Under the microscope

MEES, EPC, Colin Lillicrap, Simplified Building Energy Model, SBEM, dynamic simulation model, DSM, Tungsten Lighting
Colin Lillicrap

With MEES now in force, EPC ratings are paramount for building owners. Consultant Colin Lillicrap explains why a more detailed EPC analysis could help identify more cost-effective improvement measures – and may even result in a better rating.

Energy performance ratings are calculated using the National Calculation Method (NCM). The type of EPC certificate depends on the building. Level 3 covers small buildings such as converted domestic with heating systems less than 100kW and cooling systems less than 12kW.

And Level 4 covers small purpose-built office buildings, with heating systems greater than 100kW and cooling systems over 12kW. For these ‘simpler’ buildings the Simplified Building Energy Model (SBEM) can be used to generate Energy Performance Certificates (EPCs).

A more complex (and accurate) dynamic simulation model (DSM) engine is required for more complex Level 5 buildings, but it can also be used to assess Level 3 and 4 buildings. As CIBSE-certified level 5 Energy Assessors we use both SBEM and DSM to demonstrate the improved EPC ratings that can be achieved using DSM modelling combined with hourly weather data. A software interface allows switching between DSM and SBEM within the same building model, enabling quick direct comparisons.

The DSM and SBEM calculations are performed in very different ways. DSM simulates the building hourly throughout the year using the CIBSE test reference year (TRY) weather file, while SBEM uses monthly averages. DSM can account for shading by adjacent buildings and reflections from such buildings, which is not possible in SBEM. The DSM engine also models daylight levels more accurately and hence savings from installing daylight sensors.

The EPC rating is based on CO2 emissions and as the carbon emission factor for electricity is much higher than for gas, measures to reduce electricity consumption are more effective at improving the EPC rating.

Our consultancy carried out research on the Energy Performance of Buildings open data website to identify buildings with F or G ratings. An online survey of a sample of F and G rated building suggests that some ratings are probably inaccurate.

This was followed up with site visits to around 30 buildings to establish the age and type of construction and the services fitted. Some clear patterns emerged. In a few cases the building has been improved but the EPC has not been recalculated. The Arbnco white paper (see MEES - a timebomb under the property industry) identified that inaccurate EPCs pose a real risk to landlords.

The risk can arise in two ways. Firstly, if existing EPCs are recalculated, the rating can be reduced due to tightening regulations and the corresponding changes to the NCM. Secondly, inaccuracy can arise from incomplete surveys of the building and the use of default data held within the NCM which invariably leads to poor ratings.

In the analysis below, we consider two scenarios and identify the circumstances that lead to a low EPC rating and recommend improvement measures to comply with MEES.

Scenario 1: Small shop with simple HVAC

MEES, EPC, Colin Lillicrap, Simplified Building Energy Model, SBEM, dynamic simulation model, DSM, Tungsten Lighting
Graph 1: EPC variations with different lighting types

Shops and offices found in a typical high street are often heated by a boiler and radiators, with some having partial air conditioning using split air conditioning units. Many with F or G ratings date from the early 1900s and have solids walls. One of the most cost-effective ways to improve the EPC is to upgrade lighting. Graph 1 shows how the EPC rating varies with different lighting types with efficacy values based on manufacturers’ data rather than SBEM defaults.

The result is that this poorlyinsulated building with relatively inefficient services achieves an EPC rating of B or C provided tungsten lighting is avoided. The DSM produces slightly better ratings than SBEM in this case.

The reason for this result becomes apparent when we look at CO2 emission by end use. Tungsten lighting, typically halogen spot lamps common in the retail sector, are much less efficient than other forms of lighting. The increased heat output from the lighting increases the demand for cooling resulting in most of the energy being sourced from grid electricity leading to the G rating. For shops with halogen lighting the solution is obvious, replace the halogen lighting with modern LED lighting. In many instances there are direct LED replacement lamps that will fit into the existing luminaires. Based on LED lighting systems we have installed the cost is around £10 per m2 including installation with a payback of 2 to 3 years.

Scenario 2: Larger properties with central HVAC

The HVAC system must be selected from the list in the NCM which allocates an auxiliary energy to account for the electricity used by fans and pumps which can have a significant impact on the EPC. We have calculated the EPC rating for a number of different HVAC systems using both DSM and SBEM engines. The more detailed DSM tool consistently produces better EPC ratings.

The reasons for this are apparent when we look at the degree of shading by adjacent buildings and self-shading. A degree of over-shading is not uncommon for many buildings in towns and cities. The shading considerably reduces heat gains in summer leading to a lower cooling load and auxiliary energy. SBEM does not consider the shading effect of external buildings which gives DSM a significant advantage for coolingdominated buildings. There is a corresponding increase in the heating demand in winter (assumed to be sourced from natural gas). The net effect of the more realistic modelling is to considerably reduce the CO2 emissions.

The improved rating obtained using DSM might be sufficient to comply with MEES without any improvement. Where it is necessary to replace the HVAC system it may be useful to consider the opportunity of upgrading systems to comply with other legislation such as the F-Gas rules.

To combat the impact of MEES and reduce the risk to business landlords and their advisors should urgently arrange for the EPC rating to be recalculated for any building currently with an EPC rating of E, F or G. If an F or G rating is confirmed, it is important to identify the causes in order to assess the most costeffective improvement measures preferably using DSM to model the building.

Dr Colin Lillicrap is director of Colin Lillicrap Associates and a CIBSE registered LCC, Level 5 Energy Assessor and Lead ESOS Assessor

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