Low carbon heating in domestic buildings - technical feasibility: report

3 Construction of a model representative of Scotland's domestic building stock

In order to capture the distribution of hard to decarbonise features across Scotland's existing and future building stock, a stock model was developed. The model was produced based on a set of representative dwelling archetypes and on information about existing homes reported in the Home Analytics database.

The dwelling archetypes utilised in the stock model are defined by a list of attributes that most strongly determine the current and potential energy performance of a home, such as size, insulation type, age, existing heating system and baseline energy demand. Additional factors that directly influence the suitability of a dwelling for low-carbon heating technologies were also included, such as coastal location, space constraints and location on the gas grid or on heat networks.

After the creation of a set of dwelling archetypes, each defined by a unique combination of attribute values, all existing homes listed in the Home Analytics database were associated with the dwelling archetype that best represented their characteristics. The number of existing homes represented by each archetype could therefore be included in the model, quantifying the relevance of each archetype in the Scottish dwelling landscape.

The diagram below illustrates the methodology adopted for the development of a stock model representative of Scotland's housing stock.

1. Creation of a long list of dwelling attributes relevant for low-carbon heating suitability

2. Analysis of information available on Scotland's building stock

3. Finalisation of a dwelling attribute list and creation of dwelling archetypes

4. Mapping of real homes to the produced set of dwelling archetypes

3.1 Creation of a long list of attributes

Based on the thorough analysis of factors influencing suitability of dwellings for the considered low-carbon heating technologies as described in section 2.2, an initial long list of potential attributes was produced. A summary of all considered attributes, together with a description of how each attribute can influence dwelling suitability is summarised in Table 2.

Table 2: Investigated attributes that influence suitability for low-carbon heating

Potential attributes: Property type

Potential values: Detached house, Semi-detached house, Mid-terraced house, End-terraced house, Small block of flats/property converted into flats, Block of flats, Large block of flats, Flats in mixed use building

Influence on suitability of low carbon heating:

• Physical space constraints
• Availability of roof or outside wall space for heat pump unit or solar thermal array, land for a ground source heat array, storage for biomass/bioLPG etc.
• Flats can present challenges in coordinating works from all tenants
• High rise can be hard to treat, have restrictions on gas connection etc.

Potential attributes: Size

Potential values: Depending on floor area

Influence on suitability of low carbon heating:

• Physical space constraints for e.g. hot water storage cylinder, heat pump and hybrid heat pump units

Potential attributes: Age

Potential values: Depending on date of construction

Influence on suitability of low carbon heating:

• Age strongly correlated with aspects of construction type (e.g. solid walls, roof materials)
• Even if not listed or in conservation areas, some old/traditional buildings may be subject to similar limitations

Potential attributes: Wall type and insulation

Potential values: Solid, cavity, pre-fab, timber, stone, with various levels of insulation

Influence on suitability of low carbon heating:

• Hard to treat wall types do not have low-cost cavity wall insulation options available and/or pose technical difficulties or risks of poor performance
• Hard to fill cavities risk poor distribution of filling and resulting thermal bridges
• Unfillable cavities must be treated as solid walls, with more expensive external or internal wall insulation

Potential attributes: Roof type and insulation

Potential values: Flat, pitched, accessible/inaccessible (e.g. Mansard roofs) and others, with various insulation levels

Influence on suitability of low carbon heating:

• Homes without a loft space, or with an inaccessible loft space (including Mansard roofs) cannot install standard roof insulation

Potential attributes: Existing heating system

Potential values: Gas, Electric resistive, Electric storage, Oil boiler, Solid fuel, Community heating, and others

Influence on suitability of low carbon heating:

• Existing heating system impacts what additional works may be required on installing a low carbon heating system - for example, whether this will require a new heat distribution system, hot water cylinder, additional wiring (distributed electric heating)

Potential attributes: District heating potential

Potential values: Located in an area of high heat demand density or not

Influence on suitability of low carbon heating:

• Potential for a connection to a local low-carbon district heating network

Potential attributes: Gas network location

Potential values: On or off the gas grid

Influence on suitability of low carbon heating:

• Access (or not) to decarbonised gas through the gas grid

Potential attributes: Location (urban/rural)

Potential values: Various categorisations (e.g. Home Analytics has 8 categories): e.g. Large urban area, Remote small town, Accessible rural, etc.

