TM54 gets a refresh

Inkling are proud to have been on the working group delivering the newly published 2022 update to CIBSE TM54.

The original version was published in 2013, and has become the central protocol for evaluating the operational energy performance of buildings at the design stage. I have always liked its flexible approach, allowing modellers to exercise their intelligence and experience, on what’s appropriate for each project.

The new update stays true to that philosophy, and broadens the advice to more overtly include all building types (not just offices) and embrace more modelling tools/methods including PHPP. It emphasises the significant differences between modelling to demonstrate compliance (Part L) targets and for delivering against performance targets in use.  Also worth noting is that this revision clearly moves away from assuming a Part L2 compliance model as a starting point.

The update was prompted by the shift in the industry towards net zero targets (pick your definition), contractual performance targets, advances in modelling and reference to TM54 for predicting operational energy use in the latest Part L2 (2021) within Building Regulations.

Targets

The updated guidance makes it clear that operational targets should be set by the project owner based on their specific aspirations for the building. These could follow a Net Zero definition, LETI EUI (kWh/m²) targets, DEC rating, RIBA 2030, NABERS UK etc

Modeller responsibility is to provide the best possible modelling of the design proposals and report clearly on the analysis done against these targets. Modelling can help inform how realistic/achievable targets might be, but responsibility for delivering that in-use performance must sit with the whole design, construction and operational team, not just the modeller.

The Steps

The TM54 steps have been slightly adjusted for the update to better encapsulate the whole modelling process. They now include a step 0 on choosing suitable tools and planning the modelling approach for the project. The use of blanket management factors has been removed in favour of more analysis of the energy performance risks bespoke to each project (including risks associated with how the building might be managed). Guidance on reporting has been extended and now includes use of an implementation matrix.

Implementation matrix

One of the key additions is the implementation matrix. I believe this is a particularly valuable addition as it provides a space to summarise all the modelling inputs, where the data came from, how much variance there might be on that input, and how accurate the data is likely to be.

At early design stage many modelling inputs are guestimates based on previous practice, building regulations compliance or intentions. These are valid at this stage, but should be flagged up as such. As the design develops, inputs will get firmer with design calculations or specifications to back them up. Focus then shifts to the ‘known unknowns’ and what can be done to limit the variation of these.

Sensitivity and scenario analysis

Sensitivity analysis is recommended to explore the implications of those risks that may not have an obvious impact on energy consumption e.g. a change in glazing specification could improve the insulation lowering heating loads, but might also reduce solar gain lowering cooling loads, but increasing heating loads.

Ultimately scenarios are pieced together based on the baseline or best guess estimate for how the building will perform, a ‘high’ case that takes the upper values on inputs with a significant range (refer to implementation matrix) and a ‘worst case’ that considers everything that could go wrong at once.

These scenarios are intended to focus the design team on all the performance gap risks, and how to prevent them coming to pass for this project. We know how multi-faceted the performance gaps is so this risk management exercise is an important component in reducing the gap and delivering the designed performance.

HVAC modelling

TM54 2022 makes a case for utilising detailed HVAC modelling and gives advice on when this is appropriate. Detailed HVAC modelling is not yet done routinely in the UK, but is more commonly used within ASHRAE modelling in other parts of the world. It’s not an easy skill to pick up as it needs a good understanding of HVAC system design and control logic, as well as advanced modelling skills, but can provide powerful insights and a valuable opportunity to ‘dress-rehearse’ systems in the virtual environment pre-empting problems that would be much harder to solve if not uncovered until installation, commissioning or in use.

The TM54 guidance indicates 3 levels of HVAC modelling – steady state, template HVAC and detailed HVAC. In reality there is probably a more sliding scale, and especially as a TAS user I can use the wizard to set up an HVAC system and then tweak it to better match the proposed design as much or as little as I feel confident to do so.

Reporting

Lastly, more guidance is given around reporting on TM54 assessments. A template of headings is included in Appendix B. This should encourage some consistency in TM54 modelling reports across the industry that will make it easier for all parties to understand and find the information they need.

In conclusion

• We welcome the update and are proud to have contributed to it
• The new implementation matrix and reporting template will be helpful for building consistency across the industry
• Helpful to have a consistent modelling approach as the industry moves more towards in-use performance targets
• A LETI modelling guide is coming soon, and will draw from and align with TM54, with additional advice and support for projects targeting net zero EUI targets as advocated in the Climate Emergency Design Guide