In 2013, CIBSE issued Technical Memorandum TM54: Evaluating operational energy performance of buildings at the design stage. This document intended to explain and then close the gap between the expected performance of a new building, and the actual reality of the utility bills once in use.

According to TM54, this mismatch is often caused by two main factors. The first is that the methodology used for building compliance doesn’t take into account all energy uses, such as small power, or lifts and escalators. The second is related to site practice, and the difference between the building design and how it is actually built.

CIBSE TM54 focuses on the former, and aims to provide building designers and owners with guidance on how to evaluate energy use more fully and accurately at design stage. But why would it help to accurately predict the energy consumption of the building?

Estimating energy consumption before a property is built can flag up areas where the actual energy use would be higher than the estimated use. Assessing these areas at design stage creates the time and opportunity to solve issues before the build begins, making any alterations cheaper and easier to carry out, and ultimately making the building more energy efficient, with a lower energy bill for its occupants.

It’s also true that carrying out an energy estimation at design stage results in a more thorough understanding of the building, highlighting the areas where the occupant’s control and the BMS system play a more important role.

The importance of this issue is reflected by the latest update to the BREEAM UK New Construction 2018 scheme. The revamped Ene 01 issue within the 2018 scheme awards 4 credits for carrying out detailed energy modelling (in accordance with TM54), to understand the predicted energy use for the building.

In order to provide an estimation of the energy consumption of the building, the best place to start is an analysis of the occupants’ behaviour. It’s just as important to understand how many people are expected to spend time in the building and during what hours, what equipment they have and how often this equipment is used, than simply mapping the systems (heating, cooling, ventilation, lighting) or construction details. Once we understand potential operational demands and how the systems will be maintained, we can make more appropriate, informed decisions about building design and systems.

The final result of the estimation shows the energy calculation for each of the relevant consumption categories (lighting, lifts and escalators, small power, catering, server rooms, other equipment, domestic hot water, space heating, cooling, fans and pumps, humidification and dehumidification). Each of these categories will show a range of values (best case to worst case), that highlight the different factors affecting each particular area. This makes it easier to spot potential improvements, as well as the factors outside of the designer’s control, which helps to get a clearer idea of where best to focus efforts.

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