Influence on suitability of low carbon heating:

• Bioenergy options less suitable in urban areas due to air quality concerns; more suitable in rural areas close to sustainable sources
• Influences likelihood of external space/land for a ground array, or fuel storage facilities

Potential attributes: Consumer type (tenure) and fuel poverty

Potential values: Owner-occupied, private rented, local authority, Probability of being in fuel poverty

Influence on suitability of low carbon heating:

• Consumer type affects the likelihood of uptake of energy efficiency measures, due to affordability, alignment of tenant and landlord priorities, coordination of works, and ease of regulation (e.g. legislation of standards of private rented homes)

Potential attributes: Space constraints

Potential values: Constrained, Not constrained

Influence on suitability of low carbon heating:

• Limited internal space restricts the choice of heating system to those without large units or hot water storage, and limits installation of internal wall insulation

Potential attributes: Orientation suitable for solar thermal

Potential values: North/East/West/South, depending on roof orientation

Influence on suitability of low carbon heating:

• Roof orientation impacts suitability of solar thermal collectors

Potential attributes: High exposure to wind and rain

Potential values: Yes, No

Influence on suitability of low carbon heating:

• Impacting suitability, effectiveness and cost of wall and roof insulation

Potential attributes: Coastal location

Potential values: Yes, No

Influence on suitability of low carbon heating:

• Proximity to the coast can reduce the durability of ASHP, as high air salinity accelerates corrosion of the heat exchanger.
• Malfunction of the heat pump can be prevented by applying a coating on the heat exchanger, resulting in added costs.

3.2 Analysis of information available on Scotland's building stock

Home Analytics Scotland is a dataset of energy efficiency variables and property characteristics for the entire Scottish housing stock. It was the primary dataset used to populate the attributes in Table 2 for Scotland's existing, domestic building stock. The data contained within the database is provided down to the address level and is available to the Scottish Government and local authorities in Scotland to assist in developing, targeting and delivering policies, schemes and programmes designed to improve energy efficiency, install renewable micro-generation technologies, and alleviate fuel poverty.

The Home Analytics dataset is a combination of actual values and modelled values. Actual values are obtained from a variety of sources, such as Energy Performance Certificate (EPC) records, Home Energy Efficiency Database (HEED) and Home Energy Efficiency Programmes for Scotland Area Based Schemes (HEEPS: ABS) installation records, EST's Home Energy Check (HEC) records, SGN gas meter data, OS AddressBase, OS MasterMap Topography layer and the Scottish Census.

In cases where a property record is not available for a particular variable, models are used to impute (i.e. predict) the value of the variable based on the other building attributes and energy efficiency characteristics of the property. The methodology used to produce the final Home Analytics database depends on the underlying data sources and the variables being modelled. Typically, variables in Home Analytics are imputed using one of four types of models: spatial, statistical, derived or apportioned.

Home Analytics Scotland encompasses a wide range of physical, geographical, energy efficiency and socio-demographic factors. From the attribute list in Table 2, Home Analytics provided data on property type, size, age, listed building and conservation status, wall type and insulation, roof type and insulation, primary fuel type, gas network location, urban/rural classification, tenure, fuel poverty, roof orientation and exposure to wind driven rain. Unfortunately, it could not provide data on roof type (e.g. pitched, flat), specific heating systems (e.g. electric resistive), heat network connection suitability, coastal location or space constraints.

3.3 Archetypes representing Scotland's domestic building stock

The archetypes utilised in the stock model are determined by a list of attributes that can assume a range of values. While the set of attributes is fixed for all archetypes, each archetype is uniquely identified by a different combination of attribute values.

