Low carbon heating in domestic buildings - technical feasibility: report

A report undertaken to assess the suitability of low carbon heating technologies in residential buildings in Scotland.

2 Technical feasibility of low carbon heating

The suitability of domestic low-carbon heating depends on the available heating technologies and on their compatibility with various characteristics of the homes in question. The following sections illustrate the low-carbon heating technologies considered in this study and factors influencing their suitability.

2.1 Low-carbon heating technologies

The range of low carbon heating systems considered in this study is summarised in Table 1, followed by a brief explanation of the technologies and the assumptions considered in this study.

Table 1: Low-carbon heating technologies considered in this study

Heat pumps

1 Air Source Heat Pump (ASHP)

2 Ground Source Heat Pump (GSHP)

3 High-temperature ASHP

4 High-temperature GSHP

5 Communal ASHP

Electric resistive heating

6 Electric storage heating

7 Direct electric heating

8 Electric boiler

Bioenergy boilers

9 Solid biomass boiler

10 BioLPG boiler

11 Bioliquid boiler (B100)

Low carbon gas

12 Hydrogen boiler

13 Biomethane grid injection

Hybrid heat pumps

14 Hybrid ASHP + gas boiler (no hot water cylinder)

15 Hybrid ASHP + gas boiler (with hot water cylinder)

16 Hybrid ASHP + bio-liquid boiler (no hot water cylinder)

17 Hybrid ASHP + bio-liquid boiler (with hot water cylinder)

18 Hybrid ASHP + hydrogen boiler (no hot water cylinder)

19 Hybrid ASHP + hydrogen boiler (with hot water cylinder)

20 Hybrid ASHP + direct electric heating (no hot water cylinder)

21 Hybrid ASHP + direct electric heating (with hot water cylinder)

Heat networks

22 District heating

Combinations with solar thermal

23 ASHP + solar thermal

24 Electric storage heating + solar thermal

25 Direct electric heating + solar thermal

26 Electric boiler + solar thermal

The installation of PV technologies, thermal storage or electrical storage were not modelled in our study, but are expected to impact costs, increasing capital cost but generally reducing operating costs of the heating system.

Heat pumps

Domestic heat pumps are central heating systems that can absorb heat from the outside of a building and transfer it to the inside by means of a refrigerant fluid. The main components of a heat pump are an evaporator, a compressor, a condenser and an expansion valve. The refrigerant is circulated from the condenser, where it extracts heat at low temperature from the outside source, to the compressor, where its temperature and pressure are increased. The refrigerant then flows though the condenser, where it releases heat at high temperature to the inside of the building. After circulating through the expansion valve its pressure is finally reduced and the cycle can restart. The energy requirement of a heat pumps corresponds roughly to the electrical power needed to compress and circulate the refrigerant. The performance of a heat pump is highly dependent on the temperature of the outside source and on the temperature at which heat is delivered through the wet system.

Technologies considered in this study are air source heat pumps (ASHPs), absorbing heat from the outside air, and ground source heat pumps (GSHPs), extracting heat from the ground either through a horizontal closed ground loop or a vertical closed ground loop. Both air source and ground source heat pumps operate at optimal performance when producing heating at low temperatures. For comparison, space heating can be delivered by conventional heat pumps with optimal efficiency at temperatures around 35-40°C, while gas boilers are designed to efficiently deliver space heating at temperatures from 60°C and up to 80-90°C. Therefore, the adoption of heat pump heating requires also the installation of emitters that are larger than those utilised for a gas central heating system, in order to ensure sufficient heat is transferred from the low-temperature circulating water to the heated spaces. Additionally, in homes where the pre-existent wet system is composed of very narrow pipes, the installation of conventional heat pumps may require also the installation of new wider pipes that are capable of delivering a high flow rate.

Additionally, a high-temperature heat pump is assumed to be capable of producing output temperatures of 65°C.[5] While its upfront cost is generally higher than for a conventional heat pump, a high-temperature heat pump can deliver space heating at a temperature closer to that of a gas boiler. As a consequence, high-temperature heat pumps are unlikely to require the installation of radiators larger than those used in gas boiler heating systems, as opposed to conventional heat pumps.

