The new edition of the Domestic heating design guide has responded to a rapidly evolving heating landscape by improving technical accuracy, accountability and system performance, while aligning with current regulatory expectations, including the Building Safety Act. The 11th edition of the guide, developed by the CIBSE Domestic Building Services Panel, is intended to support designers and installers of space heating and domestic hot-water (DHW) systems in new and existing owner-occupied homes.
Reflecting current best practice, the guide retains its user-friendly structure while improving overall usability.
Key updates
The guide introduces the system criteria document, which, in line with the Building Safety Act, provides a clear, auditable record of design decisions, system intent and professional responsibility.
Heat-loss calculations, using the latest CIBSE weather data, have been updated and options around design outdoor temperatures have been expanded – guidance is now included for not just the coldest day of the year, but also the 99.6th to 95th percentile coldest day, allowing designers to make informed design decisions and reduce the risk of oversizing.
The heat-loss assessment method has been updated as per BS EN 12831-1:2017, the standard for calculating design heat load. Safety margins that designers were previously encouraged to add throughout the design process have largely been removed. A ‘safety’ margin can be added back in at the end of the design process if needed, but by doing so at this stage, the designer is much more cognisant of the scale of the margin that is added.
There is dedicated guidance on control strategies and DHW design now aligns with BS EN 12831-3:2017. Coverage of modern hydraulic and system configurations has also been expanded.
The new edition also incorporates updated U-value tables, to include up-to-date construction standards.
There is a system design section, with improved coverage of fundamental fluid dynamics principles, heat emitters other than steel panel radiators, advanced system layouts (such as 4-pipe heat generators and manifold distribution), circulator control options, and low loss headers
The guidance around assessment of air permeability has changed. Where possible, the primary recommendation is to undertake an air permeability test. As a fallback, the guide now uses the SAP 10 air permeability assessment methodology for retrofit installations.
There is expanded guidance on making sure water treatments are recorded, compatible with each other and the system, and are regularly tested using approved methods. The updated document provides guidance on sizing a system to store water at a temperature less than 60°C (no less than 50°C), when this might be appropriate, and the mitigations (including thermal disinfection procedures) that should be taken if this is followed. The guide does not offer guidance on selecting a specific model of heat generator and manufacturers’ advice should be sought on this. It does provide guidance on outputs that the heat generator should meet, on which selection can be based.
For heat generator sizing, the guide’s default position is that the heat generator should be designed for hot-water priority operation (either DHW reheat or space heating), and the cylinder reheat should be driven by the space-heating demand. Guidance for other ways of sizing is also included.
The guide does not focus specifically on heat pumps – it is heat generator neutral – but it was written with heat pumps in mind. It signposts readers to the upcoming CIBSE Domestic heat pump design and installation guide (DHPDIG), expected in Q3 2026.
Next steps
As well as the DHPDIG, the panel plans to focus on updating its Underfloor heating design and installation guide. The Rainwater harvesting guide and Solar thermal heating design guide will be reviewed and updated when appropriate.
The guide Is available at bit.ly/CJKnPo
About the author
Andy Mathews is chair of the Domestic Building Services Panel. See the video interview on the web version of this story