The choice of attributes to be included in the archetype definition was based on the long list of relevant attributes influencing the suitability of low-carbon heating reported in Table 2 and on information provided by Home Analytics on the characteristics of the Scottish housing stock. Additional relevance was placed on keeping the number of archetype and value combinations considered in our modelling to a minimum, so to reduce the computational complexity of the modelling, while ensuring an appropriate coverage and representation of the real housing stock. An overview of the selected attributes and their associated values is provided in Table 3.

Table 3: Attributes selected for the creation of dwelling archetypes

Age

Values

• Pre-1919
• 1919-1991
• Post-1991

Notes

The age group 1919-1991 was not further disaggregated, as associated potential restrictions on renovation and materials are expected to be similar among this group.

Property type

Values

• Detached
• Semi-detached
• Terraced
• Flat (block)
• Flat (other)

Notes

'Semi-detached' includes both semi-detached houses and end-terraced houses, as these are expected to have similar heat demand per unit floor area, having the same number of external walls.

'Terraced' only includes mid-terraced houses.

'Flat (block)' includes homes in large blocks of flats, composed of >15 residential dwellings located within a single building.

'Flat (other)' includes homes in smaller blocks of flats, composed of up to 15 residential dwellings located within a single building.

Size

Values

• Small (< 66 m²)
• Medium (66 - 108 m²)
• Large (> 108 m²)

Notes

Information on total dwelling floor area per habitable room (m²) collected from Home Analytics was retained in our model and categorised in three bands.

Wall insulation

Values

• SWI - Solid wall insulated
• SWU - Solid wall uninsulated
• CWI - Cavity wall insulated
• CWU - Cavity wall uninsulated with low exposure to wind and rain
• CWU exposed - Cavity wall uninsulated with high exposure to wind and rain

Notes

For efficient management of the computational parameters, the attributes 'High exposure to wind and rain' and 'Wall type and insulation' were merged.

With reference to the parameters indicated in the BRE Report 262 ("Thermal insulation: avoiding risks", 2002), 'high exposure to wind and rain' corresponds to 'very severe' or 'severe' exposure. Similarly, 'low exposure to wind and rain' corresponds to 'sheltered' or 'moderate' exposure.

Roof insulation

Values

• <100 mm
• 100-250 mm
• >250 mm
• Room in roof
• No loft

Notes

Homes with non-habitable space in the loft were assigned a value according to the thickness of their roof insulation ('<100 mm', '100-250 mm' or '>250 mm').

Homes with habitable space in the loft were assigned the value 'Room in roof' for insulation of less than 200 mm, or the value '>250 mm' for insulation of more than 200 mm.

Homes with no access to the loft (e.g. flats) were assigned the value 'no loft'.

Existing heating system

Values

• Gas boiler
• Oil boiler
• Electric
• Other

Notes

'Oil boiler' heating systems include both oil and LPG boilers.

'Electric' heating systems include electric storage and direct electric.

'Other' heating systems include: biomass boilers, solid fuel boilers, communal heating and homes with no heating system.

The type of pre-existent heat distribution system was not considered a barrier to suitability of heating technologies.

Orientation suitable for solar thermal

Values

• Yes
• No

Notes

Homes with roofs facing South, South-West or South-East were assigned the value 'Yes'.

Location

Values

• Urban
• Rural

Notes

Classification based on Scottish Government Urban Rural 8-fold Classification 2016.^[20] Attribute used to test the suitability of GSHP only.

Coastal location

Values

• Yes
• No

Notes

Homes located less than 5 km from the coastline were assigned the value 'yes'.

Gas network location

Values

• On gas grid
• Off gas grid

Notes

'On gas grid' refers to homes located in an area supplied by the gas grid. This includes both homes that already have a connection and homes that do not.

'Off gas grid' refers to homes located away from the gas grid.

District heating potential

Values

• Yes
• No

Notes

Value 'Yes' was assigned to homes located in areas where current annual heat demand density is above a threshold of 40kWh/m²/year.

