the cambridge housing model guide v 2.7 171111

A Guide to

The Cambridge Housing

Model

November 2011

Author: Martin Hughes

CAMBRIDGE

ARCHITECTURAL

RESEARCH

LIMITED

Cambridge Architectural Research 1


Table of Contents

1. Introduction....................................................................................................................................3

2. Inputs ..............................................................................................................................................7

3. Calculations ....................................................................................................................................9

4. Assumptions .................................................................................................................................12

5. Outputs .........................................................................................................................................13

6. Administration..............................................................................................................................13

7. Post processing: Modelling Gap ..................................................................................................13

8. Using the Model ...........................................................................................................................14

Annex A – Housing Data.......................................................................................................................17

Annex B - Outputs ................................................................................................................................20

Annex C - Assumptions.........................................................................................................................22

Abbreviations .......................................................................................................................................28


1. Introduction

The Cambridge Housing Model (CHM) is a domestic energy model for Great Britain and the United

Kingdom. The model is used to generate estimates of energy use for the Department of Energy and

Climate Change (DECC) Housing Energy Fact File (HEFF) and the associated Energy Consumption in

the UK (ECUK) Domestic data tables, replacing the use of the Building Research Establishment

Housing Model for Energy Studies (BREHOMES).

The primary source of input data for the CHM is the English Housing Survey (EHS). In the 2009

dataset the EHS provides data on 16,150 representative English dwellings (cases). Each of these

cases represents a quantity of dwellings in England - that is a weighting, such that their sum is equal

to the total number of dwellings in England (22.3 million in 2009). The CHM reads in the EHS

dwelling for each case and performs building physics calculations to determine energy consumption

and associated CO2 emissions, by use and by fuel type. Multiplying the energy use and CO2 emissions

by the associated weighting and summing across all cases gives total values for England. Using

appropriate England–to–GB and GB-to-UK scaling factors based on the number of dwellings in

England, GB and the UK, the approximate GB and UK energy use and CO2 emission totals can be

calculated. The CHM has further been updated to include data from the Scottish House Condition

Survey (SHCS), leading to a more accurate picture of GB homes. The 2008 SHCS provides data on just

under 9,400 representative dwellings, representing over 2.3 million dwellings in Scotland. The input

data into the CHM from the SHCS has been designed to match the form of the EHS input data, so the

CHM deals with the SHCS data in the same way it deals with EHS data. Readers should note that the

version of the model circulated with this document contains only EHS data, although we will publish

the CHM with both English and Scottish input data in early 2012.

The model is built in Microsoft Excel. Calculations are principally performed directly within

worksheets. Visual Basic for Applications (VBA) macros are used to feed data for each representative

dwelling through the model, and to record the results. The calculations used in the CHM are

principally based on the SAP 20091 worksheet, modified to include appliances and cooking energy

use. We recognise that the SAP methodology is a standardised approach for calculating the energy

performance of specific dwellings, intended primarily for checking compliance with Part L of the

Building Regulations rather than estimating actual energy consumption across the whole stock, but

SAP 2009 is the latest interpretation of the most widely-tested and widely-used framework for

assessing energy use in UK homes: BREDEM2.

1 Department of Energy & Climate Change (2010) SAP 2009: The Government’s Standard Assessment

Procedure for Energy Rating of Dwellings. 2009 edition, revised October 2010. Watford: Building Research

Establishment. 2 Anderson B R et al. (2002) BREDEM-8 model description 2001 update - with corrections, Watford: BRE.


The model consists of 8 worksheets: 6 principal sheets used for inputs, calculations, data,

assumptions and outputs, and 2 secondary sheets for administrative use and information:

Figure 1: Worksheets in the Cambridge Housing Model

Outputs are presented for each case, and at the total national level for energy use in England, by

applying weightings and summing. These national totals are adjusted to account for vacant

dwellings, such that calculated energy use figures for properties with zero occupants are re-scaled to

10% of their calculated value (those calculated values are based on a SAP calculation of the number

of occupants). Scaling to UK or GB totals must be carried out by the user.

To assist the user we have used coloured shading of cells within the model to signify inputs,

calculations, assumptions and outputs. It should be noted that this is for guidance only as the

distinction between inputs, calculations and assumptions can be blurred.

The EHS and SHCS collect large amounts of information on each of their 25,000 cases. We use some

of this data for input to the CHM. Prior to inputting this data into our model in the Housing Data

sheet, we first select the relevant datasets from the EHS & SHCS, “clean” it and run it through

Converters – one for the EHS data and one for SHCS. The “cleaning” process removes any obviously

inconsistent values from the datasets. A record of all “cleaned” values is maintained. This data is

then run through our Converters3. Some of the survey data is appropriate for use in its original form

however this is not the case for all of the data. In order to generate the required input Housing Data

some data needs to be interpreted, some combined, and some default assumptions need to be

made. A detailed description of the conversion process for EHS data will be circulated separately to

this document. Once the Housing Data has been generated it can be copied from the Converter and

pasted directly into the Housing Data sheet. The user may also input Climate Data as appropriate -

see the Inputs section below for further details.

The B Physics Parameters sheet contains a large number of variables and assumptions, primarily

taken from or based on SAP 2009. The Building Physics Model then uses the information from

3 The Converters do not form part of CAR’s work for DECC.

About the

Model

Housing Data Climate Data B Physics

Parameters

Building

Physics Model

B Physics

Outputs

Data &

Assumptions

Version History

Inputs Assumptions Calculations Outputs


Housing Data, Climate Data and B Physics Parameters to perform the model calculations –

principally based on the SAP 2009 worksheet calculations. The model can either be run for a single

case or for all cases listed in Housing Data. In either situation the results are output to B Physics

Outputs.

