NVHR and UK NCM analysis (Part L, Section 6, EPCs)

Then National Calculation Methodology (NCM) is the framework defined by the UK Government for the analysis of buildings and their systems for the purpose of analysis relating to Part L (for England, Wales and Northern Ireland), Section 6 (for Scotland), and Energy Performance Certificates (for all UK regions).

 

NCM analyses are governed by rules setting out the simulation methods to be used and the conditions applying in those simulations. Setpoints, occupancy, DHW consumption, internal gains and minimum ventilation rates are specified for a standardised set of building types and activities. Systems are organised into categories, infiltration rates are calculated by approved methods, and rules are laid down for the calculation of energy used by fans and pumps (‘auxiliary energy’). These rules mean that conditions and methods used within the NCM framework to simulate the Actual Building (a specialised NCM term) may differ from those applicable to the real building (the building as designed, occupied and operated).

 

The main sections of this document have described the approach to analysing the real building, using simulation methods tailored to the detailed representation of NVHR systems.

In order to comply with the special requirements of NCM a modified approach is required. 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 should be conducted on a copy of the real building model which is modified to accommodate the NCM requirements.

In the VE Compliance view the NVHR units and logos are not displayed, and their place is taken by provisions compatible with the NCM rules.

 

In this view, it is not possible to modify the assignment of rooms to unit groups. The standard Assign Room Group window will notify you when this is the case:

 

                                        

 

Fan heat gains from NVHR systems will be ignored in NCM analyses, which use standard prescriptions for room heat gains. The electricity consumption of the ventilation fans, must, however, be accounted for.

To represent NVHR systems working in combination with the systems providing heating (and where appropriate cooling) to the building, one or more Apache Systems must be created (or adapted from their counterparts used for the real building). The characteristics of these systems 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. Adjustments should then be applied to cater for the unique features of the NVHR systems.

 

For heated-only rooms served by NVHR systems, Cooling/vent. on the Ventilation tab should be set to Mechanical ventilation. In cases where the NVHR 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 natural 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.

 

Any rooms for which an NVHR system supplements an air conditioning system should be modelled with an appropriate air conditioning system type, and in this case Changeover mixed mode free cooling on the Apache System Cooling tab should be set to Mechanical ventilation.

Set up this way, the system will be assessed for regulatory purposes against a comparable system in the Notional Building.

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.

 

On the System tab of the Space Data dialog for each room served by an NVHR 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.

-       Free cooling flow capacity should be set to the maximum volume flow rate that the NVHR system is able to deliver to the room.

-       Is there mechanical supply in this room? (when present) should be ticked, and immediately below it the checkbox Specific fan power should be set according to the principles described below.

-       Demand controlled ventilation Type should be set to Demand control based on occupancy density (Cd = 0.85).

 

Fan power used by an NVHR 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 Space Data dialog for each 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.

 

Where heating and cooling is supplied by a separate system providing mechanical air distribution there will be SFPs associated with the central system and/or the local air delivery. Where appropriate these SFPs should be modified to take account of the small additional fan power incurred by an NVHR system.

 

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. MacroFlo analysis should be disabled by ensuring that the MacroFlo link? checkbox is unticked on the Simulation Settings tab of the Building & System Data dialog.

 

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.