Finally, a communal ASHP system refers to a single ASHP unit delivering heat to multiple flats or terraced houses, assuming a network shared between 6 dwellings.

This study assumes that with the installation of heat pump technologies heating demand is met by the heat pump itself, while hot water demand is met by electric on-demand devices, such as electric taps.

Electric resistive heating

Direct electric heating involves the production of heat from electricity through a resistive element and its delivery via radiators, panel heaters or infrared heaters.

Panel heaters and electric radiators are convector heaters, as they heat the air directly and generate passive convection currents that transfer heat across a room. Infrared heaters, or radiant heaters, transfer heat predominantly via infrared radiation to the surfaces in a room, while the surrounding air is heated indirectly by the room's warm surfaces.[6]

For this study, direct electric heating is one of the investigated low-carbon heating options, but also one of the counterfactual heating technologies already present in Scottish homes. Only convector heaters are therefore considered for direct electric heating and radiant heaters are not included, as their use is not very common in Scottish homes. Electricity use of direct electric heating is assumed to occur during the day and is therefore subject to the higher tariffs of peak-time electricity.

Electric storage heating also produces heat from electricity though a resistive element, but typically occurs overnight, taking advantage of the lower electricity tariffs during off-peak times. The heat is absorbed and stored by high thermal mass bricks and later released during the day by a fan blowing air over the heated bricks. An independent heating unit is installed in each room.

Electric boilers produce heat from electricity and transfer it to water, delivering space heating though a wet heating system, either through radiators or through underfloor heaters. Additionally, the boiler may also produce hot water, when in combination with a hot water cylinder.

This study assumes that with the installation of electric resistive heating technologies hot water demand is met by electric on-demand devices, such as electric taps.

Bioenergy boilers

Bioenergy boilers operate in the same way as a conventional natural gas or LPG boiler, burning fuels to heat water in a wet heating system.

A solid biomass boiler can burn wood pellets, wood chips or logs to heat up water and deliver space heating via a wet heating system or produce hot water in combination with a hot water cylinder, similar to a conventional gas or electric boiler. Solid biomass requires a large availability of storage space, determined by the relatively low energy density of the fuel and by fuel delivery logistics.

A bioLPG boiler is not different from a conventional LPG boiler. Evidence suggests that biopropane can be used as a drop-in fuel in LPG boilers without the need of adaptation. The use of this technology requires the installation of a gas cylinder for the storage of bioLPG.

The use of bioliquid boiler (B100), burning 100% biodiesel was also investigated. While the overall configuration of a bioliquid boiler is similar to that of a standard oil boiler, bioliquid cannot be utilised as a drop-in fuel in existing oil boilers, unless it is utilised in a fuel blend (e.g. B30K, composed of 30% biodiesel and 70% kerosene).[7] An oil boiler utilising 100% biodiesel requires a few dedicated adaptations, such as an optimised design for the burner. Additionally, the installation of a preheated fuel tank may be required, as biodiesel must generally be stored at a temperature between 5˚C and 15˚C, to ensure it maintains a low viscosity[7] . Particular attention must also be paid to the compatibility of the materials used in the boiler, pipes and storage tank that come in contact with the biodiesel, as some have been reported to degrade more easily than when exposed to conventional diesel.[7]

The use of domestic resources for bioenergy in Scotland has the potential to more than double from the current value of 6.7 TWh per year to 14 TWh per year by 2030. However, there is strong market competition and practical constraints which limit the availability and suitability of certain feedstock types. This report has not taken into consideration the availability of bioenergy feedstocks.

Low-carbon gas boilers

Low-carbon gas boilers are heating devices burning low-carbon fuel delivered by the gas grid. The main options that are commonly considered for the decarbonisation of the gas grid are the use of hydrogen or biomethane, either to be used pure or to be blended with natural gas.

Hydrogen boilers investigated in this study are assumed to be burning 100% hydrogen. The technical challenges of burning hydrogen, compared with the combustion of natural gas, are related to a higher flame speed and the associated risk of light-back, as well as a higher creation of NOx and the higher risk of explosion of unburned gas.[8] The layout of the burner and other components of the hydrogen boiler are therefore adapted to accommodate these technical requirements.

