Analysing the real building and the Actual Building
The preceding sections of this user guide describe the approach to analysing the real building, using simulation methods tailored to the detailed representation of systems.
In order to comply with the special requirements of NCM a modified approach is required for NCM modelling of Roof Terminals. This involves analysis of the Actual Building and the buildings which the software derives automatically from it – the Notional Building and (for EPC purposes) the Reference Building. These analyses are performed in the VE Compliance view of the VE, where tools are provided to facilitate the input and editing of the additional data items.
Because some of the data required for NCM is shared with the analysis of the real building, NCM analysis is conducted on a copy of the real building model modified to accommodate the NCM requirements.
In the VE UK Compliance view a model with Passivent Roof Terminals should have the Apache simulation options set to Macroflo ON (it is normally OFF).
For heated-only rooms served by Passivent Roof Terminal systems, Cooling/vent. on the Cooling tab should be set to Natural or Mechanical ventilation (Airstract iAT® and Hybrid Plus Airstract®).
For Airscoop® mixed mode (mechanical cooling) an Apache System should be created with an appropriate air conditioning system type. The characteristics of this system should be entered via the UK NCM system data wizard, starting with the selection of a suitable system type from the UK NCM system type menu e.g. Passive chilled beam or Split system and in this case Changeover mixed mode free cooling on the Apache System Cooling tab should be set to Natural ventilation.
For Roof Terminals where fans are also operational i.e. Airstract iAT® and Hybrid Plus Airstract® the ApHVAC networks cannot be utilised for NCM compliance, however the electricity consumption of the ventilation fans, must, however, be accounted for.
Close attention should be paid to the settings on the System adjustment, Metering Provision and System controls tabs of the UK NCM system data wizard, since these have a critical effect on performance as assessed by the Regulations.
When the Apache Systems have been created, they must be assigned as appropriate to each room served by a Passivent Roof Terminals system. These assignments can be made (to the System field on the System tab) either in the Building Template Manager or in the room’s Space Data dialog.
On the System tab of the Room Data dialog for each room served by a Passivent Roof Terminals system:
- System and Auxiliary vent. system should be set to the appropriate Apache System.
- DHW system should be set to the Apache System created for this purpose.
- Demand controlled ventilation Type should be set to Demand control based on occupancy density (Cd = 0.85).
Where the Passivent Roof Terminals system provides all the ventilation, the Air supply setting on the Ventilation tab of the UK NCM system data wizard should be set to Local ventilation-only units, such as window/wall/roof units serving a single area. The mechanical ventilation component of the system will be taken into account via SFP adjustments described below. It should be noted that the aspects of the system which provide benefit in the form of reduced summertime temperatures do not explicitly receive credit for this benefit under the current NCM Criterion 3 rules, which in contrast to earlier editions of the Regulations are expressed exclusively in terms of reduced solar gain. However, this aspect of performance will form an important part of the design as assessed by the performance of the real building.
Set up this way, the system will be assessed for regulatory purposes against a comparable system in the Notional Building.
Fan power used by a Passivent Roof Terminals system is represented within the NCM framework as a component of auxiliary energy. This is the sum of contributions from pumps and fans. Pump power is a function of system category and subject to limited user control. Fans fall into two categories – central and local – for each of which a specific fan power (SFP) must be specified.
In the case of heating-only systems for which ventilation is provided locally the SFP parameter on the Apache System dialog (System specific fan power (SFP)) should be set to zero and a suitable SFP value entered on the System tab of the Room Data dialog for each room (ventilation & extract > local mechanical exhaust in room?). This value should represent the average SFP applicable to the room ventilation during operating hours, accounting for the fact that for many hours the fan will be off. A suitable value (which will typically be very small) should be derived from detailed simulations performed (under equivalent assumptions) for the real building. For Airscoop (standlone) this value will be zero.
A separate Apache System, with suitable operation parameters, should be created to provide domestic hot water (DHW), and assigned to the rooms for this purpose.
For the model used for NCM analysis, infiltration is set using parameters on the Building & System Data dialog.
On the Simulations Settings tab of the Building & System Data dialog (accessed via the button Set Building & System Data at the bottom of the screen):
SunCast link? should be ticked if a suitable shading file has been generated.
MacroFlo link? should be ticked.
HVAC link? should be unticked (in contrast to the setting in Apache View).
Radiance link? should be unticked.
Natural ventilation air exchange? should be unticked.
With regard to the numerous other data inputs relating to NCM presented in the VE Compliance view, reference should be made to the general guidance provided on this aspect of the software.
