Release Notes

The release notes below highlight significant changes that have been made in each version of the Heat Loss Calculator. Some of the changes may affect the reported heat loss by a small margin.

v3.0.0 - 01 April 2026

New features

• The automated External Design Temperature now offers the choice between 99% (CIBSE Column A) and 99.6% (CIBSE Column B).
• A user-specified Design Air Change Rate can now have a pressured difference specified as well (either infiltration or at 4 Pa/50 Pa).
• Automated U-value lookups now display the data source and reference table alongside each element type.
• The air permeability value used in the calculation has been added to output result page for clarity.
• Heat loss calculations based in Jersey or Guernsey will now use reference temperatures from the Jersey Met Office.

Improved features

• An assumed airtightness method now defaults to using BS 12831 Annex B air permeability values (previously it was based on DHDG ventilation rates).
• The calculated ventilation envelope area now correctly matches the CIBSE TM23 definition.
• External U-value lookups now uses reference data from RdSAP 10 (June 2025).
• Internal U-value lookups now uses reference data from Domestic Heating Design Guide (2026-01).
• U-value lookups for certain room types (basements, conservatories and room in roof) are now more accurate.
• When adding a new element type (wall/floor/ceiling/door/window), the age band will now default to the same as the main dwelling.
• Room level minimum air change rates have been updated to Domestic Heating Design Guide (2026-01).
• Neighbouring building / unheated space temperatures have been updated to Domestic Heating Design Guide (2026-01).

Removed features

• The Number of Occupants input field has been removed as it is no longer used.

Bug fixes

• It is no longer possible to pick an element age band that is older than the main dwelling.
• "Ground (basement wall)" adjacent type is now only shown for rooms in the basement.

v2.1.1 - 06 October 2025

Bug fixes

• Walls that were completely covered by a door/window were causing a divide by zero error when calculating the net area.

v2.1.0 - 03 June 2025

New features

• When manually specifying a U-value, a user can decide whether the U-value already includes a thermal bridging factor or not.
• A warning message will be displayed when selecting MVHR as the ventilation system without using a measured air permeability.

Improved features

• Additional unheated space categories were added for "Internal Cupboard", "Garage (built-in)", "Garage (attached)" and "Circulation Area".

Bug fixes

• The geometric parameter used in "Ground floor (insulated)" U-values was being calculated incorrectly.

v2.0.4 - 28 May 2025

Bug fixes

• The gross wall area for walls containing openings was being calculated incorrectly in certain situations.

v2.0.3 - 27 January 2025

Bug fixes

• Altitude adjusted external temperatures were not being used in certain situations.

v2.0.2 - 09 May 2024

Bug fixes

• Party wall U-values (of unknown construction) for flats/maisonettes were not using the correct reference data.

v2.0.1 - 24 April 2024

Bug fixes

• The age band of the element (not main building) was being incorrectly used for determining the thermal bridging factor of external elements.

v2.0.0 - 23 April 2024

New features

• New dwelling input fields were added: Degree Days, Dimension Specification.
• New ventilation input fields were added: Zone height above ground (for flats/2nd zones only).
• New room input fields were added: Intermittent Heating (%) and Heat Gains (W).
• Space Heating Load (incorporating heating up/heat gains) and Annual Energy were added to the output result.

Improved features

• Number of sheltered sides can now be specified individually for each ventilation zone.
• Room detail page now displays each elements width/height (instead of calculated area) for better user experience.

Bug fixes

• Unheated interior spaces marked as "no external walls", e.g. airing cupboards, are no longer included in the external envelope area.
• Annex E (U-value adjustment for ground contact) was not being applied to manually entered U-values.