Biomethane grid injection consists of blending a portion of biomethane into the gas grid. The type of heating technology required in the case of biomethane grid injection will depend on the future decarbonisation of the gas grid. In fact, partial or total decarbonisation could be achieved in future though the supply of a gas blend composed of hydrogen and natural gas in varying proportions. In the case of biomethane grid injection, a portion of biomethane would also be added to the blend. While blends with hydrogen concentration below 20 mol% are expected to be compatible with combustion in conventional gas boilers,[9] blends with higher hydrogen content would require the installation of a hydrogen boiler.

Hybrid heat pumps

Hybrid heat pumps are low-carbon heating systems that combine a heat pump with a different heating technology, thus integrating the low-carbon performance of a heat pump with the reliability of an additional heating unit as backup for the colder winter months. As heat pump efficiency depends on both the outside temperature and the temperature at which it delivers heat, the two technologies of a hybrid system are operated alternatively, choosing the technology that offers the highest efficiency and level of thermal comfort at a given time.[10]

The hybrid heat pump systems considered in this study combine an ASHP with either a gas boiler, bioliquid boiler, hydrogen boiler or direct electric heating. Their suitability was analysed both in combination with a hot water cylinder or standalone with no production of hot water. It is assumed that 80% of the annual space heating demand is met by the heat pump and the remaining 20% by the additional heating unit. Hot water demand is entirely met by the additional heating unit, except for hybrid heat pumps with direct electric heating, for which hot water demand is assumed to be met by electric on-demand devices, such as electric taps.

Heat networks

District heating networks deliver heat from a common energy source to a large number of homes through a pipe network. A low-carbon heat network can be operated using a range of technologies such as a heat pump, biomass boiler, or solar thermal unit, or by recovering waste heat from industrial processes.[11] Centrally generated hot water or steam is distributed through an underground pipe network and is delivered to a heat exchanger in each home to produce space heating and hot water on demand.

Solar thermal

Solar thermal collectors can be installed alongside various heating technologies to support the production of hot water. Considered technologies in this study are the combinations of solar thermal with ASHP, electric storage heating, direct electric heating and electric boilers, all requiring the connection to a hot water cylinder to supply hot water. For these combinations, it is assumed that 60% of hot water demand is delivered by the solar thermal system and the remaining 40% is met by the heating system.

2.2 Factors influencing suitability of low-carbon heating

2.2.1 Technical factors

The suitability of homes for the low-carbon heating technologies considered in this study is determined by a range of potential barriers.

Space constraints

Lack of internal space for the installation of large units or large hot water cylinders can affect the suitability for the installation adoption of heat pumps and other heating technologies associated with a hot water cylinder.

Scarce availability of external space can impact the suitability for installation of external components of the heating system, such as a horizontal ground loop for a ground-source heat pump, a gas cylinder for the storage of bioLPG, or a biofuel tank required by a bioliquid boiler. Additionally, it can constitute an obstacle to the implementation of biomass heating, which requires external space for the storage of the fuel. Finally, the lack of wall space for an external unit or a suitably orientated roof can influence the suitability of an air-source heat pump or solar thermal collectors.

Dwelling type

Communal heat pump systems are most cost-effective when installed in homes located close to each other, such as terraces and flats, due to the lower cost of piping and associated groundwork.

Heat demand

Peak heat demand and peak specific heat demand are two important parameters influencing the suitability of various low-carbon heating technologies. Peak heat demand is here defined as the maximum heat demand of a home at a given time, typically occurring on the coldest winter day and measured in W. This measures the amount of heat that must be supplied to a home to maintain thermal comfort. Peak specific heat demand is calculated as peak heat demand divided by the total floor area of the habitable rooms and is measured in W/m2. Large specific heat demand is generally associated with homes that are poorly insulated and/or located in cold climates. Large heat demand, on the other hand, can be a result of both large specific heat losses and of large dwelling size.

Heat pumps in dwellings with large peak specific heat demand (typically above 150 W/m2) are at risk of not meeting thermal comfort, as this requires the installation of very large radiators and/or the heat pump to produce space heating at a higher temperature - and reduced energy performance. The average peak specific heat demand across Scottish homes is 87 W/m2 and only ~1% of Scottish homes are estimated to currently have peak demand above 150 W/m2.

