What is the new Home Energy Model or HEM?
The Home Energy Model, or HEM, is to be introduced in the UK to replace the Standard Assessment Procedure (SAP), which is currently used to demonstrate that new homes comply with Part L of the Building Regulations and to produce EPC's. HEM is still under development with a continuing industry consulation and is intended to be implemented alongside the Future Homes Standard in 2025.
The HEM, along with SAP (10.2) are building energy models which are tools to help model, with reference to building physics, how buildings perform under a set of variables, usually modelling energy balances and thus giving estimates or indications of potential energy consumption. Other building energy models, include PHPP which is specifically used to comply with the Passivhaus standard and ESP-r, which was a research based and now open source building modelling tool. In some ways these have been considered more specialist tools, whilst SAP is the standard tool required to evidence compliance with Building Regulations for dwellings.
SAP and its non-domestic partner SBEM might be considered as simpler tools, and perhaps intended intitially as a result to be more accessable. However its replacement which is still under consultation and development (HEM) is in many ways aiming to achieve some of the high accuracy functions of tools such as ESP-r, whilst retaining some simplicity through the use of wrappers. These different tools have a wide variation in capability, complexity and function, and as part of the consultation and validation process in the development of HEM the Department for Energy, Security and Net-Zero carried out an "Inter-Model Comparison Summary Report", the full report can be found here whilst the extracted table below shows a brief comparison of the various tools.
ESP-r | PHPP | SAP 10.2 | HEM | |
Thermal zones | Multiple, unrestricted | Single zone | Multiple – restricted to living and non-living zones | Multiple, unrestricted. however the FHS assessment will be restricted to a living and non-living zone only. |
Simulation type | Dynamic | Steady state | Steady state | Steady state |
Simulation timestep | High resolution (i.e. half-hourly or less). | Monthly | Monthly | High resolution (down to half-hourly). |
Simulation areas | Building physics, system performance of PV array (but not heating systems etc). | Building physics and system performance of various services. | Building physics and system performance of various services. | Building physics and system performance of various services. |
Validation | Demonstrated to have good accuracy vs field test data. Comparable to other detailed simulations tools when subject to the Building Energy Simulation Test (BESTEST) and other empirical trials. |
Has demonstrated accurate simulation space heating demand and annual energy use of high- |
The current calculation method for Part L compliance and EPC ratings. It is not regarded to |
N/A – new model |
Role | Comparator model for dynamic calculations of zonal operative temperature, solar energy, and |
Comparator model for monthly space heating demand, elemental |
Reference model to demonstrate the step change expected for compliance |
N/A |
Table 1 Summary of the modelling packages applied in this inter-model validation study. (Extract from "Inter-Model Comparison Summary Report - Home Energy Model Validation" December 2023.
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