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2. Inputs

Housing Data

The principal input to the model is the data contained within the worksheet Housing Data. This

sheet contains one row for each case, with columns representing descriptive data for that case –

such as the dwelling ID (house code), the weighting for this dwelling, the type of dwelling, the

dwelling age band, numbers of occupants, building dimensional data, information on the heating

and hot water systems, levels of insulation and glazing, and so on. A full list of the information

contained in the Housing Data sheet is shown in Annex A.

The input data can be pasted directly into the Housing Data sheet from row 7 onwards. The

appropriate data must be placed in the corresponding column for each case. Rows 4 and 5 in this

sheet are headers for each column.

There are a number of active cells within this worksheet.

• Cell A5 counts the number of cases (16,150 for the 2009 EHS data). The default for this cell is to

COUNT(A7:A60000), however if more than 60,000 cases are to be used then this COUNT should

be extended as appropriate.

• Cell B5 sums the number of dwellings – that is the weighting for each case. This sum gives the

total population of dwellings considered in the model (22.3 million for the EHS data, the total

number of English dwellings). The default for this cell is to SUM(B7:B60000), however if more

than 60,000 cases are to be used then this SUM should be extended as appropriate.

• The second row is used to identify the data associated with a single case code, for use when only

a single case run is performed, by entering the appropriate house code from the list of input

Housing Data into the appropriate cell at the top of Building Physics Model and pressing Enter.

This row is automatically populated when the user presses this button.

The top row is inactive, but is used by the macro that loops through the Housing Data when

performing a full model run of all of the Housing Data. This row is automatically populated when the

user presses the Run Calculation button at the top of Building Physics Model.

Climate Data

Climate Data contains a number of inputs and also a number of calculations. The three key pieces of

input climate data are:

• Monthly External Temperature (oC) by region

• Monthly Average Wind Speed (m/s) by region

• Monthly Average Horizontal Solar Radiation (W/m2) by region


As a default we have used data that we believe is appropriate here, however the informed user may

wish to overwrite this data with their own. There are also several additional pieces of climate data

that the informed user may wish to input:

• Latitudes (o North) for each region

• Monthly Solar Declination (o)

• Solar Hot Water Collector Setting - Tilt of collector and Orientation: user selections from a drop-

down menu

• Ratio of Monthly Solar Radiation to Annual Average Solar Radiation for a series of collector tilts:

horizontal, 30 o

, 45 o

, 60 o

and vertical.

However we would emphasise that only an informed user should change any of this information, if

they believe that they have more relevant data. Readers should note that changes to any of the

above data will be applied to all cases in any given model run.

The top of the Climate Data sheet contains a summary of the climate data used for the currently

referenced case. In addition, towards the middle and end of the sheet are a number of calculations

in accordance with SAP 2009, which are referenced.

B Physics Parameters

Within B Physics Parameters there are a number of user-selection inputs which the informed user

can vary. We would emphasise that only an informed user should consider changing these values.

Also it should be noted that typically these user-selected values will be applied to all cases in any

given model run. Below is a list of the user-selection inputs:

• Window U-value (W/m2K) - SAP Table 6e: Curtain Effect factor.

• Wall U-value (W/m2K): England and Wales (semi-exposed*) - SAP Table S6: Thermal resistance of

unheated space (Ru).

• Hot Water Usage Calculation - Dwelling is designed to achieve a water use target of ≤125 litres

per person per day (all water use, hot and cold): User selection Yes or No re-scales the “Base

Rate” and “Per person” values.

• Hot Water Storage Loss: Temperature Factor - SAP Table 2b: Cylinder Thermostat Factor.

• Internal Heat Gain Type - Heat Gain Setting: user selection from a drop-down menu.

• Fuel Costs: Domestic Hot Water (DHW) System Electricity Price: High-rate Fraction.

• Fuel Costs: Secondary Heating System Electricity Price: High-rate Fraction.

• Fuel Costs: Mechanical Ventilation System Electricity Price: High-rate Fraction.

• Fuel Costs: Other Electricity Uses Electricity Price: High-rate Fraction.


• Effective Air Change Rate Calculation Parameters: there are a series of user selections from a

drop-down menu.

Building Physics Model

There is a single input option in Building Physics Model relating to running the model for a single

case: the user needs to enter the appropriate House Code in the section “Calculation for a single

house”, prior to pressing the button Enter.

3. Calculations

The calculations undertaken in the model are primarily based on the SAP 2009 worksheet and other

associated SAP calculations. References to all SAP calculations are shown in the model in the format

[Ref]. The majority of model calculations are undertaken in Building Physics Model. Other

calculations are also made in Climate Data and B Physics Parameters. SAP 2009 should be referred

to for further details of these calculations.

As the focus for SAP is ‘regulated’ energy use (comprising space and water heating, fixed lighting,

ventilation and pumps), calculations for determining the energy use and associated CO2 emissions

for electrical appliances and cooking are not explicitly stated in SAP 2009. These features are

therefore additionally considered in the CHM, based on BREDEM 8 and SAP as follows:

• Electrical Appliances calculations are as per the appliances energy use [L11] - [L12] SAP 2009

calculations which form part of section (5) Internal Gains calculations. (Note that within SAP

these calculations are only used as part of the internal gains calculations.)

• For the cooking calculation we have used the equations in section 5 of BREDEM 8, but with some

adjustments. In the model, we provide two options of cooking systems: (1) Gas hob and electric

oven and (2) electric cooker.

We assume dwellings that use gas for main heating and/or Domestic Hot Water (DHW) would

use gas for cooking, otherwise electric cooker is applied.