From this list, a set of ~140,000 different dwelling archetypes was produced, each representing a unique combination of all chosen attribute values. Each archetype constitutes therefore a simplified representation of a dwelling, equipped with a unique set of physical and geographical characteristics that are useful in the assessment of low-carbon heating suitability.

In order to calculate the stock of each dwelling archetype in the Scottish landscape, all 2.66 million existing homes in Scotland entered in the EST Home Analytics database were aggregated based on the attributes reported in Table 3. The number of existing homes represented by each archetype was therefore included in the model, providing information on the amount of real homes that may be associated with the same suitability constraints of a particular archetype.

A large number of the initial ~140,000 dwelling archetypes resulted to be associated to none of the existing homes. These 'empty' archetypes were therefore discarded and further analysis on the suitability of low-carbon heating was performed only on the remaining set of ~54,000 useful archetypes that resulted to represent existing homes. An overview of the 20 most common dwelling archetypes on the gas grid and off the gas grid is reported in Table 4 and Table 5 respectively.

Table 4: Characteristics of the 20 most common archetypes on the gas grid

Archetype ranking	Property Type	Size	Age	Wall Insulation	Roof Insulation (mm)	Heating system	Solar orientation	Coastal location	DH potential	Stock
1	Semi detached	Medium	1919 to 1991	CWI	More than 250	Gas boiler	Yes	No	No	34,973
2	Semi detached	Medium	1919 to 1991	CWI	More than 250	Gas boiler	Yes	Yes	No	27,196
3	Semi detached	Medium	1919 to 1991	CWI	100 to 250	Gas boiler	Yes	No	No	17,189
4	Flat (other)	Small	Pre 1919	SWU	None	Gas boiler	Yes	Yes	Yes	16,463
5	Semi detached	Medium	1919 to 1991	CWI	100 to 250	Gas boiler	Yes	Yes	No	13,083
6	Terraced	Medium	1919 to 1991	CWI	More than 250	Gas boiler	Yes	No	No	11,725
7	Flat (other)	Medium	1919 to 1991	CWI	None	Gas boiler	Yes	Yes	No	11,285
8	Flat (other)	Medium	1919 to 1991	CWI	None	Gas boiler	Yes	Yes	Yes	11,091
9	Flat (other)	Medium	Pre 1919	SWU	None	Gas boiler	Yes	Yes	Yes	10,788
10	Flat (other)	Medium	1919 to 1991	CWI	None	Gas boiler	Yes	No	No	10,740
11	Semi detached	Medium	1919 to 1991	CWI	More than 250	Gas boiler	No	No	No	10,563
12	Flat (other)	Small	Pre 1919	SWU	None	Gas boiler	No	Yes	Yes	10,430
13	Detached	Large	Post 1991	SWI	More than 250	Gas boiler	Yes	No	No	9,606
14	Flat (other)	Small	1919 to 1991	CWI	None	Gas boiler	Yes	Yes	Yes	9,445
15	Semi detached	Medium	1919 to 1991	CWI	More than 250	Gas boiler	No	Yes	No	8,769
16	Flat (other)	Small	1919 to 1991	CWI	None	Gas boiler	Yes	Yes	No	8,498
17	Flat (other)	Small	1919 to 1991	CWI	None	Gas boiler	Yes	No	No	8,383
18	Terraced	Medium	1919 to 1991	CWI	More than 250	Gas boiler	Yes	Yes	No	8,337
19	Semi detached	Medium	1919 to 1991	CWI	More than 250	Gas boiler	Yes	No	Yes	7,862
20	Semi detached	Medium	1919 to 1991	CWU exposed	More than 250	Gas boiler	Yes	No	No	7,588

