The Home Energy Model (HEM) is being developed by DESNZ to replace SAP as the government’s
calculation methodology for estimating energy use in domestic buildings. The aim is to provide
greater accuracy, especially for the modern technologies that are expected to be increasingly used
to replace fossil fuel-based systems.
HEM is expected to be introduced in 2026 for the demonstration of compliance with the Future
Homes Standard (FHS) for new build homes. It will also be used for EPCs and possibly other purposes
that support government policies (e.g. the fuel poverty strategy, carbon budgets and the energy
transition).
There will be a ‘core engine’ will be available on-line for anyone to use and is based on open source
reference code. ‘Wrappers’ will be developed to interact with the ‘core engine’ for each purpose, i.e.
to collate input data for processing and to calculate the outputs required.
The FHS wrapper will define the inputs required for the FHS compliance calculations – these will be a
combination of:
standardised inputs (occupancy conditions, the notional building specification); and
user inputs (details of the dwelling design).
process the outputs to compare the notional building and the new building design (DER/TER,
etc.)
The combination of the FHS ‘wrapper’ and the HEM ‘core engine’ will be referred to as the HEM:FHS
assessment methodology, which will be used to assess compliance with the Future Homes Standard.
It is expected that the energy assessor accreditation schemes will develop user-friendly interfaces to
facilitate the interaction with HEM.
Compared to SAP, HEM will use much more detailed calculations, and therefore more detailed input
information, to better represent actual energy use. To achieve this, the two key differences are:
Half hourly calculations of energy use (instead of monthly)
Based on operative temperature (rather than air temperature)
There will also be an expanded and more detailed product database (PCDB) and a streamlined
process for adding new products to this.
The other main differences from SAP are that HEM:
Includes estimates of all use of energy in the home, including cooking and appliances
Takes account of the capacity (sizing) of all systems.
Takes more account of thermal mass and where it is located, including the thermal mass of
heat emitters
Accounts for full details of hot water system distribution pipework and its insulation
Enables variable set temperatures and stratification of hot water cylinders to be taken
account of
Includes more detail on heat gains, although typically overall gains will be lower.
As a result, there will be additional information that will need to be input into the HEM software
compared to SAP 10. This can be grouped under four headings: general details, dwelling fabric,
windows and heating/hot water system.
General details:
Altitude – this is the height above sea level in metres
Distant shading – the height and distance away of any obstacles that cause shading, plus the
directions of each edge of the obstacles (in degrees)
Dwelling total floor area and volume At present, these are not calculated from other inputs
in the demonstration software
Dwelling fabric:
Thermal mass for each element (internal and external)
o Areal heat capacity/kappa value – five options from very light to very heavy (not
calculated)
o Mass distribution class – five options depending on location of insulation
Orientation of each external wall and roof – in degrees
Calculated U-value for party walls, based on the party wall construction (not defaults as in
SAP)
Windows
The height of the base of each window above the ground
The openable area of each window
Details of trickle vents
The depth of window reveals to assess shading, and details of any other shading to
individual windows
Heating and hot water systems:
Thermal mass of heat distribution system
Number of radiators and their convection fraction
Flow temperature, flow rate and temperature drop through heat emitters
Ecodesign controller class
Length, diameter and insulation of hot water system pipework, including hot water
distribution pipework as well as primary pipework and any external pipework (e.g. from heat
pump)
This additional information only covers heat pumps as the heating system, so there may be further
additional items once other heating systems are incorporated into the model.