In BREDEM 8 Table 5.1, there are cooking fuel use and the associated gains for various cooking

systems. However, in SAP 2009 Table 5 there is only one equation for the calculation of internal

heat gains for cooking. In order to derive the equations for different cooking systems to be used

in the CHM, we compared the SAP 2009 typical gains equation with the BREDEM 8 gains

equations:

• SAP 2009 - Typical gains: 35+7N

• BREDEM 8 - Gas hob and electric oven: 59.7+12N

• BREDEM 8 - Electric cooker: 48.5+9.7N


Here N is the number of occupants. It should be noted that SAP 2009 gives a lower gains than

BREDEM. This reflects the improved efficiency in cooking systems in the last decade, and

changes in household cooking trends. Working out the reduction factors by dividing the BREDEM

8 constants by SAP 2009 constants:

• For Gas hob and electric oven: 59.7/35=1.71; 12/7=1.71

• For Electric cooker: 48.5/35=1.39; 9.7/7=1.39

So the SAP 09 internal gain equation can be expressed as BREDEM 8 divided by 1.71 for gas

cooker and BREDEM 8 divided by 1.39 for electric cooker. We have applied the same reduction

factors in the cooking fuel calculations in the new model, which results in the constants you see

in the B Physics Parameters sheet:

• For gas hob and electric oven:

o BREDEM 8: 1.49+0.3N (gas) becomes 0.87+0.18N

o BREDEM 8: 0.85+0.17(electricity) becomes 0.50+0.10N

• For electric cooker:

o BREDEM 8: 1.70+0.34N becomes 1.22+0.24N

Furthermore, as the SAP methodology is a standardised approach for calculating the energy

performance of individual dwellings and not specifically intended for calculating household energy

use at a national (e.g. England) or UK level, a series of modifications have been made to the original

SAP worksheet calculations. The key changes are:

• Demand temperature and heating regimes – a default demand temperature of 19oC is assumed

for the living area for all dwellings, as opposed to 21oC in SAP. The rest of dwellings’ demand

temperatures are based on the SAP calculations, but using the 19oC living area demand

temperature. The SAP heating regimes have been retained. It is recognised that demand

temperatures and heating regimes vary considerably between households however there is

limited recent data on either of these parameters4. The 19

oC living area demand temperature

and the SAP heating regimes are used here as a simple proxy to typical user behaviour.

• Climate data - monthly external temperature (oC), monthly average wind speed (m/s), and

monthly average horizontal solar radiation (W/m2) - is applied at a regional level, as opposed to

the national level in SAP. England is divided into the nine old Government Office Regions (GORs).

• For monthly external temperatures, data is used for the specific year under consideration: in the

current version of the model 2009 external temperature data is used. This is as opposed to SAP

values, which are averaged over decades.

• The number of occupants in each representative dwelling is taken from the EHS data. This is as

opposed to the SAP methodology where a calculation is used to approximate the number of

4 There is some limited data suggesting an average living area demand temperature close to 19

oC: Kane, T.,

Firth, S.K., Allinson, D., Irvine, K.N., Lomas, K.J. 2011. Understanding occupant heating practices in UK

dwellings. World Renewables Energy Congress 2011; Energy End-Use Efficiency Issues.


occupants, based on dwelling floor area. However for representative dwellings identified as

having no occupants in the EHS, the CHM does use the SAP occupancy calculation.

• For representative dwellings identified as having no occupants in the EHS it is assumed that the

dwelling is vacant. In the 2009 EHS data this corresponds to more than 1 million vacant dwellings

in England. For the CHM calculation of national/GB total energy, the consumption for vacant

properties (based on the default SAP calculation for number of occupants) is reduced so that it is

only 10% of the originally calculated energy consumption value. This calculation is performed

post-processing, and is a simple approximation to account for reduced energy use in vacant

dwellings, second homes, etc.

• The levels of heat loss due to party walls and thermal bridge have been reduced to 25% of the

SAP calculated values for pre 2003 dwellings. This reflects the much greater significance of

thermal bridges and party walls in better-insulated modern homes, which has been incorporated

into the SAP calculation. It was felt that homes constructed before 2003, with poorer thermal

performance, are less sensitive to thermal bridges and party wall losses.

• Wall u-values for filled cavity walls have been amended from the original SAP Table S6 values so

that they are now 0.65 for homes built before 1983 (SAP dwelling age categories 1 to 6

inclusive). This reflects research commissioned by the Energy Saving Trust which found that

cavity wall insulation achieves an average of 38% less than the expected improvement in

thermal resistance (Doran & Carr, 2008).

It should be recognised that because of these differences any SAP-like ratings calculated in the CHM

are not equivalent to SAP ratings, which use a Standardised Assessment Procedure. To make this

clear, these calculated values are called ‘CHM values/ratings’ in the CHM building physics calculation

and output worksheets.

We welcome feedback on these changes to the SAP asssumptions.

In addition to the worksheets the model also contains three macros:

• Workbook_Open(): This generates the pop-up box that greets the user when first opening the

model, stating that this is the Cambridge Housing Model and showing the version number. The

version number is referenced from the worksheet Version History – see below for further

details.

• SimulateSingleHouse(): This is linked to the Enter button at the top of Building Physics Model,

and runs the calculation for a single case when the appropriate House Code is entered (note that

the code must be one of the codes listed in Housing Data). The subroutine sets the variable

SingleHouseCode in cell A2 of Housing Data, to be equal to the variable SingleHouseCodeInput

input by the user into the House Code box under the heading “Calculation for a single house

(enter House Code below)” at the top of Building Physics Model. This automatically populates

the remaining cells in row 2 of Housing Data. This range is named SingleHousingDataInput. The

corresponding range in row 1 of Housing Data is HousingDataInput and the macro sets

HousingDataInput to be equal to SingleHousingDataInput – that is row 1 of Housing Data is

populated with the input Housing Data for user-selected House Code. By default the Building

Physics calculations in the model are set to look to row 1 of Housing Data as the default input


data setting, therefore the model is automatically populated with the correct input data and the

calculations are automatically performed. Row 1 of B Physics Outputs automatically looks to the

appropriate cells in the calculation sheets for the model outputs. Therefore the outputs for the

single case selected can be found here.