Table 5: Characteristics of the 20 most common archetypes off the gas grid

Archetype ranking	Property Type	Size	Age	Wall Insulation	Roof Insulation (mm)	Heating system	Solar orientation	Coastal location	DH potential	Stock
1	Detached	Large	Pre 1919	SWU	Room in roof	Oil boiler	Yes	No	No	9,075
2	Semi detached	Medium	1919 to 1991	CWI	More than 250	Electric storage	Yes	No	No	3,937
3	Detached	Large	Post 1991	SWI	Room in roof	Oil boiler	Yes	No	No	3,780
4	Detached	Large	Pre 1919	SWU	Room in roof	Oil boiler	No	No	No	3,162
5	Detached	Large	Post 1991	SWI	More than 250	Oil boiler	Yes	No	No	3,160
6	Detached	Large	Pre 1919	SWU	More than 250	Oil boiler	Yes	No	No	3,089
7	Semi detached	Medium	1919 to 1991	CWI	More than 250	Electric storage	Yes	Yes	No	2,917
8	Detached	Large	1919 to 1991	SWU	More than 250	Oil boiler	Yes	No	No	2,267
9	Semi detached	Medium	1919 to 1991	CWI	100 to 250	Electric storage	Yes	No	No	1,821
10	Detached	Large	Pre 1919	SWU	Room in roof	Oil boiler	Yes	Yes	No	1,815
11	Detached	Large	Pre 1919	SWU	100 to 250	Oil boiler	Yes	No	No	1,784
12	Flat (blocks)	Medium	1919 to 1991	SWI	None	Electric storage	Yes	Yes	Yes	1,757
13	Detached	Large	1919 to 1991	SWU	100 to 250	Oil boiler	Yes	No	No	1,739
14	Terraced	Medium	1919 to 1991	CWI	More than 250	Electric storage	Yes	No	No	1,709
15	Detached	Large	Post 1991	SWI	More than 250	Oil boiler	No	No	No	1,691
16	Detached	Large	Pre 1919	SWU	More than 250	Oil boiler	No	No	No	1,680
17	Flat (blocks)	Small	1919 to 1991	SWI	None	Electric storage	Yes	Yes	Yes	1,649
18	Semi detached	Medium	1919 to 1991	CWI	100 to 250	Electric storage	Yes	Yes	No	1,637
19	Detached	Large	1919 to 1991	SWU	Room in roof	Oil boiler	Yes	No	No	1,598
20	Detached	Large	Post 1991	SWI	Room in roof	Oil boiler	No	No	No	1,581

3.4 Scotland's housing stock in 2017

3.4.1 Calibration of total number of homes

While Home Analytics counts ~2.66 million homes in its database, figures by Scottish Government report ~2.46 million households in Scotland in 2017.^[21] The difference in the total figures is likely due to differences in the definition of a dwelling and likely aggregation of multiple units.

In order to render the stock model compatible with the official figures by Scottish Government, a calibration was performed by applying a multiplication factor to the number of homes assigned to each dwelling archetype, so that the total number of homes included in the model would add to 2.46 million.

3.4.2 Results

An initial useful output of the stock model is a representation of the characteristics of the current Scottish housing stock, based on a set of interesting attributes. Figure 1 reports the breakdown of homes in the stock according to their age, property type, size, wall insulation, roof insulation, existing heating system and heritage status. The assessment of the current state of the Scottish housing stock is based on information available for the year 2017.

Figure description:

Vertical bar chart showing that Scotland’s housing stock in 2017 consisted of approximately 2.46 million homes, which are broken down: by age (479,000 were built pre-1919, 1,521,000 between 1919 to 1991 and 460,000 post 1991); by property type (151,000 are flats (blocks), 845,000 flats (other), 246,000 terraced, 697,000 semi-detached and 521,000 detached); by size (625,000 are small, 1,225,000 medium and 609,000 large-sized); by amount of wall insulation, by amount of roof insulation, by heating system (1,843,000 have a gas boiler, 184,000 oil boiler, 340,000 electric storage heating and 93,000 other heating systems); and by whether they have designated heritage status.

3.5 Scotland's housing stock in 2040

The suitability of low-carbon heating technologies within the Scottish housing stock will change over time as homes become more energy efficient.

