This publication is an update to AG 15/2002 and as with the previous guide provides basic reference information on mechanical building services systems for construction clients and professionals in other areas of the construction industry. This guide will cover the key areas of heating, ventilation, air conditioning and controls and since the previous edition of this guide, information has been added on: - pumps - pressurisation - low-carbon heat sources - MVHR systems - commissioning, handover and project evaluation. For construction clients, the guide provides a simple insight into the main system options discussed during the briefing process. It can also help clients to identify and raise technical questions which they feel are relevant to their organisation's specific needs. For construction professionals, the guide provides a quick reference to building services systems. To ensure the guide is simple and quick to use, a brief overview of each system is provided, followed by a list of key points. Photographs and simple drawings are used to help explain the appearance and operation of each system.