• SimulateEntireStock(): This is linked to the Run Calculation button at the top of Building Physics

Model, and runs the calculation for all cases listed in Housing Data. The macro operates in a

similar way to SimulateSingleHouse(), except that here the macro must loop through all of the

House Codes in Housing Data. Starting at row 7 in Housing Data, this row is copied and pasted

into row 1. As before, the values in row 1 automatically populate the calculations within the

model, and the outputs are automatically generated and reported in row 1 of B Physics Outputs.

This macro now copies the information in row 1 of B Physics Outputs and pastes it into row 7 of

the same sheet. By looping through the full set of Housing Data in this way (next moving to row

8, then row 9 and so on until the final entry), outputs are generated for all cases and recorded in

B Physics Outputs. The macro also includes a “percentage complete” message in the Excel status

bar, showing the real-time percentage of the model run that is complete, and at the end of the

run outputs an information box stating the model run time. Finally the macro performs “sum

product” calculations to generate total energy use figures, adjusted for vacant dwellings.

4. Assumptions

The vast majority of assumptions used in the model are taken from SAP 2009 and are explicitly

expressed within the B Physics Parameters and B Physics Model worksheets. SAP 2009 should be

consulted for further details. However a number of additional assumptions are made in the CHM,

and these are stated in Data & Assumptions and are also presented in Annex C here.

Data & Assumptions

This sheet contains details of the data used in the model, any non-SAP 2009 calculations, and also

any assumptions that are not explicitly expressed within the model and/or referenced to SAP 2009

or related sources.

The bottom half of Data & Assumptions also contains a list of the named variables & ranges used in

the model. Named variables & ranges have been used in a number of places, rather than simple cell

referencing, because names can be used to aid the ease of understanding for the user. In addition

the list of named variables & ranges is hyperlinked to help identify what a specific name relates to.


5. Outputs

All model outputs are contained within the sheet B Physics Outputs, and are listed in Annex B here.

The primary outputs are energy consumption by use and by fuel type, and the associated CO2

emissions by use and by fuel type, for each of the cases. In order to generate totals for each subset

of the English population of dwellings associated with a single case, relevant values must be

multiplied by the dwelling weighting (which is an input in Housing Data but is also restated in B

Physics Outputs). The total energy use and CO2 emissions for England (say) is the sum of all of these

EHS weighted values. To approximate GB or UK energy use and emissions these figures can be

multiplied by appropriate England-to-GB or GB-to-UK scaling factors. The user must do scaling to GB

or UK totals.

Total national level energy use figures for England are calculated here, post-processing of the

individual representative dwelling calculations. This is achieved by applying weightings and

summing. Here these totals are adjusted to account for vacant dwellings, such that calculated energy

use figures for properties with zero occupants are re-scaled to 10% of their calculated value (where

calculated values are based on a SAP estimate of the number of occupants).

6. Administration

The remaining “administrative” worksheets are:

• About the Model: The model opens onto this sheet. There is a brief introductory description of

the model, a simple design flow diagram, hyperlinks to each of the worksheets, a more detailed

description and a colour key relating to inputs, calculations, assumptions and outputs in the

model.

• Version History: This is a list of the versions of the model that have been generated over time.

The user will note that the current version number is boxed, and is defined as the named

variable CHM_Version. This should always be the case for the current version, as it is this version

number that is used in the information box that pops up when the user first opens the model.

We suggest that to retain this protocol, if the user generates a new version of the model, a line

should be inserted above the “current” version, details of the “current “version copied to this

blank line, and the details of the newly current version of the model written over the top of the

information in the bottom line.

7. Post processing: Modelling Gap

The CHM is used to generate domestic energy consumption figures for GB and the UK, which are

reported in the HEFF and the associated ECUK domestic data tables. Here energy consumption is


reported by end-use and by fuel type. The Digest of UK Energy Statistics (DUKES) publishes country-

wide totals of actual energy consumption broken down by sector, including totals for domestic

energy consumption by fuel use. Therefore model outputs can be compared to the DUKES data.

In order to report results in the HEFF and ECUK data tables, it is desirable to align the modelled CHM

estimates with the measured DUKES data. To generate the breakdowns reported in the HEFF and

ECUK, CHM-to-DUKES adjustment factors are calculated at the fuel type level and applied to the

CHM estimates. In this way a breakdown of UK domestic energy consumption by fuel and end-use is

generated that aligns with the 2009 DUKES energy consumption figures by fuel.

8. Using the Model

When you first open the model you will be prompted to enable the macros, which you must do. You

will then see an information box informing you that this is the Cambridge Housing Model and telling

you the version – click OK. The model will now be open in About the Model.

If the model is already populated with appropriate data then you may simply proceed to the top of

Building Physics Model and run the model – either for all cases by pressing the button Run

Calculation, or for a single case by entering the appropriate House Code and pressing Enter.


If all cases are run the status bar of the model will show the model progress, and at the end a pop-up

box will advise the user of the run time. Results will be output to B Physics Outputs from row 7

onwards. For a single case the run time should be almost instantaneous and the results will be

output to row 1 of B Physics Outputs.

You should remember that all outputs (with the exception of the adjusted, total energy use figures)

relate to the relevant case only, and that outputs must be multiplied by the appropriate weightings

(also stated in B Physics Outputs) in order to generate outputs for the full population of dwellings.

