Calculation does not complete for unheated buildings
Posted by Andrew Bairstow on 06-Jul-22 04:44 PM
Interfaces to SBEM v6.1b (including DesignBuilder 7.1) raise an error and do not complete the calculation for buildings which have no heated zones.

This is because SBEM checks that the model includes a non-zero setting for Foundation Area (as defined in the 2021 NCM).

The Foundation Area calculation uses zone areas of zones which have an HVAC System assigned

DesignBuilder calculates the Foundation Area automatically.

So it will be calculated as zero for an unheated building.

SBEM checks to ensure that the Foundation Area is not zero during the calculation and stops it if that is the case

How to generate an XML for an Unheated Building

The EPC Conventions do not currently provide advice on generating an XML for unheated buildings for SBEM v6.

Some Accreditation Schemes are advising to set Foundation Area to 1m² (the minimum value accepted by SBEM) in order to unblock XML generation.

This is also our advice.

Note that Foundation Area does not affect the EPC Rating, it is only used in deriving the Notional Building TER.

The TER is used in deriving the benchmarks 'Newly Built' and 'typical' which appear on the EPC.

You can manually set Foundation Area in Project Details:

Note this is unlikely to have any significant effect on the Newly Built and Typical ratings, thus it should be safe to force the Foundation Area to 1m².

For example, for the location 'London' a Foundation Area of 1m² will represent a reduction of 4.3kg of CO2 emissions for the Notional Building classified as Side-Lit (eg office) and 8.6kg of CO2 if classified as Top-Lit (eg warehouse) (equivalent to a reduction of 0.043 or 0.086 in the TER for a building of area 100m²). For larger buildings the TER reduction will be less (0.0043 or 0.0086 for 1000m²).

For an Unheated warehouse of 400m² (with T8 lighting and gas DHW)-ie using Designbuilder defaults

BER = 5.0438kgCO2/m²
SER (Reference Building) = 3.8369kgCO2/m²
Rating = 5.0438/3.8369*50=65.7275=66

With a Foundation Area of 1m²
TER = 1.07981kgCO2/m²
Newly Built rating = 1.07981/3.8369*50=14.07139=14
Typical= 14.07*4=56.28=56

With a Foundation Area of 0m² (correct setting for Unheated Building)
TER = 1.10129kgCO2/m²
Newly Built rating = 1.10129/3.8369*50=14.3513=14
Typical= 14.3513*4=57.4052=57