Tips for Passing Part L2a

The following measures should be considered when attempting to achieve a pass for the Building CO₂ Emission Rate (Criterion 1). Many of them will also benefit summer temperatures (Criterion 3). They are listed in rough order of importance.

Substantial reductions in building emission rate can be achieved by improving HVAC efficiency. The UK NCM System Data Wizard provides options for changing a range of system settings and parameters. For systems that are not covered there, it is permissible to enter appropriate parameters in the Apache System dialog, provided they can be justified on the basis of manufacturers’ data.

Fuel choice has a major effect on carbon dioxide emissions.

Specific fan power (SFP) is a parameter that has a significant effect on system performance.  The Non-Domestic Heating, Cooling and Ventilation Compliance Guide defines requirements for a number of features including SFP. These are potentially onerous requirements that need to be taken into account from the start of projects.  Designers need to be careful to allow large enough plant and riser space to ensure that these requirements can be met. Note that default values for SFP in the UK NCM System Data Wizard (which mirror those in iSBEM) tend to be somewhat high, often exceeding the Criterion 2 requirement. These therefore need to be edited.

The ‘System adjustment’ and ‘Control corrections’ options in the wizard produce worthwhile improvements in performance.

DHW systems should be examined for efficiency opportunities.

Efficient lighting and lighting controls offer substantial potential for bringing down emission rates and moderating summer temperatures. Any saving in lighting consumption has a double benefit, directly reducing electrical load and by reducing room gains having a favourable effect on summertime cooling loads and temperatures.

Reducing glazing area, introducing shading and optimising glazing type may all improve the building’s performance dependant on the heating/cooling load balances. The implications for natural lighting, should, however be borne in mind.

Since infiltration is reduced to low levels by Criterion 2, and minimum ventilation levels are laid down by the room activities, the main opportunity for using ventilation control to reduce carbon emissions and summer temperatures is free cooling.

It is possible to use ApacheHVAC in compliance simulations. However, the inputs to this program needs to be set with care in order to ensure that the ventilation levels and schedules specified for the room activities are accurately reproduced. In both cases profiles will need to match that of the NCM usage schedules, which means that special models will need to be created for the compliance simulations.

The settings entered in this box on the Building & System Data ‘Building & System Performance’ tab produce useful reductions in the Building Emission Rate (BER).

Improving U-values will tend to reduce heating, but may increase cooling demand in air-conditioned buildings. Increasing the solar performance of the glass by reducing the solar gains into the building can reduce cooling requirement, but may also increase heating demand.

To assess which option (U-value or solar performance) will have the greatest effect, perform a test run on the actual building and look for the dominant load. Buildings with high internal gains and large areas of glazing will almost certainly be cooling dominated.

Increasing the thermal mass of the building, for example using exposed ceiling slabs, will tend to reduce cooling demand and moderate summer temperatures.

The air permeability of the building envelope has to conform to the standard set by Criterion 2, namely a permeability of 10 m³/hr/m² at 50 Pa (corresponding to an air leakage index of 15 m³/hr/m²⁾. Guidance in CIBSE TM23 indicates that this permeability typically gives rise to an air infiltration rate of about 0.25 ach. An improved permeability can be assumed to produce an improved infiltration rate in proportion.

Note that while reducing infiltration rates will tend to lower heating energy, it may increase cooling energy by reducing the building’s ability to dispose of internal heat gains.