If the model is not already populated with appropriate data then you must paste the correct data

into the relevant worksheets. Most obviously this involves Housing Data, from rows 7 onwards. Note


that the columns relate to the specific type of information that is required (e.g. House code,

weighting, dwelling type, age, etc.). Our Converter automatically generates data in the correct

format.

If other data is to be changed, e.g. Climate Data, you must overwrite the existing data, taking care

not to change anything else.

See earlier notes for details of other possible user inputs. However we reiterate that only the

informed user should make further changes to the data or assumptions currently used in the model.


Annex A – Housing Data

Below is a list of information contained within the Housing Data sheet. Each piece of information

relates to a column in the worksheet.

Housing Code

Number of Dwelling

SAP Age band

Tenure Type

Dwelling Type

Occupant - Adult

Occupant - Children

Region

Basement Area

Basement Storey Height

GF Area

GF Storey Height

1F Floor Area

1F Storey Height

2F Floor Area

2F Storey Height

3F Floor Area

3F Storey Height

Room in roof Area

Room in roof Storey Height

Chimneys - Main heating

Chimneys - Secondary heating

Chimneys - Other

Open flues - Main heating

Open flues - Secondary heating

Open flues - Other

Intermittent fans

Passive vents

Flueless gas fire

Structural Infiltration

Floor Infiltration

Draught Lobby

Windows and doors draught stripped

Sides sheltered

Ventilation System

Door Area

Door U-value

Windows 1 Type


Windows 1 Area

Windows 1 Frame

Windows 1 Overshading

Windows 1 Orientation

Windows 2 Type

Windows 2 Area

Windows 2 Frame

Windows 2 Overshading

Windows 2 Orientation

Roof Window Type

Roof Window Area

Roof Window Frame

Roof Window Orientation

Basement Floor Construction

Basement Floor Heat Loss Area

Basement Floor Exposed Perimeter

GF Construction

GF Heat Loss Area

GF Exposed Perimeter

Exposed Floor Construction

Exposed Floor Heat Loss Area

Living area fraction

Basement Wall Construction

Basement Wall Area

External Wall Construction

External Wall Area

Semi-exposed Wall Construction

Semi-exposed Wall Area

Roof Construction

Loft Insulation

Roof Area

Room in roof Construction

Room in roof Heat Loss Envelope Area

Party Wall Construction

Party Wall Area

Party Floor Construction

Party Floor Area

Party Ceiling Construction

Party Ceiling Area

Internal Wall Construction

Internal Wall Area

Internal Floor Construction

Internal Floor Area

Internal Ceiling Construction


Internal Ceiling Area

DHW System

DHW Boiler with Central Heating

DHW Electric System Type

DHW Electric System Tariff

DHW - Community Heating Tariff

DHW - Community Heating Fuel Type

DHW - Community Heating CHP Fraction

DHW - Community Heating CHP Fuel

DHW System Efficiency

DHW Cylinder Volume

Cylinder Insulation Type

Cylinder insulation Thickness

Primary Pipework Insulation

Cylinderstat

Solar DHW

Solar DHW in Cylinder

Solar DHW Storage

Main Heating System

Main Heating - Electric Tariff

Main Heating - Community Heating Tariff

Main Heating - Community Heating Fuel Type

Main Heating - Community Heating CHP Fraction

Main Heating - Community Heating CHP Fuel

Main Heating - Heater Flue

Main Heating - Oil Pump Location

Main Heating - Heat Emitter

Main Heating Efficiency

Main Heating Control - Programmer

Main Heating Control - Room Thermostat

Main Heating Control - TRVs

Secondary Heating System

Low Energy Lighting

EHS Age band

Wall Thickness


Annex B - Outputs

Below is a list of outputs contained within the B Physics Output sheet. Each output relates to a

column in the worksheet.

Housing Code

Dwelling Type

Number of Dwelling

Total Energy excluding Appliances & Cooking

Total Energy excluding Space Cooling

Energy Rate (CHM)

Total CO2 excluding Appliances & Cooking

Total CO2 excluding Space Cooling

CO2 Rate (CHM)

Total Primary Energy excluding Appliances & Cooking

Total Primary Energy excluding Space Cooling

CHM 2009 Rating

CHM 2009 Rating Band

CHM 2009 EI Rating

CHM 2009 EI Rating Band

Dwelling Heat Loss

Mean Internal Temp

Energy Consumption (kWh/year): Gas

Energy Consumption (kWh/year): Oil

Energy Consumption (kWh/year): Solid

Energy Consumption (kWh/year): Biomass

Energy Consumption (kWh/year): Electricity

Energy Consumption (kWh/year): Renewable

Energy Consumption (kWh/year): Space Heating - main

Energy Consumption (kWh/year): Space Heating - secondary

Energy Consumption (kWh/year): Water Heating

Energy Consumption (kWh/year): Space Cooling

Energy Consumption (kWh/year): Lighting

Energy Consumption (kWh/year): Electrical Appliances

Energy Consumption (kWh/year): Cooking

Energy Consumption (kWh/year): Pumps and fans

CO2 Emission (kgCO2/year): Gas

CO2 Emission (kgCO2/year): Oil

CO2 Emission (kgCO2/year): Solid

CO2 Emission (kgCO2/year): Biomass

CO2 Emission (kgCO2/year): Electricity

CO2 Emission (kgCO2/year): Space Heating - main

CO2 Emission (kgCO2/year): Space Heating - secondary


CO2 Emission (kgCO2/year): Water Heating

CO2 Emission (kgCO2/year): Space Cooling

CO2 Emission (kgCO2/year): Lighting

CO2 Emission (kgCO2/year): Electrical Appliances

CO2 Emission (kgCO2/year): Cooking

CO2 Emission (kgCO2/year): Pumps and fans


Annex C - Assumptions

Most assumptions used in the model are taken from SAP 2009 and are explicitly expressed within

the B Physics Parameters and B Physics Model worksheets. Readers should consult SAP 2009 for

further details. However a number of additional assumptions are made in the CHM and a number of

changes have been made from the original SAP formulations, all of which is stated in Data &

Assumptions and repeated here. All assumptions should be considered in the context of SAP 2009

and the worksheets in the CHM.

