Factsheet: Low & Zero Carbon Technologies

A closed-loop ground source heat pump system is used to extract heat from the ground for the purpose of space heating and hot water. The system comprises a ground-buried pipework system to transfer heat into a fluid (water), which is piped to a heat pump that upgrades the heat to a temperature suitable for use in the building. The energy transferred from the ground to the heating system by the heat pump is typically many times higher than the electrical energy required by the heat pump, so the imported energy and associated carbon emissions for heating energy generation are substantially reduced relative to gas-fired heating or direct electric.

The closed loop in the ground is arranged either vertically as a series of boreholes, spaced typically 5-10m apart, or horizontally as a series of trenches up to 2m deep, typically 5-10m apart. Horizontal arrays take up more external space but are typically cheaper than a vertical array achieving equivalent output.

The ground-source array is typically arranged on manifolds that are piped back to the ground-source heat pumps located internally. The system would also include circulation pumps and a thermal energy store to store heat and avoid over-cycling of the heat pump.

Traditionally, heat pump systems have been limited to low-grade heat generation (up to about 50°C). This is better suited to buildings with underfloor heating systems or modern, well-insulated buildings where heating demands throughout the year can be met with lower temperature heating. Systems are now available that can achieve higher temperatures of up to about 60°C, which use CO2 as the refrigerant and have lower embodied carbon than conventional units. These are appropriate for domestic hot water generation or the space heating of existing buildings which require higher heating flow temperatures. These systems still operate more efficiently when generating lower grade heat.

An open-loop ground-source system operates on the same principles as closed-loop, although rather than circulating water within the ground, groundwater is abstracted from the ground from which heat is removed prior to discharge either back to the ground or at surface level.

Groundwater typically maintains a temperature range  of 10-14°C throughout the year,  so it can be cooled comfortably by about 5°C (to remove heat) without much impact. Open-loop systems can also be used in reverse for cooling in the summertime, which is often beneficial to ‘recharge’ the groundwater temperatures.

As the groundwater is already at the temperature required, less overall ground contact is required in an open loop system relative to a closed loop system of equivalent output. For example, two wells of 100m depth abstracting and recharging 5 l/s of groundwater could provide about 100kW of heat output, whereas two vertical loop boreholes of equivalent depth would provide up to about 10kW of heat output.

An open loop system is far more sensitive to the local ground conditions however, particularly the presence and flow of groundwater at an adequate depth beneath the site. Open-loop boreholes require careful hydrogeological consideration and associated construction to ensure that they are engineered appropriately. As they impact groundwater conditions (even when recharging afterwards), they are subject to licensing from the Environment Agency and test records need to be provided as part of the licensing process.

Open-loop groundwater systems comprise the abstraction and recharge (if required) boreholes with associated pumps and well head chambers. The groundwater is passed across filtration equipment (to avoid silting) and a plate heat exchanger from which heat is transferred to the heat pump system. The heat pump principles are then like the closed loop. As with closed loop, the systems are more efficient for low-grade heat generation although more recent CO2 heat pumps can achieve higher temperatures.

Learn more here: The viability of Ground Source Heat Pumps in Historic Buildings.

Surface water source heat pumps allow the harnessing of renewable energy from the sea, rivers, canals, and lakes. They represent a large opportunity to deliver low carbon heating and cooling to buildings.

Surface water source heat pumps work by extracting low grade heat from the water and converting it to useful temperature heat to use in buildings. The reverse process can also be used for cooling in the summer, where heat is taken from the building and absorbed into the water. Systems where heating and cooling are provided simultaneously are also possible.

SWSHPs can be sub divided into closed and open loop schemes.

In an open-loop system, water is extracted from the source and then returned at a different temperature. This type of technology is flexible as abstraction rates can be adjusted to match demand.

In an open-loop system there is potential of debris and corrosion in the pipework and heat exchangers hence anti corrosion measures and filtration is required. Additionally, the separation distance between the abstraction and discharge points needs to be sufficient to avoid short circuiting of the system.