Additionally, a large peak heat demand may be unsuitable for any technology that generates heat from electricity, such as direct and storage electric heating, heat pumps and hybrids. Large peak heat demand of cold winter days may result in an electricity demand triggering the fuse limit of the building, rendering it unsuitable to electric heating technologies. While the efficiency of direct heating and storage heating is assumed to be 100%, the performance factor of heat pumps is expected to decrease with the external temperature. Therefore, while on cold winter days electricity demand will generally increase due to a larger space heating demand, in the case of heat pumps the electricity demand increment will be exacerbated by a reduced performance of the heating technology. In this study it was assumed that heat pump technologies will operate at average external temperature of ~8°C and minimum external temperature of -10°C.[12]

While there is not sufficient information available on the fuse limit of individual Scottish homes, it is assumed that typical values will lay in the range of 30A to 100A,[13] the latter being the maximum fuse rating available for a single-phase domestic connection. Load increases to up to 100A generally involve the replacement of the fuse alone and are associated with little to no cost, depending on the state of the connection cables and on the network operator (not exceeding a few hundred £[14] ). For load increases above 100A, an upgrade to a three-phase connection is also possible for individual dwellings. This is however associated with significant costs, expected to be of the order of a few thousand £, and may require several weeks to be completed, especially if a permit for digging the power cables is required from the local authority.[15] Where an upgrade of the fuse limit would result to be too costly or undesirable, an alternative option would be the installation of an electric battery, to support the supply of power to the heating device, or of a heat battery, to support the delivery of heat to the home alongside the heating system.

The implementation of heating technologies that rely on electricity may not only face suitability obstacles in certain homes but may also represent a burden for the distribution network. Additional costs for network reinforcement must be considered.

In this study, peak heat demand was estimated from the yearly heat demand, assuming peak heating load factor of 16%. In other words, peak heat demand was assumed to be the power that would be provided by the heating system if it were operating for about 3 hours and 50 min per day and delivering the yearly heat demand over the course of one year. Note that this assumption has a large impact on assessment of the number of homes that may be affected by peak heat demand constraints. In fact, a larger peak heating load factor, would result in smaller peak heat demand and therefore also a smaller number of homes in which the implementation of electric resistive heating or heat pumps may trigger the fuse limit or contribute to the risk of not meeting thermal comfort.

Coastal location

Air-source heat pumps located close to the sea are subject to a reduced lifetime, due to the accelerated corrosion of the heat exchanger caused by the salinity of air. Malfunction of the heat pump can be prevented by applying a coating on the heat exchanger; however, this adds to the capex costs of the appliance and may increase operational costs due to maintenance.

Geological characteristics

Local geological characteristics may influence the implementation of GSHP, impacting the suitability for the installation of a vertical ground loop.[16]

Gas grid and district heating network proximity

Dwellings located in areas away from the gas grid are not suitable for hydrogen boilers and biomethane grid injection. Similarly, the connection to a district heating network may not be available for homes in areas of low heat density.

Air quality restrictions

Restrictions on air quality may affect the suitability of fuel combustion appliances. In fact, biomass boilers are responsible for the emission of a substantially larger amount of particulate matter (PM2.5) per kWh of heat than gas boilers,[17] while high-temperature boiler systems such as hydrogen boilers may produce a high level of NOx emissions,[18] adversely affecting local air quality, typically most critical in urban areas. This factor was not included in our suitability assessment.

Noise pollution

Concerns around noise pollution may discourage the implementation of heat pumps, especially in densely populated urban areas, where multiple units may need to be installed in close proximity. This factor was not included in our suitability assessment.

Complementary measures

The implementation of certain low-carbon heating technologies may involve complementary measures, such as the installation of additional equipment, leading to additional costs and disruption for the occupants:

  • Installation or replacement of wet heating system, required when replacing electric resistive heating with low-carbon boilers or generally when installing a heat pump;
  • Installation of a hot water cylinder;
  • Local network reinforcement;
  • Replacement of cooking appliances, required when disconnecting from the gas grid;
  • Replacement of electrical wiring or gas pipework;
  • Installation of a fuel tank or biomass storage.