The user may note several references to a "CHM Rating" (short for "Cambridge Housing Model

Rating") or "CHM" energy or CO2 calculated values, in the titles in B Physics Output and in sections 9

- 13 inclusive in Building Physics Model. Here the CHM prefix is used to differentiate the value from

a corresponding SAP value. All such CHM values are calculated in a similar manner to the

corresponding SAP values, however there are some differences in our underlying Building Physics

calculations:

• Assume a default demand temperature of 19oC. It is recognised that demand temperatures and

heating regimes will vary between occupants and over time; this use of 19oC is a simple proxy for

realistic demand temperatures / heating regimes.

• Numbers of occupants are taken from the EHS data where this information is available; where

this is not available the SAP 2009 calculation is used.

• Where the EHS does not have a record of the number of occupants in a dwelling it is assumed

that the dwelling is vacant; in the 2009 EHS dataset there are more than 1 million vacant

dwellings. Here the energy consumption for vacant properties is reduced so that it is only 10% of

the originally calculated value. This calculation is performed post- processing and is only applied

to the total energy use calculations; reported outputs at the individual representative dwelling

level are for 100% of the originally calculated values.

• A regional breakdown of dwelling locations is used; this is specifically relevant in terms of

regional climate data.

• We use year-specific mean external temperatures; that is for, say, the 2009 EHS dataset the

associated external temperature data is also 2009.

• In Section 3 of Building Physics Model, "Heat Loss and Heat Loss Parameters", we have added

[29b] a "Semi-Exposed Wall" element and [30a] of a "Room in Roof" element.

• We have reduced the levels of heat loss due to party walls and thermal bridge for pre 2003

dwellings; dwelling from 2003 onwards remain unchanged, but for pre 2003 dwellings the party

wall heat loss is reduced to 25% of the calculated value.

• At DECC’s request, changed the Wall u-value for "Filled cavity / Cavity with insulation

(internal/external)" so that it is now 0.65 for representative dwelling of SAP dwelling age

categories 1 to 6 inclusive; this applies to the Scotland data table, as well as the England & Wales

table.

• We have added an additional wall category, "Metal Frame"; the parameters for this category are

based on the following assumptions: SAP ages 1-3: framed, single galvanised steel + cavity +

plasterboard; SAP ages 4-5: framed, single galvanised steel with 25mm EPS; SAP ages 6-8:

framed, single galvanised steel with 50mm EPS; this is based on Szokolay Steven V. (2004)

Introduction to architectural science: The basis of sustainable design. Oxford, UK: Architectural


Press, p.253 (Data sheet D.1.3); for SAP ages 9-11: data follows SAP Table S6 'System build (as

built)' (Type 12).

In addition we have used the following assumptions:

• After [8] it is stated "if pressurisation test has been carried out or is intended, proceed to [17],

otherwise continue from [9] to [16]." Here it is assumed that NO pressurisation test has

been/will be carried out, therefore [9] to [16] are calculated here. (Note: at some future point

when the year of construction is included in the EHS data, the calculation will be adapted to

include further assessment of homes built after 2005.)

• [17] relates to air permeability if a pressurisation test has been done, and value [18] relates to

[17]. However here we assume that no pressurisation test has been done, therefore [17] is

omitted, and [18] = [16] as stated in SAP 2009.

• [23b] is calculated “if exhaust air heat pump using Appendix N” (using equation [N4]). Here it is

assumed that this is NOT the case, therefore “otherwise” is assumed, and [23b] = [23a].

• [25] can be calculated “If Appendix Q applies in relation to air change rate, the effective air

change rate is calculated via Appendix Q instead”. Appendix Q “provides a method to enable the

SAP calculation to make use of the characteristics of technologies that are not included in the

published SAP specification. This procedure may only be used for technologies whose

characteristics have been independently assessed and which are described on the web page

www.bre.co.uk/sap2009.” It is assumed that this is NOT the case here, and so [25] Effective air

change rate is calculated using [24].

• In the calculation of [36], Thermal bridges, it is assumed that “details of thermal bridging are not

known”.

• For calculation [43], there is the option to “reduce the annual average hot water consumption

by 5% if the dwelling is designed to achieve a water use target of not more than 125 litres per

person per day (all water use, hot and cold).” Here, in B Physics Parameters we have included a

user-specification box which allows the user to change the response to this question between

“No” (NO 5% reduction) or “Yes”. The calculation is amended automatically when the user

makes this choice. Note that our default setting is “No”, and that furthermore this choice applies

to all dwellings in the model run.

• In the SAP 2009 worksheet, in the assessment of water storage loss ( [47] – [49] or [50] – [54])

we have assumed that the manufacturer’s declared loss factor is NOT known – and hence

calculate [50] – [54]. This further means that [55] is equal to [54].

• In the calculation of [51], if the DHW System is a Combi or there is no cylinder, the storage loss

factor [51] is assumed to be 0.

• In the calculation of [52], Volume factor, from Table 2a the calculation VF = (120/Vc)1/3 is used –

point (2) of the notes for the table – rather than a value from the table.