An open-loop system needs a licence issued by the Environment Agency for abstraction if quantities exceed 20m3 per day as well as a separate licence for discharge to a water source.

Closed loop systems work by circulation a refrigerant through a heat exchanger to absorb heat from the water source. Unlike an open loop system, no filtration is necessary hence maintenance requirements are lower, although the loop positions need to be considered to ensure water courses/bodies are not affected. No extraction licence is required for closed loop schemes.

The Environment agency would need to be consulted for any of the systems with regards to the access to the water source, but a licence is not required for closed source systems.

Air-source heat pumps follow a similar principle to ground-source or water-source heat pumps except heat is extracted from the atmospheric air rather than the ground or water.

As the system must be exposed to the air it can have greater space requirements above ground, with consequential visual and acoustic impacts relative to ground systems. Also, the system is reliant on the temperature of the atmospheric air, which is far less stable than that of the ground or surface water and is naturally very low during the heating season. Due to this, air-source heat pumps have a lower efficiency than ground or water-source heat pump systems.

As with ground-source, the systems are more efficient when generating low-grade heat only, although systems have been introduced recently that can generate higher temperatures up to 80oC. These use CO2 as the refrigerant and could be suitable for older buildings with a more limited retrofit.

The system comprises fans to draw air across a heat exchanger containing water or a refrigerant which is warmed up. This is circulated to the heat pump system which upgrades the heat to feed into the building heating circuit.

The system is typically enclosed in the same packaged unit or has a split arrangement where the air-to-water heat exchanger can be separated from the heat pump which may be located internally. The split arrangement offers the benefit of reduced external plant space and often reduced noise impacts where there is additional space to locate the heat pumps internally. As the heat pumps used in the split arrangement are like ground source, the air-to-water heat exchanger can also be used to supplement output from a ground-source heating array (reducing ground pipework required). A split arrangement system is generally not suitable for freezing conditions, so it needs to be backed up by another heating method.

As well as the heat pump plant, an air-source heating system would include a thermal energy store, distribution pipework, distribution pumps and packaged controls. Heat pump plant is typically modular with a few units arranged in an array to meet the total required duty. External plant typically requires screening for visual and acoustic treatment, with adequate space internally for air circulation.

Learn more here: Heat Pumps in Historic Buildings – Air Source Heat Pumps Case Studies.

A DHW (Domestic Hot Water) heat pump is a system designed specifically to heat water for use in kitchens, bathrooms, and other domestic applications. It works by extracting heat from the surrounding air—either from inside or outside the building—and transferring that heat to a water tank using a refrigeration cycle. This process is highly energy-efficient, as it uses electricity primarily to move heat rather than generate it directly.

DHW heat pumps are considered a low-carbon alternative to traditional electric or gas water heaters, especially when powered by renewable electricity. They are most effective in well-ventilated areas with a consistent air temperature and are commonly used in residential buildings, apartments, and some commercial settings. By reducing reliance on fossil fuels and lowering energy consumption, DHW heat pumps contribute to more sustainable and cost-effective hot water production.

Stale air must be continuously removed from inside a building to maintain a healthy indoor environment. This can be done either naturally—through windows, ventilation grilles, and other openings—or mechanically using ventilation systems. However, as buildings become increasingly thermally efficient and airtight, natural ventilation alone is no longer sufficient, so mechanical ventilation is used to supplement it.

To reduce heat loss when warm indoor air is expelled, mechanical ventilation systems are designed to recover heat from the outgoing stale air and use it to warm the incoming fresh, cooler air. Exhaust Air Heat Pumps (EAHPs) enhance this process by boosting the heat recovered from the exhaust air. This technology allows the system to provide not only ventilation but also space heating and domestic hot water, all from a single unit.

By integrating these functions, EAHP systems eliminate the need for traditional 'wet' central heating systems with radiators, as well as the need to connect the building to a gas supply.

