L1 Carbon Index Method (ADL1 paragraphs 1.27-1.28)

The L1 Carbon Index Method lays down two requirements: a Carbon Index Requirement based on the SAP^[8] procedure, and a Maximum U-value (Table 3) Requirement. The ‘additional requirements’ of L1 – ADL1 paragraphs 1.30-1.45, referred to in the Summary Guide as ‘Additional checks by builders’ – are, for convenience, also treated here under the heading of the Carbon Index Method.

 “The Carbon Index adopted in this method is defined in the SAP, and the requirement would be met if the Carbon Index for the dwelling (or each dwelling in a block of flats or converted building) is not less than 8.0...” (ADL1, paragraph 1.27).

The Government’s Standard Assessment Procedure for Energy Rating of Dwellings (2001 edition) (‘SAP 2001’^[8]) defines a method for producing an energy cost rating (the SAP rating) and a carbon index (the CI) for a dwelling, based on calculated annual energy for space and water heating. The L1 Carbon Index Method is based on the carbon index calculated by this method.

Input data for the SAP and CI calculation is taken from the Building Regulations input dialogues, the model geometry and the assigned thermal attributes. Data specific to the SAP & CI calculations (referred to for brevity as SAP calculations) forms the bulk of the L1 data input on the Part L Building & System Data dialogue. Some of the SAP data relating to the heating system is shared by the Elemental and Target U-value Methods. Data specific to the SAP calculations is indicated with green text.

The SAP/CI calculation is set out in SAP 2001 as a worksheet with numbered boxes. The software fills out this worksheet automatically, using data provided in the interface. Tab headings on the Part L Building and System Data dialogue – for example ‘SAP 5 Internal gains’ – identify data relating to different sections of the worksheet.

The following sections describe how the software fills out the SAP worksheet using the input data.

Where SAP 2001 offers a choice between using tabulated values or a formula (for instance SAP Table 1), the software always uses the formula.

Dimensions, areas and volumes are calculated by the Part L convention (ADL1 paragraph 0.15) which ignores the width of internal partitions and floors. Ground floor area is taken to be the total floor area in contact with the ground. Data for higher floors is grouped together under the heading ‘Other floors’.

Data for this section is taken from the ‘SAP 2. Ventilation rate’ tab.

Fabric data is extracted from the model. Only the most commonly used window, wall and roof types are listed individually. Other types are grouped together, the worksheet displaying their total area and average U-value.

Data for this section is drawn from the ‘SAP 4 & 10. Water heating’ tab and the ‘Heating system’ tab.

Where the worksheet notes refer to ‘instantaneous water heating’, ‘no stored water’ or ‘no tank’ the program uses the check-box setting ‘Is there a hot water tank or other storage device?’. Where it refers to community heating the choice is dictated by the ‘Heating system’ setting on the Heating System tab (echoed by a check-box on the ‘SAP 4 & 10’ tab).

The setting of box 42 – Water storage loss factor (Table 2) – uses the setting for ‘Hot water storage type (SAP Table 2)’ and follows the notes to SAP Table 2. An exception applies in the case of community heating with no storage tank (see note to box 42).

Box 48 – Primary circuit loss (Table 3) – gets a value from SAP Table 3 using the setting for ‘Water heating system type’, modified for a combi boiler using the settings ‘Combi boiler?’, ‘Keep-hot facility type’ and ‘Controlled by time clock?’.

In the case of a combi boiler with an electric keep-hot facility, box 49 omits the contribution from box 48 – see footnote ‘**2)’ to SAP Table 3.

Box 50 – Efficiency of water heater – takes a value dependent on the water heating system. If the setting for ‘Water heating system’ is ‘Provided by the space heating system (but not heat pumps)’ box 50 is set to the efficiency of the space heating system adjusted by Table 4c (‘Adjusted efficiency (SAP Table 4c)’ on the Heating System tab). (The note to box 50 stipulates ‘Use value from Table 4a or 4b...’, but it is clear from a footnote to SAP Table 2 that database or manufacturers’ efficiencies are to be used, where available, for hot water systems.)

If water heating is provided by heat pumps, section SAP Appendix G, section G1 Water heating requirement, applies. In this case, water heating is partly or entirely provided by an immersion heater, and accordingly:

‘Water heating system (SAP Table 4a)’ should be set to ‘Immersion heater (single)’ or ‘Immersion heater (dual)’;

‘Water heating fuel (SAP Table 12)’ should be set to the appropriate electricity tariff.

‘Water heating system type’ should be set to ‘Electric immersion heater’.

For a ground source or water source heat pump, box 50 is assigned, in place of efficiency, the heat pump’s seasonal performance factor (SPF) taken from the ‘Efficiency’ column of SAP Table 4a, with data from the Heating System tab. In this case, the formula given in SAP Appendix G is used to calculate box 51 – Energy required for water heating. For an air source heat pump, box 50 is set to 100%.