• In the calculation of value [53] using SAP 2009 Table 2b, the notes for Table 2b include the use of

a value to multiply the Temperature Factor by, for calculations involving (a), (b). Here we assume

(a): “Multiply Temperature Factor by 1.3 if a cylinder thermostat is absent.” However the user

has the ability to change this to (b) in the “Cylinder Thermostat Factor” box in our calculations

in the B Physics Parameters.

• In the calculation of [61] we assume the use of Table 3a, NOT 3b nor 3c; within Table 3a we

further assume that the instantaneous combi type is WITHOUT keep-hot facility.

• In the calculation of value [63], we use Appendix H NOT Appendix G.

• In the SAP 09 worksheet, in the calculation of value [63] we use default collector parameters

from Table H1 of SAP 2009, assuming the collector is "flat plate, glazed".

• In Climate Data, for the Solar Hot Water Collector Setting we have enabled a user input to

change the setting from amongst a selection in a drop-down box. However we have used a

default setting of Tilt: 30 deg, Orientation: South. This setting is used in the determination of the

“Ratio of Monthly Solar Radiation to Annual Average Solar Radiation”, also in Climate Data. This

is further used in the calculation of [63]. Note that this Solar Hot Water Collector setting applies

to all cases in a given model run.

• In the SAP 2009 Appendix H calculation [H6], (which we use to calculate [H7] in Building Physics

Model, which is further used to calculate [H17] and [63]), we have assumed the selection

“Modest: 20% - 60% of sky blocked by obstacles” from Table H4.

• In the SAP 2009 worksheet, relating to section 5 Internal gains, in Table 5 there is a choice: (A)

Typical gains or (B) Reduced gains. In B Physics Parameters we have enabled a user-selection for

this "Heat Gains Setting". Our default choice is (A) typical, however the user can switch this

selection. Note that this setting applies to all cases in a given model run.

• In the calculation of [70], using Table 5a: if Ventilation System Type = 2 we assume a specific fan

power = 0.8 litre/s, whereas if Ventilation System Type = 5 we assume specific fan power = 2

litre/s.

• The 9 regions used in the EHS are the 9 English Government Office Regions (GORs); in extending

our consideration to the SHCS we also consider 3 Scottish GORs.

• Where regional data has been mapped from the SAP 2009 regional breakdown to our GOR-

based regional breakdown, the mapping is as follows: Region 1 (North East) - Borders; Region 2

(Yorkshire and the Humber) - North East; Region 3 (North West) - West Pennines and North

West; Region 4 (East Midlands) - East Pennines; Region 5 (West Midlands) - Midland; Region 6

(South West) - South, South West and Severn; Region 7 (East of England) - East Anglia; Region 8

(South East) - Thames and South East; Region 9 (London) - Thames ; Region 10 (Western

Scotland) - West Scotland; Region 11 (Eastern Scotland)- East Scotland; Region 12 (Northern

Scotland) - North East Scotland & North West Scotland.


• In Climate Data, the Monthly Average Horizontal Solar Radiation (W/m2) data comes from

BREDEM-8 Table D13 (which uses the same regional breakdown as SAP 2009 - and has therefore

been mapped to our GOR-based breakdown as above).

• In the calculation of [74] to [82] inclusive, in SAP 2009 a calculation is made for each “applicable

orientation”. Here we have already identified the orientations for Window 1, Window 2, and the

Rooflight (in the input Housing Data) – therefore we just apply the calculation for these 3 cases.

Much of the orientation calculations is undertaken in Climate Data.

• For the calculation of [103] gains (space cooling), SAP 2009 states that for solar gains use the

applicable weather region based on Table 10, rather than the non-regional Table 6a. However in

both instances we use our own regional weather data as specified in Climate Data.

• For the calculation of [105], leading to [107], assume that the cooled area fraction is equal to the

living area fraction - which is a Housing Data input.

• For the calculation of [203] - [205]: Assume there is NO "second main system", therefore

[204]=[202] and [203]=[205]=[207]=[213]=0.

• In SAP 09, for the calculation of the adjusted values for [206] and [216], we have only considered

two scenarios for this adjustment: (i) Condensing boiler with under-floor heating: we assume

this is the case if the main heating system is gas, if the DHW system in NOT with the boiler, and if

there is under-floor heating; and (ii) No thermostat control of room temperature: we assume

this is the case if this is NOT a Community Heating System and there is NOT a room thermostat -

and for space heating if there is a boiler, and for DHW if it is a non-combi boiler. This is because

the other elements in Table 4c (load compensator, weather compensator…) are not covered by

our input data, and similarly for control system, we don't have information about boiler

interlock.

• For the calculation of [208], we use data from "Table_SecondaryHeatingEfficiency" in B Physics

Parameters; this data is based on Table 4a as follows: (2) for "Gas Fire" we assume the "Gas fire

or wall heater, balanced flue" natural gas option; (3) for "Gas Coal effect fire" we assume the

"Flush fitting live fuel effect gas fire, open fronted" NOT fan assisted, natural gas option; (5) for

"Open fire" we assume an average of the "open fire in grate" and "open fire with back boiler",

natural gas options.

• For the calculation of [303], the SAP 2009 worksheet gives you the opportunity to specify up to

four heat sources in addition to the community CHP. Here we have allowed for just a single

additional heat source, therefore our calculations go up to [303b] rather than [303e], and

similarly for up to [304b], [307b] and [310b].

• In SAP 09, for the calculation of values [305] and [305a] using

"Table_CommunitySystemChargingMethodFactor", we have used Table 4c, including inferring

some values: for 'room thermostat and TRVs' assume values for 'TRVs', and if

'programmer+thermostat+TRVs' assume values for 'programmer+TRVs'.

• For the calculation of [306], the values we use from Table 12c are an average of the distribution

factors for different heat distribution systems.


