New HEM Technical Documents published

This is only going to be exciting for us high-level Home Energy Model nerds, but new Home Energy Model Technical Documents have just been published on the DESNZ website.

These docs explain the logic behind HEM.. a bit like the SAP methodology manual.

https://www.gov.uk/government/publications/home-energy-model-technical-documentation

This update DOESN'T confirm what compliance looks like, nor what the notional specification looks like... Imagine being given a car manual before you've got the car... or know how to drive it!

The Tech Docs were originally published with the 2023 Future Homes Standard consultation. Things have progressed a lot since then... this update will bring us up to date and give us some insight into how the final version of HEM is going to work.

A different approach:

When an assessor is creating Part L calculations and SAP spits out unexpected results, they can look through the calculation worksheets and read through the SAP methodology. 99% of the time, they can find the reason and fix it.

I’m going to miss this when SAP is retired.

The Home Energy Model doesn’t produce simple worksheets that assessors can scrutinise, and it doesn’t have a single methodology you can thumb through.

Instead, HEM has Technical Documents. These were updated last week as the UK Government gets ready for its official launch.

There are 18 volumes of these Technical Documents, each explaining a different section of HEM’s calculation approach. Topics include heating cycles, fabric heat loss, ventilation and weather conditions.

With SAP, all the equations were in one handy document, but HEM relies on a network of various ISO standards and CIBSE Guidance.

There’s very little original material in HEM… this is a good thing because we know it’s been built on tried and tested scientific data. But the disadvantage is you need a working knowledge of all these ISO documents to understand why HEM is doing what it’s doing.

And being able to unpick the SAP Worksheet will also be assigned to history. 

HEM is far too complex to generate a printout one can follow with a notepad and a bit of Excel wizardry.

Each HEM model runs over 17,500 iterations (one for every 30 minutes throughout a year); using the results from the previous cycle to feed the next.

Translating this into a physical document would require several trees worth of paper per plot.

HEM will still create BREL documents for Part L compliance, and will generate Energy Performance Certificates. But we don't yet know how the data from HEM will be presented for Building Control checks, and we don't yet know how easy it will be to analyse the results.

We're expecting more details about the roll-out of HEM and the Future Homes Standard in the coming weeks. Stay tuned!