The Energy Efficient Scotland Route Map, published in May 2018, sets a target for all owner-occupied homes to reach EPC band C by 2040, where technically feasible and cost effective. Energy efficiency measures that will therefore need to be implemented between 2017 and 2040 in homes with lower energy efficiency performance include measures that will modify some of the characteristics of a home. In particular, modifications that are relevant for this study include the type of insulation and annual space heating demand.

In order to assess the influence of the implementation of expected energy upgrade measures on the characteristics of the existing building stock in 2040, EST's Portfolio Energy Assessment Tool (PEAT) was used to forecast future changes. A detailed description of the analysis performed with PEAT is reported in the appendix.

The influence of the implementation of energy efficiency measures considered by PEAT on the characterisation of the housing stock primarily affects two of the modelled attributes: wall insulation and roof insulation. Other attributes are more tied to the characteristics of the home structure and can therefore not be modified. Figure 2 reports the breakdown of homes in the stock in terms of the types of wall insulation and roof insulation in 2040 and compares these with the corresponding figures for 2017, before the implementation of the energy efficiency upgrade measures.

Figure description:

A horizontal bar chart shows the expected upgrade in wall and roof insulation in Scotland’s 2.46 million homes between the years 2017 and 2040. The bar chart to the left breaks down wall insulation and shows that SWI homes will increase from 494,000 in 2017 to 1,124,000 in 2040, 714,000 SWU will decrease to 83,000, 175,000 CWU will decrease to 27,000, 232,000 CWU exposed will decrease to 36,000 and 846,000 CWI will increase to 1,190. The bar chart to the left breaks down roof insulation and shows that 98,000 homes with less than 100 mm in 2017 will decrease to 2,000 in 2040, 536,000 between 100 to 250 mm will decrease to 53,000, 874,000 with more than 250 mm will increase to 1,698,000, 295,000 with room in roof will decrease to 49,000 and 658,000 homes with no loft will remain the same.

The number of homes with wall insulation (Solid Wall Insulated or Cavity Wall Insulated) is expected to increase from 54% of the stock in 2017 to 94% in 2040, with nearly two thirds of wall insulation upgrade interventions performed on solid wall homes.

While 27% of dwellings in Scotland are not connected with their building's roof and therefore require no roof insulation (value "no loft"), the number of homes with roof insulation of less than 250 mm thickness or with poorly insulated room-in-roof is expected to reduce from 38% of the stock in 2017 to 4% in 2040.

3.6 Technical suitability of selected low-carbon heating technologies

In order to assess which dwelling archetypes of the stock model are suitable for the investigated heating technologies, assumptions were made around the compatibility of the technologies with the dwelling characteristics described by the archetype attributes. An overview of these assumptions is reported in Table 6. Findings around the suitability of each archetype were later utilised to assess the suitability of Scotland's building stock in 2017 and 2040.