A decentralised air source heat pump is a type of heating and cooling system where individual heat pumps are installed in multiple zones or rooms of a building, rather than relying on a single, central unit to condition the entire space. Each unit extracts heat from the outside air (even in cold weather) and uses it to heat a specific area, or reverses the process to provide cooling.

Unlike centralised systems, which distribute heated or cooled air through a duct network from a single source, decentralised air source heat pumps operate independently. They are typically mounted on walls, floors, or ceilings in the spaces they serve. These systems are often seen in the form of split units or mini-split systems.

Decentralised systems offer several advantages, including better control over individual room temperatures, increased energy efficiency due to reduced distribution losses, and easier installation—especially in retrofits or buildings without existing ductwork. However, they may require more maintenance if many units are installed and can be more expensive upfront when scaling to cover an entire building.

This approach is commonly used in residential buildings, apartments, hotels, small offices, and retrofit projects where individual control and zoning are important.

District heating, also known as a heat network, is a system that supplies heat to multiple buildings from a single central source. This heat is distributed in the form of hot water through a network of insulated pipes. As a low-carbon and cost-effective solution, district heating plays a key role in the transition to cleaner energy, a fact recognised by the UK government through initiatives like the Green Heat Network Fund (GHNF).

Currently, district heating supplies around 2% of the UK’s total heat demand but its potential is much greater. A wide range of technologies can serve as heat sources for these networks, including power stations, energy-from-waste (EfW) facilities, industrial processes, biomass and biogas boilers, combined heat and power (CHP) plants (including gas-fired systems), fuel cells, heat pumps, geothermal energy, electric boilers, and solar thermal arrays.

District heating systems are often more energy-efficient than individual heating systems because heat is generated at a central plant and distributed where needed, rather than being produced separately in each building. They are also considered more cost-effective, as multiple users share the costs of operating and maintaining the central plant. When powered by renewable energy sources such as solar or geothermal, these systems can significantly reduce greenhouse gas emissions.

In addition to environmental and economic benefits, district heating requires less physical space within each building, freeing up valuable real estate. These systems are generally more reliable, with easier access for maintenance and repair, making them an increasingly attractive option for future-proofing heating infrastructure in the UK and beyond.

A heat sharing network is a low-temperature system designed to distribute heat between different floors within a building or among multiple buildings. This setup allows heat to be recycled efficiently by transferring excess heat from buildings that need to reject it to those that require heating. For example, an office building’s air conditioning system can discharge heat into an ambient loop instead of releasing it directly into the atmosphere. This captured heat can then be used by neighbouring buildings.

The buildings are connected to a shared piped water circuit, where a heat pump in each building can either extract heat when heating is needed or reject heat when cooling is required. Additionally, the heat sharing network is linked to a common ground source, such as a borehole array, which provides a backup heat supply if all connected buildings require heating simultaneously.

A hybrid heating and hot water system combines two or more technologies to generate heat, such as combining a heat pump with a boiler. This may be appropriate where the constraints of the building or site make it unsuitable for single system.

Solar photovoltaic panels use photons in sunlight to create an electrical current. They are typically placed in an exposed location where they have access to bright, unshaded sunlight. Based on current developments in this technology, the maximum efficiency of panels is around 25%. In the UK, the optimal position for a panel is south facing and pitched between 20-30° to the horizontal. Panels should be arranged to avoid shading from adjacent buildings, trees etc.

A grid-tied system typically comprises panels, an inverter and charge controller. Generated electricity can be used directly when there is an on-site demand, and surplus electricity can be sent back to the grid. Off grid systems utilise batteries to charge during the day and provide the entire daily electrical load. They are not as efficient as a grid-tied system due to the additional components.

Solar thermal heating is an established technology for extraction of heat from incident solar radiation. Solar thermal panels are orientated in an exposed location, typically at roof level, through which water is circulated to extract heat from the sunlight. The system can be highly efficient, particularly compared to solar photovoltaics, with heat capture of up to 50% of the incident solar radiation. It can also capture heat from diffuse radiation when the sun is hidden behind clouds and generate heat in wintertime (although this is more limited).