For other types of water heating system, box 50 is set to a value from the ‘HOT-WATER-ONLY HEATING SYSTEMS’ section of SAP Table 4a. The appropriate line of the table is determined by the ‘Water heating system (SAP Table 4a)’ setting, where the options ‘Immersion heater (single)’ and ‘Immersion heater (dual)’ cover the case ‘Independent electric water heating system’.

Boxes 51 and 52 are set using the formulae indicated on the worksheet, with the exception relating to heat pumps identified above.

Box 53 – Lights, appliances, cooking and metabolic (Table 5) – is set as a function of total floor area using the SAP Table 5 formula. (Note that early editions of ADL1 contain an error in the expression for N; the correct version is given at the foot of SAP Table 1.)

Box 53a – Additional gains from Table 5 – is set using data from the ‘SAP 4 & 10’ tab. The value is incremented by 10 or 25 W for each type of gain ticked on this tab, as specified in the footnotes to SAP Table 5.

Boxes 54 and 55 are calculated by the formulae indicated on the worksheet.

For glazing in each of the 9 orientation bands, solar gains are calculated using SAP Tables 6a (Access factor) and 6 (Flux).  Access factor is set as an attribute of the glazing type in APcdb. The value for rooflights is always 1.3.

Where more than one type of glazing is used in a given orientation band, the method follows the guidance given in SAP 2001 section 6.3: the values displayed for ‘Flux (Table 6)’ are area-weighted averages, and the values displayed for ‘Access factor (Table 6a)’ are (area × flux)-weighted averages, thus ensuring that the Gains total is correct for each orientation band.

Glazing connecting a heated room with an unheated, separated conservatory is treated as external glazing for the purpose of the solar gain calculation, in accordance with SAP 2001 section 3.3.3.1.

For box 70 – Mean internal temperature of the living area (Table 8) – and box 72 – Adjustment for gains – the ‘heating type’ parameter for Table 8 and the Responsiveness (R) are taken from Table 4a or 4d, with input from settings on the Heating System tab.

For box 71 – Temperature adjustment from Table 4e, where appropriate – and box 74 – Temperature difference between zones (Table 9) – the Heating System setting ‘Heating system controls (SAP Tables 4e & 4c)’ is used as input to Table 4e.

Early editions of SAP 2001 have a printing error in the calculation for box 78: the correct calculation is (box 70) ÷ (box 37).

The nearest value is taken from Table 10 (interpolation is not applied).

Sections 9 and 10 of the worksheet apply to dwellings served by systems other than community heating systems. Such systems are referred to in L1 as ‘individual’ or ‘conventional’ heating systems. Dwellings with community heating are identified by the setting ‘COMMUNITY HEATING SCHEME’ assigned to the ‘Heating system (Elemental Method and SAP Table 4a)’ field on the Heating System tab.

Box 83 – Efficiency of main heating system, % – is set to the value displayed for ‘Adjusted efficiency (SAP Tables 4e & 4c) (%)’ on the Heating System tab. This includes a correction for system controls.

Where there is a secondary system or an electric CPSU, nonzero values are written in boxes 82 and 84. The presence and type of any secondary system is specified by the two fields labelled ‘Secondary system’ on the SAP 9 & 10 Space-heating tab. The presence and type of any electric CPSU is specified by fields on the Heating System tab (Heating system = ‘CENTRAL HEATING SYSTEM...’, Heating system type = ‘Electric boiler’, Electric boiler type = ‘Electric CPSU in heated space).

Electric CPSUs are treated by the method set out in SAP Appendix F, where their on-peak input qualifies as a secondary system for the purpose of box 82. The nearest values of C_max is taken from Table F1, with no interpolation applied. 100% is entered in box 84 – Efficiency of secondary heating system.

For other types of secondary system, values for boxes 82 and 84 are obtained from Table 11 using information on the main system and secondary system provided on the SAP 9 & 10 Space-heating tab. The ‘Main system’ setting on this tab is set automatically from the main system data provided on the Heating System tab, as follows:

When Heating system is ‘CENTRAL HEATING...’ or ‘WARM-AIR SYSTEM’, ‘Main system’ is set to ‘Boiler’, except where ‘Heating system type’ is ‘Heat pumps’, when the setting is ‘Heat pump’.

When Heating system is ‘STORAGE RADIATOR SYSTEM’, ‘Main system’ is set to ‘Fan assisted storage heaters’ or ‘Electric storage heaters not fan assisted’, depending on the Heater type.