Table 6: Technical suitability matrix

Heating systems		Heat demand	Dwelling type					Space constraint		District heating		Solar thermal		Coastal location		Gas network
			Detached	Semi-detached	Mid-terrace	Flat (block)	Flat (other)	Yes	No	Yes	No	Yes	No	Yes	No	On gas grid	Off gas grid
1	ASHP	(1)	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	Y	(4)	Y	Y	Y
2	GSHP	(1)	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	Y	Y	Y	Y	Y
3	High-temperature ASHP	(2)	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	Y	(4)	Y	Y	Y
4	High-temperature GSHP	(2)	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	Y	Y	Y	Y	Y
5	Communal ASHP	(1)	N	N	Y	Y	Y	Y	Y	Y	Y	Y	Y	(4)	Y	Y	Y
6	Electric storage heating	(2)	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y
7	Direct electric heating	(2)	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y
8	Electric boiler	(2)	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y
9	Solid biomass boiler	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y
10	BioLPG boiler	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	N	Y
11	Bioliquid boiler B100	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	N	Y
12	Hydrogen boiler	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	N
13	Biomethane grid injection	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	N
14	Hybrid HP + gas	(2)	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	Y	(4)	Y	Y	N
15	Hybrid HP + gas (hwc)	(2)	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	Y	(4)	Y	Y	N
16	Hybrid HP + bioliquid	(2)	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	Y	(4)	Y	N	Y
17	Hybrid HP + bioliquid (hwc)	(2)	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	Y	(4)	Y	N	Y
18	Hybrid HP + hydrogen	(2)	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	Y	(4)	Y	Y	N
19	Hybrid HP + hydrogen (hwc)	(2)	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	Y	(4)	Y	Y	N
20	Hybrid HP + direct el.	(2)	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	Y	(4)	Y	Y	Y
21	Hybrid HP + direct el. (hwc)	(2)	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	Y	(4)	Y	Y	Y
22	District Heating	Y	Y	Y	Y	Y	Y	Y	Y	(3)	N	Y	Y	Y	Y	Y	Y
23	ASHP + solar	(1)	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	N	(4)	Y	Y	Y
24	Electric storage + solar	(2)	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	N	Y	Y	Y	Y
25	Direct electric + solar	(2)	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	N	Y	Y	Y	Y
26	Electric boiler + solar	(2)	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	N	Y	Y	Y	Y

Legend:

Y - Suitable for all dwellings

(…) - Suitable for some dwellings under specified conditions. See numbers below.

N - Unsuitable for all dwellings

(1) Up to 150 W/m² peak demand or fuse limit

(2) Fuse limit

(3) 20% of dwellings in an area with a heat network assumed not to connect

(4) Added cost to account for the need for anti-corrosive coatings

Heat demand - Specific heat loss

Homes with specific peak heat loss rate above 150 W/m² were considered unsuitable for the use of conventional heat pumps, as the heat delivered even by large emitters would unlikely meet the heating demand, due to the low temperature at which space heating is provided.

Heat demand - Fuse limit

The installation of electric resistive heating or heat pumps was considered unsuitable in homes with large heat demand, if resulting in peak electrical power consumption exceeding the maximum fuse limit of 100A. Due to the uncertainty of the actual fuse limit values of individual Scottish homes, a sensitivity analysis over a range of potential values was performed. Fuse rating values chosen for the sensitivity analysis were 60A, 80A and 100A. While typical values of fuse rating for single-phase domestic connections in Scotland range from 30A to 100A, modern cut-outs are assumed to have a fuse rating of 60A or higher,^[22] which can be upgraded to up to 100A typically at no cost for the homeowner. Fuse rating values smaller than 60A were considered not sufficiently interesting for the sensitivity analysis, as these are expected to be found in a limited portion of the domestic building stock.

Dwelling type

Communal ASHP were assumed to be unsuitable for detached, semi-detached and end-terrace homes. All other types of dwellings were considered suitable, due to the reduced length of pipes needed for the connections and thus higher cost-effectiveness.

The dwelling type was also utilised to estimate the overall suitability of homes for the installation of GSHP and high-temperature GSHP. In addition to the space, peak specific heat demand and fuse limit constraints, the suitability of a dwelling for GSHP and high-temperature GSHP is also influenced by the geology of the location and on the availability of outside space. In particular, the installation of a vertical ground loop may be impacted by local geological characteristics^[23] whereas the installation of horizontal ground loop arrays is predominantly influenced by the availability of sufficient outside area, roughly estimated to be around twice the surface area of the dwelling.^[24] Given the lack of information on the geological characteristics of the locations of homes provided by Home Analytics, an overall assessment of the suitability of GSHP and high-temperature GSHP was performed, solely on the basis of the building type and the location in urban or rural areas.

Detached homes are more likely than other dwelling types to have the available outdoor space for the installation of a horizontal ground loop. Additionally, outdoor space availability is more likely to be scarce for the same set of dwelling types in urban areas, when compared to rural areas. A similar approach was taken when considering the installation of a vertical ground loop, as the access and operation of heavy machinery required for the drilling is incompatible with a small garden, if present. An overview of our assumptions on GSHP suitability is summarised in Table 7.