2.2.2 Heritage factors

Additional barriers need to be considered when assessing the suitability of low-carbon heating technologies and energy performance upgrades in heritage homes and in old dwellings (pre 1919).

Heritage homes are defined here to include both Listed buildings (Category A, B, C) and homes in Conservation areas, which are respectively buildings and areas of architectural or historic interest, benefiting from statutory protection under the Planning (Scotland) Act 1997. Planning consent is required to make changes to the external appearance and, for listed buildings, to the internal fixtures of these homes.

A recent study[19] by Element Energy for the Committee on Climate Change on hard to decarbonise homes included a high-level analysis of the technical suitability of low-carbon heating technologies and energy performance upgrade measures for heritage homes in the UK, providing both a qualitative and a quantitative appraisal.

Due to the complexity and case-by-case nature of the barriers to retrofit in heritage and old homes, as well as the high level of simplification that would be required to perform a quantitative analysis, this study will only provide a qualitative assessment of the suitability of Scottish heritage homes to low carbon heating and energy efficiency measures. This approach was supported by consultation with experts at Historic Environment Scotland.

Low-carbon heating technologies generally encounter fewer obstacles than energy efficiency measures in their implementation in heritage and old homes, as they require less disruption and integration of new materials. Following considerations on the barriers to implementation of low-carbon heating technologies and energy performance upgrade measures in heritage and old homes were provided by Historic Environment Scotland.

Barriers to low-carbon heating technologies

  • Solar thermal: Both aesthetics and technical aspects of the installation, such as the roof material, the weight of the collectors and the location of the pipes, can be an obstacle to suitability. Suitability will need to be assessed on a case-by-case basis through the application for planning permission, where required.
  • Bioliquid boiler: A potential barrier to the implementation of bioliquid boilers is represented by a potential limitation to the installation of a bioliquid tank outside a heritage home, due to aesthetics.
  • GSHP, district heating: Excavation works and laying pipes on heritage properties can raise complexities due to e.g. archaeological findings. Nevertheless, these obstacles have little impact on suitability of the technology and mainly increase the cost of the installation.
  • ASHP, hybrids: The main restrictions are related to the placement of the outdoor unit such that it does not affect the external appearance of the dwelling.
  • Less visible or more standard technologies (communal heating or boilers) are considered to be feasible in all buildings.

Barriers to energy performance upgrade measures

  • Wall insulation: The suitability of wall insulation is highly dependent on the wall configuration and on the type of insulating material used. Suitable insulating materials include wood fibreboard, hemp fibre and foam, while phenolic or plastic materials are commonly not accepted. Unfortunately, suitable wood fibreboard panels are often thin and less insulating than phenolic or plastic materials, and result in lower energy savings. It is additionally important to tailor the insulation solution to the wall structure, in order to prevent thermal bridges, to maintain weather-proofing of the external walls and to allow for air circulation to prevent condensation. External wall insulation, in particular, may require the extension of exterior elements, such as pipes and windowsills, in order to maintain the original appearance of the façade. Cavity insulation is generally not suitable, as cavities, where present, are often non-standard.
  • Window glazing: Secondary glazing is broadly preferred to the substitution of the existing windows with double glazing in listed buildings, as the characteristics and value of the original panels are not replicable.
  • Door insulation: Poorly insulated old doors should not be replaced with new doors but rather upgraded through draught proofing. This measure is in fact sufficient to significantly reduce heat losses and improve the energy performance of a home, while preserving the heritage value of the original door.
  • Roof insulation: Obstacles are associated with roof insulation are generally minor. For slate roofs, slate vents should be installed, in order to prevent condensation, as the board onto which slates are mounted will become colder after roof insulation. Alternatively, another available solution is over the roof insulation.
  • Ventilation: Ventilation measures are generally well tolerated in heritage and old homes.
  • Overheating prevention: Overheating prevention measures are rarely applicable to heritage homes, due to their high impact on the outer appearance of the dwelling.


Email: zeroemissionsheat@gov.scot

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