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TM54: Evaluating Operational Energy Performance of Buildings at the Design Stage
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TM54: Evaluating Operational Energy Performance of Buildings at the Design Stage

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There has been a growing awareness for some time that many ‘low energy buildings’ use more energy than the designers thought they would. As energy costs have risen, this awareness has started to spread to building owners, who hear much about low energy buildings and subscribe to programmes that rate the design of the building, only to find that their ‘low energy design’ turns out to be have a typical energy bill. The performance of low energy designs is often little better, and sometimes worse, than that of an older building they have replaced, or supplemented.

This phenomenon is not restricted to the UK, but has been observed as far afield as the US and Australia. There is a mismatch between the expectations around the performance of new buildings and the reality of the utility bills. This difference between expected and realised energy performance has come to be known as the ‘performance gap’.

There are two main reasons for this performance gap. The first is that the method of calculating energy use for the purposes of compliance does not take into account all the energy uses in a building. In particular, it does not address energy used by lifts and escalators, for catering facilities, or for server rooms. This energy use can be substantial: in one case study, the National Trust HQ at Swindon, it was found that 60% of the energy use, that for the server room and the catering, was used in just 3% of the floor area, and more than doubled the operational energy use over the design estimates.

The second reason for the performance gap is related to site practice. To deliver a building that uses as much energy as expected requires that the design is built as intended, the engineering systems are commissioned effectively and the operators and occupiers of the building understand how to operate and maintain the building so that it delivers the expected performance.

This Technical Memorandum (TM) addresses the first reason. It provides building designers and owners with clear guidance on how to evaluate operational energy use more fully, and accurately, at the design stage. After a brief introduction, which explains the need for the guidance in more detail, it explains the importance of making an accurate estimate of the operating hours and likely occupancy of the building. It then sets out how the operational energy required for the building can be estimated. As well as covering lighting, heating, ventilation and cooling and provision of hot water, it also considers lifts and escalators, small power loads, catering, server rooms and other plant and equipment.

In each case, it provides guidance on how to make more accurate estimates based on the intended use and operation of the building. As well as enabling the designer to estimate energy use more accurately, it will highlight any areas where actual energy use may be higher than a typical building, allowing this to be considered at the design stage.

This guidance will help to turn low energy designs into low energy buildings that achieve the design energy targets. It is one of several CIBSE actions to promote more effective assessment of energy performance. CIBSE, along with the RIBA and other partners, has been instrumental in the development of the Carbon Buzz website, which provides a practical tool for designers to compare their low energy designs against others on the website, as well as providing benchmarks of expected and realised performance.

A Preface from Justin Snoxall, Head of the Business Group, British Land comments: This CIBSE TM is therefore very timely. It encourages building designers to consider the implications of their design for operational energy use. It provides guidance on how to address energy at the design stage, not just to comply with Building Regulations and EPC ratings, but also to improve the understanding of operational energy performance.

Phil Jones
Independent Energy Consultant & Chairman of the CIBSE Energy Performance Group
TM54 is a really excellent and important publication and a big step forward for the energy performance sector. It's not just guidance, it provides a step-by step methodology for designers to follow that brings consistency to estimation. It also has lots of helpful numbers on inputs to the method, giving support designers.

Also really important are the ideas about presenting the results to clients to help them really understand true performance and avoid the performance muddle we have got ourselves into.

What we need now are ways to encourage designers to use the methodology but even more importantly ways to get clients to ask for their designers to produce estimates based on TM54.




1 Introduction

2 Aims of this document

3 Scope of this document

4 Why estimate energy use?

4.1 Current situation
4.2 Evaluating energy use

5 Risks of evaluating energy use at the design stage

6 Principles of evaluating energy use

6.1 Principles of CIBSE TM22
6.2 Using dynamic simulation models
6.3 Assumptions and simplifications in models
6.4 Using benchmarks at early design stages

7 The methodology
7.1 Step 1: Establishing floor areas
7.2 Step 2: Estimating operating hours and occupancy factors
7.3 Step 3: Evaluating lighting energy use
7.4 Step 4: Evaluating energy use for lifts and escalators
7.5 Step 5: Evaluating energy use for small power
7.6 Step 6: Evaluating energy use for catering
7.7 Step 7: Evaluating energy use for server rooms
7.8 Step 8: Evaluating energy use of other equipment
7.9 Step 9: Evaluating energy use of domestic hot water
7.10 Step 10: Evaluating internal heat gains
7.11 Step 11: Evaluating energy use for space heating,cooling, fans and pumps
7.12 Step 12: Evaluating energy use for humidification and dehumidification
7.13 Step 13: Estimating management factors
7.14 Step 14: Running scenarios
7.15 Step 15: Sensitivity analysis
7.16 Step 16: Review against benchmarks
7.17 Step 17: Presenting the results

8 Post occupation


Principal authors: David Cheshire (AECOM); Anna Carolina Menezes (AECOM)

Contributors: Mark Bacon (AECOM); Andrew Cripps (AECOM); Xavier Fulbright (AECOM); Brian Graham (AECOM); Anna Holding (AECOM); Martin McLaughlin (AECOM); Ant Wilson (AECOM)

Task Group
Chair: David Hughes
Members: Dave Cheshire (AECOM); Andrew Cripps (AECOM); Hywel Davies (CIBSE); John Field (TEAM (EAA Ltd.)); Phil Jones (Building Energy Solutions); Anna Carolina Menezes (AECOM); Philip Oliver (Kyoob Ltd.); Ian Pegg (Johnson Controls); Mike Smith (BSRIA); John Ward (SES)
Referees: Eszter Gulacsy (Mott MacDonald); David Kingstone (Buro Happold); Brian Spires
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