SAP to HEM Transition Guide for Assessors

If you are a SAP assessor, the transition to the Home Energy Model (HEM) requires practical preparation across several areas: learning a new methodology, adapting to significantly greater data requirements, understanding the ECaaS calculation platform, and adjusting your workflow for longer assessment times. This guide walks through each change in detail and provides a practical checklist for getting ready. The key message is that HEM is not simply an update to SAP; it is a fundamentally different calculation engine, and the sooner you begin preparing, the smoother the transition will be.

Timeline: When You Need to Be Ready

The transition from SAP to HEM is directly tied to the Future Homes Standard (FHS) implementation timeline. The government has confirmed a dual methodology period during which both SAP 10.3 and HEM can be used to demonstrate compliance:

Which methodology applies is separate from which standard applies. Work with a building notice, initial notice, or full plans application given before 24 March 2027 may still be built to Part L 2021, provided it commences before 24 March 2028. Work notified on or after 24 March 2027 must meet the FHS immediately. For full details on the transitional arrangements, see our Transitional Arrangements page.

New Data Requirements

The most immediate practical impact for assessors is the step change in data requirements. HEM's half-hourly dynamic simulation requires far more granular input data than SAP's monthly calculation. Below is a detailed breakdown of the new data points you will need to collect.

Site-Specific Data

• Site altitude, which affects external temperature and pressure calculations at each timestep
• Noise nuisance potential, which influences assumptions about window-opening behaviour in the ventilation model
• Detailed orientation data: HEM's half-hourly solar calculation is sensitive to precise building orientation

Building Geometry

• More precise measurements than SAP requires, as HEM models the thermal envelope in greater detail
• Zone definitions: SAP uses two zones (living area and rest of dwelling). The FHS assessment fixes the dwelling at a single zone; the core engine can model multiple zones, but they are thermally uncoupled
• Thermal bridging details: the default y-value route SAP allowed has been removed for HEM, so junction lengths and psi-values must be modelled, calculated, or taken from standard values

Hot Water Outlets

• Individual outlet specifications: every tap, shower, and bath must be individually specified
• Tap flow rates: the actual flow rate for each hot water outlet
• Shower types: mixer, electric, or pumped, with specific characteristics
• Pipework lengths and insulation: detailed hot water distribution pipework, including pipe diameters, lengths, and insulation levels

Product-Specific Data

• Heating equipment: Specific make and model from the PCDB for every heat generator (heat pump, boiler, etc.)
• Ventilation equipment: Make and model of MVHR or MEV units, including heat recovery efficiency and specific fan power
• Heat emitters: Radiator or underfloor heating specifications
• PV systems: Panel specifications plus inverter specifications (make, model, efficiency)
• Controls: Specific heating control and thermostat details

The Assessment Workflow: SAP vs HEM

The assessment workflow changes significantly under HEM. The table below compares each stage of a typical assessment:

The longer turnaround for each calculation run means that assessors will need to be more deliberate about design iterations. Instead of rapid trial-and-error, you will need to plan specification changes more carefully and potentially batch multiple adjustments into single runs.

Understanding ECaaS

ECaaS (Energy Calculation as a Service) is the cloud-based platform that delivers HEM to the industry. Understanding how it works is essential for assessors.

How ECaaS Replaces Third-Party Engines

Under the current SAP regime, multiple software providers develop their own implementations of the SAP calculation. This has led to documented inconsistencies between providers. The same dwelling can produce different results depending on which software is used.

ECaaS eliminates this by providing a single, centralised calculation engine maintained by MHCLG. Software providers no longer build their own calculation engines. Instead, they build user interfaces that send data to the ECaaS API and display the results. This means:

• Every assessor gets the identical calculation, regardless of which software they use
• Updates to the HEM engine are applied centrally and take effect immediately for all users
• The calculation engine is built in Rust for performance, based on MHCLG's open-source implementation
• A basic web front-end is being developed for stakeholder familiarisation

What This Means in Practice

Your day-to-day experience will still involve using assessment software with a graphical interface. The difference is that when you press “calculate”, the data is sent to the ECaaS API rather than being processed locally. The calculation itself runs in under 2 seconds, though the more detailed data requirements of HEM's half-hourly simulation mean that overall assessment time is longer than SAP.

Calculation Time

One of the most notable practical differences is calculation speed. SAP calculations are near-instantaneous because the monthly method is computationally simple. HEM's half-hourly dynamic simulation across an entire year (17,520 timesteps) is significantly more demanding:

• HEM calculation time: under 2 seconds via ECaaS
• SAP calculation time: Near-instantaneous

This affects the iterative design process that many assessors are accustomed to. In SAP, you can quickly test multiple specifications to find the optimal compliance path. In HEM, each iteration requires a new calculation run, so you need to plan your approach more carefully. Experienced assessors recommend preparing a complete dataset before the first run, then making targeted adjustments based on the results.

Training and CPD

The government has not yet published final details on HEM qualification requirements. However, the scale of change from SAP means assessors should expect significant training needs:

• Methodology training: Understanding HEM's half-hourly dynamic simulation, how it differs from SAP, and how input changes affect results
• Data collection: Learning the expanded data requirements and developing efficient collection processes
• ECaaS platform: Familiarising yourself with the API-based workflow and the specific software interfaces your provider develops
• Building physics: A deeper understanding of building physics concepts (thermal mass, dynamic heat balance, pressure-driven ventilation) will help you interpret HEM results and advise clients
• Product knowledge: Understanding how specific product characteristics (heat pump COP curves, MVHR heat recovery efficiency, PV inverter efficiency) affect the calculation

Certification bodies including Elmhurst, ECMK, Quidos, and Stroma are expected to update their training programmes ahead of the FHS coming into force. Contact your certification body for details on upcoming HEM training courses.

Test Environment Limitations

When the ECaaS test environment opened for familiarisation in early 2026, it modelled only a narrow set of technologies. The list below is that early-2026 snapshot and has not been re-confirmed against later ECaaS releases:

• Only electricity can be modelled as an energy supply; gas and other fuels are not yet available
• Only heat pumps are available for heat generation; boilers and other heating systems are not yet implemented
• Only radiators are selectable as heat emitters; underfloor heating is not yet available
• Only homes with hot water cylinders can be modelled; combi systems are not yet supported

Practical Preparation Checklist

Frequently Asked Questions

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