Jakub Ostrowski and Baz Iyer
June 6, 2024
Home Energy Model outputs half hourly operative temperature for each zone modelled. This unlocks the possibility of quantifying the comfort of homes, with a model that is fast to set up and use. This is an important metric when designing low energy and carbon homes.
If comfort is not considered, it is possible to make decisions that appear high impact and low cost - but sacrifice comfort. Discomfort caused by solutions may not be spotted in the design process - creating future costs and user dissatisfaction.
For example, a large window that reduces annual heating needs (and therefore annual bills) may increase the frequency and intensity of overheating events, resulting in cooling demand. Or, a heat pump installed in a building with high heat loss may theoretically operate with a low supply temperature, but at the cost of immediate comfort - causing occupant behaviour that erodes system efficiency.
There are various ways in which a designer can ensure thermal comfort is maintained. Passive solutions are preferred such as shading, thermal mass, night-time purge ventilation or heat dissipation. Active solutions such as cooling, mechanical ventilation and heat recovery can then be used to cover any remaining demand. Based on our analysis of HEM, we are confident that any of the mentioned solutions can be modelled easily.
Home Energy Model outputs operative temperature at half-hourly intervals while SAP 10.2 and PHPP only output dry-bulb air temperature. Operative temperature is more comprehensive than dry-bulb as it considers temperature of nearby surfaces and their effect on the perception of thermal comfort.
In addition HEM models floating temperature, calculated by a set of heat balance equations. This enables HEM to model and output short-term temperature variations. If a system cannot meet the heating/cooling demand within a timestep, HEM reflects this in the internal temperature.
This is in contrast to SAP 10.2 and PHPP, which assumes that the heating and cooling demand is always met. This means SAP and PHPP reflect a fixed temperature across a whole year. HEM’s floating temperature has been shown to produce results closer to the ESP-r dynamic modelling engine than SAP or PHPP, as shown below.
Finally HEM can split a house into an unlimited number of zones, each with their own heating and cooling setpoints. In contrast, SAP 10.2 and PHPP only model two zones. This is problematic, as it does not reflect how certain buildings perform in reality, and masks problems in isolated spaces.
There are limits to the Home Energy Model’s current implementation. Unlike dynamic models, HEM cannot easily measure phenomena which accrue over longer periods of time - night-time ventilation and thermal mass are complex physics behaviours which may be understood with the BS EN ISO 52016-1:2017 standard. When validated against ESP-r, HEM fell short of accurately accounting for the heat loss and inertia of energy contained within thermal mass. HEM’s validation studies have noted these gaps, and it is likely this will be fixed in further development stages.
Further, operative temperature alone is not sufficient to understand thermal comfort. HEM does not currently model humidity: a significant factor in the perception of comfort and a lever to remove latent heat gains. Mould is also a significant issue in the UK housing stock, and has been linked to health problems such as pulmonary diseases. Understanding humidity would enable mould risk to be assessed and systematically eliminated.
At the time of writing, the Home Energy Model has only been used for a single regulated implementation, the Future Homes Standard. Other regulations will also require energy modelling - for example, the MCS room-by-room heat loss calculation required for the Boiler Upgrade Scheme (BUS).
The MCS calculations have many of the flaws of SAP and PHPP models in measuring thermal comfort. As an open source model, HEM could easily be adapted for BUS calculations - replicating the room-by-room approach of MCS calculations, but providing higher fidelity analysis that ensures better outcomes for occupants. By measuring operative temperature, HEM can enable better decisions by heat pump installers and homeowners.
We’re building Vulcan - the best way to use the Home Energy Model. Vulcan enables users to identify points of discomfort across any individual space in a home. This supports decisions that not only deliver economic and climate outcomes, but result in comfortable and happy people.
Calculating space heating and cooling demand within the Home Energy Model: A technical explanation of the methodology
https://assets.publishing.service.gov.uk/media/65783cab254aaa0010050ae5/hem-tp-04-space-heating-and-cooling-demand.pdf
The Home Energy Model: Making the Standard Assessment Procedure fit for a net zero future
https://assets.publishing.service.gov.uk/media/65e1f99a2f2b3b001c7cd879/home-energy-model-consultation.pdf
HEM-VAL-01 Inter-model comparison
hem-val-01-inter-model-comparison.pdf (publishing.service.gov.uk)