• For the calculation of [217], see our table "Table_MainHeatingEfficiencySummer" for the

efficiencies taken from SAP Table 4b.

• For the calculation of [230a] and [330a] using our "Table_VentilationSystemElectricity" using

Tables 4f and 4g, we have made the following assumptions: for Mechanical - positive input

ventilation from outside and Mechanical - whole house extract ventilation assume specific fan

power = 0.8 litre/s; for Mechanical - balanced whole house ventilation with or without heat

recovery assume specific fan power = 2 litre/s and air throughput = 0.5 ach.

• For the calculation of [230f], it is assumed that there is no keep-hot facility; therefore [230f]=0.

• For the calculation of [230g] and [330g], it is assumed that the solar water heating pump is

electrically powered.

• In our calculation of Cooking Use, after [232] and [332] in Building Physics Model, assume "Gas

hob and electric oven" if main heating/ DHW system uses gas, otherwise assume "Electric

cooker and oven'".

• We have not considered Energy saving/generation technologies SAP calculations [233] - [235]

inclusive, or [333] or [334]; subsequently we have also not considered the "Appendix Q items"

which are repeated throughout the SAP worksheet.

• For the calculation of [311], SAP states "If DHW by immersion or instantaneous heater within

dwelling" - we assume this is the case if this is NOT Community heating, and only if the DHW

system is an electric boiler or "other electric".

• In our table "Fuel Costs: Main Heating Gas, Oil and Solid Systems": our figures for Solid boiler

house coal / anthracite is an average of the constituent figures in Table 12; also our figures for a

Biomass boiler assume Wood pellets (bulk supply for main heating) from Table 12.

• In our table "Fuel Costs: Secondary Heating Gas, Oil and Solid Systems": our figures for open fire

assume Manufactured smokeless fuel from Table 12a.

• In SAP 09, for the calculation of value [247], for 'Other electric' systems (i.e. not single or dual

immersion), assume high rate fraction = 0.7 if 7-hour tariff and 0.4 if 10-hour tariff - based on

the data in Table 12a.

• In our table "Fuel Costs: Main Heating System Electricity Price" we assume the following, based

on SAP Table 12a: (i) for "Electric boiler", Off-peak tariff (Economy 7) assume a high rate fraction

of 0.9, Off-peak tariff (Economy 10) assume a high rate fraction of 0.5; (ii) for "Electric storage",

Off-peak tariff (Economy 7) assume a high rate fraction of 0.2, Off-peak tariff (Economy 10)

assume a high rate fraction of 0.5; (iii) for "Electric room heater", Off-peak tariff (Economy 7)

assume a high rate fraction of 1.0, Off-peak tariff (Economy 10) assume a high rate fraction of

0.5; (iv) for "Warm air - electric", "Ground source heat pump" and "Air source heat pump"

assume the standard tariff.

• For Space Cooling calculations it is assumed that space cooling is electric powered.

• For [361] CommunityHeatingCHP_ElectricalEfficiency, we assume a value of 30%.


• For [362] CommunityHeatingCHP_HeatEfficiency, we assume a value of 50%.

• Note that we assume that there is no CO2 emission associated with Community heating scheme:

Water heating by immersion heater or instantaneous heater [375].

• In Climate Data, the latitude and Monthly Average Horizontal Solar Radiation (W/m2) data for

Northern Scotland (here Region 12) is based on data from SAP Table 10 and BREDEM 8

respectively, assuming North East Scotland.

• Age Band Mapping: SAP age bands are used to lookup values in SAP tables, such as wall U values.

However several of the EHS age bands map onto 2 different SAP age bands, for example EHS age

band 4 covers the period 1919 to 1944 inclusive, whilst the SAP age bands 2 and 3 cover the

periods 1900 to 1929 and 1930 to 1949 inclusive, respectively. To identify the appropriate values

from SAP tables, in instances where the known EHS band relates to 2 SAP age bands, the

proportion of the EHS age band that lies within each of the corresponding 2 SAP age bands is

determined, based on the number of years in an individual EHS age band that falls into each of

the 2 SAP age bands. Values from the SAP tables corresponding to both SAP age bands are

identified, and a new value is calculated proportionally based on the two SAP values and the

proportions of the two SAP age bands relating to the single EHS age band. For example, for a

representative dwelling with EHS age band 4, and appropriate values from a SAP table of 1.0 for

SAP age band 2 and 2.0 for SAP age band 3, the used value is calculated as (0.4231 x 1.0) +

(0.5796 x 2.0). See table "AgeMapTable" in B Physics Parameters.

• For Appendix P: Assessment of Internal Temperature in Summer (these calculations are not

integral to SAP and do not affect the calculated SAP rating or CO2 emissions) we have enabled a

user-selection drop down menu in B Physics Parameters for Cross ventilation, Window opening,

Curtain and blinds, and Overhangs depth; but as a default we have selected choices of 2 - Not

Possible, 3 - Windows open half the time, 4 - Dark-coloured curtain or roller blind, and 1 - No

overhang shading, respectively.


Abbreviations

BRE Building Research Establishment

BREDEM Building Research Establishment Domestic Energy Model

BREHOMES Building Research Establishment Housing Model for Energy Studies

CAR Cambridge Architectural Research

CHM Cambridge Housing Model

DECC Department of Energy & Climate Change

DHW Domestic Hot Water

DUKES Digest of UK Energy Statistics

ECUK Energy Consumption in the UK

EHS English Housing Survey

GB Great Britain

GOR Government Office Region

HEFF Housing Energy Fact File

HQ Head Quarters

RdSAP Reduced SAP

SAP Standard Assessment Procedure

SHCS Scottish House Condition Survey

UK United Kingdom

VBA Visual Basic for Applications

the cambridge housing model guide v 2.7 171111

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