When Heating system is ‘ROOM HEATER SYSTEM’, the ‘Main system’ setting depends on ‘Room heater system type’ and ‘Heater type’. When ‘Room heater system type’ is ‘Gas’, the setting is ‘Gas room heaters’. When ‘Room heater system type’ is ‘Solid Fuel’, and ‘Heater type’ is any of the options beginning ‘Open fire...’, the setting is ‘Coal fires or other electric systems’. In all other cases the setting is ‘Coal or electric room heater’.

When Heating system is ‘OTHER SPACE-HEATING SYSTEMS’ (which means Electric ceiling heating) ‘Main system’ is set to ‘Coal fires or other electric systems’

Boxes 87a-8e are filled in accordance with SAP Table 4f using data from the ‘Additional gains (SAP Table 4f)’ section of the SAP 9 & 10 Space-heating tab and elsewhere:

Box 87a is incremented by 0.47 if ‘Central heating pumps?’ is ticked (note that these include CPSU pumps and heat pumps – see SAP Appendices F and G), and by 0.35 if ‘Boiler oil pump?’ is ticked. The value is then multiplied by 1.3 if there is no room stat

Box 87b is set to 0.16 if ‘Gas boiler with flue fans?’ is ticked.

For a warm air system (Heating System tab), box 87c is set to 0.002 × dwelling volume (box 6).

For a whole-house mechanical ventilation system (SAP 5. Internal gains tab), box 87d is set to 0.004 × dwelling volume (box 6).

For a combi boiler with an electric keep-hot facility (SAP 4 & 10. Water heating, ‘Combi boiler?’ and subsidiary fields), box 87e is set to 0.8 or 2.0 depending on the setting for ‘Controlled by time clock?’.

For box 88 – Space heating - main system – the fuel price is obtained from SAP Table 12 using the Space heating fuel specified on the Heating System tab. In this context the option ‘Electricity (7-hour tariff)’ relates to the Table 12 entry ‘Electricity (off-peak)’ and the option ‘Electricity (10-hour tariff)’ relates to the Table 12 entry ‘Electricity 10 hour tariff (off-peak)’.

For box 89 – Space heating - secondary system – the fuel price is obtained from SAP Table 12 using the Secondary system fuel specified on the SAP 9 & 10 Space-heating tab.

Boxes 90-94 (water heating) depend on settings on the SAP 4 & 10 Water heating tab and, where the main space heating system is used for water heating, the characteristics of this system.

If the setting for ‘Water heating system (SAP Table 4a)’ is ‘Provided by space heating system (but not heat pumps)’, the fuel price for water heating (box adjacent to box 94) is set equal to the fuel price for the main space heating system (box adjacent to box 88), with an exception in the case of a system using an electric CPSU. In this case a 10-hour electricity tariff is assumed, with on and off-peak prices taken from Table 12.

If the setting for ‘Water heating system (SAP Table 4a)’ is ‘Immersion heater (single)’ or ‘Immersion heater (dual)’ (which are appropriate settings when space heating is provided by heat pumps) the on and off-peak prices are again taken from Table 12, taking into account the tariff specified in ‘Water heating fuel (Table 12)’.

For other settings of ‘Water heating system (SAP Table 4a)’ (‘From a heat exchanger built into a gas warm-air system’ or ‘Single/Multi-point gas water heater’) the fuel price for water heating is assumed to be the same as the price for space heating.

Where off-peak electricity is used for water heating, box 90 – On-peak percentage (Table 13 or Appendix F /G ) – is set using Table 13, or Appendix F (in the case of an electric CPSU), or Appendix G (in the case of  ground or water source heat pumps). An attempt to apply Table 13 where there is no water tank or other storage device (an unrealistic situation) results in the on-peak percentage being set to 100.

The price for heating pump and/or fan energy appearing to the left of box 95 is always set to 19.69 £/GJ (Table 12, Electricity (standard tariff)).

Box 96 – Additional standing charges (Table 12) – is the sum of the standing charges for fuels used in the main system, the secondary system and the water heating system. If the same fuel is used for more than one purpose the standing charge is not duplicated.

Box 99 – Energy cost factor (ECF) – and  box 100 – SAP rating – are calculated from previous worksheet entries using the formulae provided.

The section of the worksheet appearing under ‘Community heating’ is given the heading ‘9* Space-heating requirement’ in the program output for consistency of presentation. This section takes the place of ‘9 Space-heating requirement’ when community heating is in use, as identified by the setting ‘COMMUNITY HEATING SCHEME’ assigned to the ‘Heating system (Elemental Method and SAP Table 4a)’ field on the Heating System tab.

In the program output, the guidance note ‘If CHP is not involved enter ‘0’ in box (83*) and ‘100’ in box (84*)’ appearing at the top of this section of the worksheet is clarified to ‘If CHP/recovered heat is not involved enter ‘0’ in box (83*) and ‘100’ in box (84*)’.