Solar thermal systems can generate high grade heat e.g. suitable for heating of domestic hot water or for heating older buildings. They are typically sized and configured to serve domestic hot water loads to make best use of the heat collected in the summertime. Systems are sized to provide up to around 40-45% of the domestic hot water load to ensure that generation from the systems is not wasted.

The systems typically comprise the solar thermal collectors, distribution pipework, a circulation pump, a solar thermal heat store (located internally) and a packaged control system.

Biomass fuel refers to organic materials—such as wood, plant matter, manure, and household waste—that can be used to generate energy. Biomass is considered a renewable energy source when it comes from sustainably managed supplies that are regularly replenished and harvested in a way that protects biodiversity and avoids negative environmental or social impacts.

When sourced and managed correctly, biomass is seen as carbon neutral, since the carbon dioxide (CO₂) released during combustion is roughly equal to the amount absorbed by the plants during their growth. While burning biomass still emits CO₂, the overall environmental impact is significantly lower than that of fossil fuels like gas or oil. However, if biomass is burned improperly, it can release other harmful pollutants, including nitrogen oxides and particulate matter.

There are two main types of biomass heating systems: biomass stoves, which heat individual rooms, and biomass boilers, which connect to central heating and hot water systems. Replacing a coal-fired system with a biomass boiler can enable significant reductions in annual carbon emissions but may be more expensive to run than a modern condensing gas boiler.

Different types of biomass fuel are suited to different building types. Wood chips are typically used for larger buildings or district systems, while pellets are easier to handle and offer more automation, functioning much like gas or oil boilers. Log boilers, by contrast, are usually fed by hand and require more manual effort, although logs can be more cost-effective where there is a reliable local supply.

Learn more here: Biomass Energy. 

MVHR offers a highly efficient system that simultaneously provides fresh air to your building while recovering heat from air that is leaving the building, maintain good indoor air quality and utilising exhaust heat that would otherwise be wasted. Fresh air is drawn into the system, passing through filters that remove pollutants. At the same time, stale air is extracted from inside passing through a heat exchanger that transfers warmth from outgoing air to the incoming air, without the two air streams mixing.

In colder months, the recovered heat can significantly reduce loads on a building’s heating system. During warmer months, a bypass function allows fresh filtered air to enter without additional heating. While and MVHR system are a great choice for conserving energy and supplying consistent ventilation for healthier indoor environments, however they are not designed to cool or heat a house independently, though they do assist in maintaining a more stable indoor temperature.

Generally, these systems reduce energy consumption, lower heating bills and reduce carbon consumption. They improve indoor air quality by removing pollutants, allergens, and excess moisture from indoor spaces. The removal of stale air reduces the risks of mould, dampness, and the accumulation of indoor pollutants, leading to healthier living environments.

A runaround coil, also known as a run-around coil system, is a type of energy recovery system used in heating, ventilation, and air conditioning (HVAC) to transfer heat between two separate air streams without allowing the air to mix. The system consists of two heat exchanger coils connected by a closed loop filled with a heat transfer fluid, typically water or a water-glycol mixture. One coil is placed in the exhaust air stream, where it absorbs heat from the warm air leaving the building. The heated fluid is then pumped through the loop to the second coil, which is located in the supply air stream. Here, the fluid releases its heat to the incoming cold air, thereby pre-heating it.

In hot weather, the system can also work in reverse to pre-cool incoming air using the cooler exhaust air. One of the key advantages of a runaround coil system is that it prevents cross-contamination between the air streams, making it ideal for clean environments such as hospitals and laboratories. It is also relatively simple and reliable, with few moving parts, and can often be retrofitted into existing HVAC systems. However, it tends to be less efficient than some other heat recovery methods, such as heat wheels, and requires pumps and piping for fluid circulation, which adds to maintenance and energy consumption. Runaround coil systems are commonly used in buildings like hospitals, laboratories, schools, and office complexes.