Reception + air supply from AHU1

SYS003 = 24 VRF Packaged Units serving 200 bedrooms + air supply from AHU2 (heating only)


The components of an air conditioning system are those items on which there is a series of questions in the Air Conditioning Inspection Report.

Cooling Plant  Cooling Plant
Air Handling Plant Air Handling Units
Terminal units Grilles, Diffusers, Fan Coils, VAV Boxes etc.
Controls Control for Chillers, AHUs and Terminal Units

If there are 3 air handling units in system 1 (SYS001) they can be referenced as AHU1, AHU2 and AHU3. If there is a second system with air handling units they can be referenced as AHU1, AHU2 and AHU3, or AHU4, AHU5 and AHU6 - the EA can decide.

Each air conditioning component has a unique reference by a combination of the VOL/SYS/Component id.



See further examples in Appendix 1: AC CL 1.5 Appendix 01