Box 82* – Overall system efficiency of the heating plant – is set to the value displayed for ‘Adjusted efficiency (SAP Tables 4e & 4c) (%)’ on the Heating System tab. The value is 100% plus a correction for system controls.

Box 83* – Fraction of heat from CHP unit or fraction of heat recovered from power station – is the value entered in the box with this label on the SAP 9* & 10* Space-heating (community heating) tab.

Box 85* – Distribution loss factor (Table 12a) – is set using Table 12a and the radio button options on the SAP 9* & 10* tab.

For a whole-house mechanical ventilation system (SAP 5. Internal gains tab), box 88* is set to 0.004 × dwelling volume (box 6).

An asterisk is added to the section number in the program output to indicate the use of community heating. This section takes the place of ‘10 Fuel costs’ when community heating is in use.

Depending on the setting of ‘Water heating system (SAP Table 4a) on the SAP 4 & 10 Water heating tab, water heating is provided by either the community heating scheme (boxes 91* and 92*) or by immersion heater (boxes 94* and 94a*). No other options are permitted.

If water heating is provided by the community heating scheme, the fuel prices appearing to the left of boxes 89* and 90* are set to 3.17 and 4.55 £/GJ, respectively (figures for community heating from SAP Table 12), and boxes 94* and 94a* are set to 0.

If water heating is provided by immersion heater, the fuel prices appearing to the left of boxes 89* and 90* are set to 0, and the on-peak and off-peak prices appearing to the left of boxes 94* and 94a* are set to values from SAP Table 12 appropriate to the tariff specified in ‘Water heating fuel (SAP Table 12)’. Box 93* – On-peak percentage (Table 13) – is set using Table 13. An attempt to apply Table 13 where there is no water tank or other storage device (an unrealistic situation) results in the on-peak percentage being set to 100.

The price for heating pump and/or fan energy appearing to the left of box 95* is always set to 19.69 £/GJ – the Table 12 value for Electricity (standard tariff).

Box 96* – Additional standing charges (Table 12) – is the sum of the standing charges for community heating and (where applicable) off-peak water heating. If the same fuel is used for both the standing charge is not duplicated.

An asterisk is added to the section number in the program output to indicate the use of community heating. This section takes the place of ‘11 SAP rating’ when community heating is in use.

Box 98* – Energy cost deflator (Table 12) is set unconditionally to 1.05. Box 99* – Energy cost factor (ECF) – and  box 100* – SAP rating – are calculated from previous worksheet entries using the formulae provided.

The section number 12 has been added to this heading for consistency of presentation.

This section of the worksheet applies to dwellings with individual (conventional) heating systems, and those with community heating provided by community boilers.

The boxes under the heading ‘Energy (GJ/year)’ are either copied from previous boxes or (in the case of community boilers) calculated from previous boxes using simple formulae specified on the worksheet.

The boxes under ‘Emission factor (Table 15)’ are evaluated using SAP Table 15 with a suitable fuel as input. The fuels used in this context are ‘Space heating fuel’ on the Heating System tab (for individual heating systems), ‘Community boilers fuel (SAP Table 15)’ on the SAP 9* & 10* tab (for community boiler systems) and, where applicable, ‘Secondary system fuel’ on the SAP 9 & 10 Space-heating tab and ‘Water heating fuel (SAP Table 12)’ on the SAP 4 & 10 Water heating tab.

The section number 12* has been added to this heading for consistency of presentation.

This section of the worksheet applies to dwellings served by community heating using CHP or heat recovered from power stations (possibly supplemented by community boilers).

In the case of CHP, boxes 101* and 102* are set using the corresponding parameters on the SAP 9* & 10* tab, and the fuel used in Table 15 for box 103* - CO₂ emission factor for the CHP fuel from Table 15 – is ‘CHP fuel (SAP Table 15)’. If a nonzero value is entered for heat to power ratio on the SAP 9* & 10* tab, box 106* is set to this value. Otherwise it is calculated from the ratio specified on the worksheet. Box 107* is calculated using the worksheet formula.

In the case of heat recovered from power stations, the procedure laid down in SAP Appendix C2 is followed. 0 is entered in boxes 101* to 106* and 5 is entered in box 107*. Box 113* is set to 0.

The boxes labelled ‘Table 15’ in the Emission factor column are evaluated using SAP Table 15 with a suitable fuel as input. The fuels used in this context are ‘CHP fuel’ and ‘Community boilers fuel (SAP Table 15)’ on the SAP 9* & 10* tab (for CHP and community boilers, respectively), ‘Water heating fuel (SAP Table 12)’ on the SAP 4 & 10 Water heating tab (where appropriate), and the emission factor (19.69) for standard tariff electricity (for fan and pump electricity).