Table 7: GSHP and high-temperature GSHP suitability

Dwelling type	Dwelling type suitability	Scotland's total stock	Suitable stock
Urban - Detached homes	10%	222,000	22,000
Urban - Terraced or semi-detached homes	5%	639,000	32,000
Urban - Flats	2%	825,000	17,000
Rural - Detached homes	78%	299,000	233,000
Rural - Terraced or semi-detached homes	56%	305,000	171,000
Rural - Flats	56%	170,000	95,000

Suitability assumptions for rural areas are in accordance with the '"high" technical assumptions' scenario of suitability for the RHI Phase II.^[25]

Space constraint

In order to identify dwellings in which space is constrained, total dwelling floor area per habitable room was calculated. Homes with total dwelling floor area per habitable room smaller than 18m² were classified as space constrained, in line with the assumptions of the recent work for the CCC on 'Hard to Decarbonise Homes'.^[26] These dwellings were assumed to be unsuitable for the installation of conventional, high-temperature and hybrid heat pumps, due to their additional requirement of a large hot water cylinder for the production of hot water.

District heating

The potential availability of district heating for Scottish homes was assessed on the basis of local heating demand, assuming that district heating networks will be put in place if not already existing in areas where demand for heat is sufficiently high and concentrated. District heating potential was attributed to all homes located in areas in which current annual heat demand density is above a threshold of 40kWh/m²/year. Heat demand density was calculated both at Data Zone level and on each square of a grid with 1 km² resolution, with either of the two tests being sufficient to qualify a home for district heating potential. Data Zones offer good spacial resolution in urban areas, but span over much larger areas outside of cities. The additional assessment of heat density over the 1 km² mesh is therefore useful to identify rural areas of localized high heat density that would otherwise not have emerged from the datazone analysis. From this analysis, 32.5% of the Scottish housing stock results compatible with the connection to a potential district heating network. The same analysis performed exclusively on data zones would have produced a smaller compatibility of 27.3% of the stock.

Finally, 80% of homes for which a connection to district heating is possible were assumed to be suitable for district heating, based on expected connection rates and prospective deployment of heat networks in areas with high heat density in line with the assumptions of the recent work for the CCC on 'Hard to Decarbonise Homes'.^[26]

Solar thermal

The suitability of a dwelling for solar thermal was based on the orientation of its roof, and suitability was assigned to homes with roofs facing South, South-West or South-East.

Coastal location

The relative distance from the coast was calculated for each home based on its geographical location. Homes located less than 5 km from the coastline were assigned 'Coastal location' value 'yes'.

While the coastal location of a home alone was not considered a sufficient barrier to the suitability of any technology, additional costs were assumed for the use of air-source heat pumps, due to the necessary measures required to prevent accelerated corrosion of the heat exchanger. Additional costs are expected to arise from the capex premium for the installation of a 'coastal' model of heat pump or from the application of a corrosion protection coating on the heat exchanger of a conventional model.

Gas Network location

Homes located outside of areas supplied by the gas grid were considered unsuitable for the adoption of hydrogen boilers, biomethane grid injection, and any of their combinations with heat pumps in a hybrid heating system, as these technologies rely on the fuel being delivered by the gas grid.

On the other hand, homes located in areas supplied by the gas grid were assumed to be incompatible with bioLPG and bioliquid boilers. However, no suitability restriction was posed on biomass boilers, which are assumed to be compatible with both homes on and away from the gas grid.

Current advice by the Committee on Climate Change (CCC)^[27] discourages the extensive use of bioenergy for domestic heating, especially in homes where alternative heating solutions are available. Given the similarities in technology and operations of both bioLPG and bioliquid boilers with hydrogen boilers, it was assumed that hydrogen boilers would be preferred to bioLPG or bioliquid boilers in homes located on the gas grid. However, due to the technical differences between biomass boilers and hydrogen boilers, it was assumed that biomass boilers could represent a valid alternative to hydrogen boilers also in some homes located on the gas grid.