Kensa Shoebox ground source heat pump

The Kensa Shoebox is a compact ground source heat pump, small enough to fit inside an airing cupboard or kitchen cabinet, and it is the product most closely associated with retrofitting flats and tower blocks in the UK¹. Where a conventional ground source heat pump needs plant space, the Shoebox sits inside the dwelling it serves, drawing heat from pipes in the ground or in water outside and delivering it to radiators or underfloor heating. Kensa describes the Shoebox range as the first ground source heat pumps that can be fitted inside a dwelling³.

It comes in 3kW and 6kW models¹, delivers around 300% efficiency, meaning over three units of heat for every unit of electricity used⁴, and is designed to last 20 to 25 years, with the underground pipework expected to perform for over 100 years⁴. For a household, it removes the gas supply or night storage heaters entirely, but it keeps the home dependent on electricity, a supplier and a tariff, so running costs still follow the electricity price.

A ground source heat pump that fits in a cupboard

The Shoebox is a ground source heat pump in the true sense of the term: it extracts low-grade heat from the ground, or from water, through collector pipes and upgrades it to useful temperatures for space heating and hot water. What sets it apart is where it lives. In the Manchester retrofit at Lullington Close, each home received a compact Shoebox and a hot water cylinder in a cupboard⁴. In the Chadwell St Mary tower blocks, it commonly replaced ageing night storage heaters, fitting easily into an airing cupboard in each apartment⁹.

The range includes variants for particular jobs. The Shoebox Twin is a 6kW compact model, and Kensa describes the Shoebox range as the first ground source heat pumps that can be fitted inside a dwelling³. The Shoebox NX, a later design, is described as a compact powerhouse for space constrained homes, designed to fit in cupboards¹⁰. Some of Kensa's ground source heat pumps, including the Shoebox, can be configured to deliver passive cooling, which Kensa calls a relatively un-mechanical means of removing excess heat because it only uses a circulation pump¹¹. For the wider context on how these systems extract heat from the ground, see ground source heat pumps.

3kW and 6kW versions: dimensions and technical details

The core Shoebox range is offered in 3kW and 6kW models¹. The Shoebox NX is offered in 3kW and 5kW models⁵, and the Shoebox Twin is a 6kW unit³. These are small outputs by heat pump standards, and that is deliberate: the Shoebox is sized for individual flats and apartments rather than whole-house plant. At Fountainbridge in Edinburgh, a mix of 3kW and 6kW units, eight of the larger and six of the smaller, served 14 homes alongside Sunamp thermal batteries¹³.

A 6kW Shoebox was also specified for a 35m² classroom space on a canal barge, attached to an open loop collector extracting heat from the water, showing the range works as a water source as well as a ground source system¹⁴. Sizing depends on the heat loss of the dwelling, and Kensa can do a heat loss survey of your home to tell you how much heat you need¹⁵. For how output relates to property size, see heat pump sizing.

Efficiency: around 300%, or over three units of heat per unit of electricity

Kensa's ground source heat pumps deliver around 300% efficiency, producing over three units of heat for every unit of electricity used⁴. The ground source system is 300% efficient in the sense that for every unit of electricity, two to three units of free renewable energy are accessed from underground⁹. Independent guidance for heat pumps generally gives an average of 300 to 400%¹⁶, so the Shoebox sits at the lower end of the normal band for the technology, which reflects its small output and retrofit applications.

The strongest recorded figure is from the Shoebox NX. A self-build bungalow installation achieved a seasonal coefficient of performance of 5.35, meaning that for every 1 kilowatt of electricity consumed, well over five units of heat were produced⁶. The installer on that project explained that ground temperature of around 10 degrees underpins the figure, and that this ups the efficiency to a seasonal coefficient of performance of 5.35 when in heating mode⁷. That is a single installation rather than a range-wide claim, and it was achieved with ground temperature around 10 degrees, which is typical of UK ground conditions.

Because heat is generated at the point of use, inside the dwelling, there are no heat losses through the pipework, which increases the system's efficiency compared with schemes that circulate hot water between buildings¹⁷. For what these figures mean and how they are measured, see heat pump efficiency: COP, SCOP and SPF.

Shared ground loops with individual pumps: how the system operates

The Shoebox is the product at the heart of Kensa's networked ground source heat pump model, also referred to as shared ground loops¹⁸. Multiple properties are connected to a network of boreholes, with a heat pump in each home, providing heating, hot water and cooling²⁰. At Daisyfield in Blackburn, the scheme used Kensa Shoebox heat pumps on a Shared Ground Loop Array, with the system collecting energy from the underlying rock and distributing it at ambient temperature to the heat pumps². An independent account of the Chadwell St Mary scheme describes the same architecture: a ground source heat pump per dwelling, each connected to a Fifth Generation District Heating ambient loop²¹.

The critical difference from a communal heat network is ownership of the equipment. Each Shoebox is wired to its own private electrical supply¹⁷, and with a Kensa ground source heat pump in each home, residents can choose their energy supplier and tariff¹⁰. Kensa states this provides residents with heating and hot water independence and the ability to freely switch suppliers¹⁷. There is no communal heat bill and no heat supplier to negotiate with; the shared part of the system is the ground array itself.

Kensa can also fund, own and maintain the shared ground array on site, leaving the customer responsible only for the heat pump and the internal distribution system¹³. For a fuller explanation of this arrangement, see shared ground loop networks.

Suitable settings: apartments, new homes and swapping out storage heaters

Kensa positions the Shoebox as ideal for flats, apartments, new builds and Networked Heat Pump schemes¹. The Shoebox NX is described as ideal for flats and 2 to 5 bed properties², and an installer notes it is very well suited to a shared ground array serving a block of flats⁷. The common thread is limited space: the unit is small enough to fit inside an airing cupboard¹⁷, and can be fitted inside airing cupboards or kitchen cabinets².

Its biggest established use is replacing electric night storage heaters in tower blocks. At Chadwell St Mary, nearly 300 council flats had storage heaters replaced with Shoebox heat pumps and heat batteries⁹. In the Heat the Streets project, existing off-gas heating systems were replaced with Kensa's Shoebox heat pumps and new radiators throughout, to maximise performance²². At Airey Close in Shropshire, the retrofit gave residents heating and hot water independence and the ability to freely switch suppliers¹⁷. In a Gentoo Group tower block retrofit, the scheme reduced carbon emissions by 70%²³.

For households on storage heaters, this is the clearest independence gain: the Shoebox removes the dependence on overnight charging windows and expensive day-rate electricity, replacing it with a heat source that runs efficiently at any hour. What remains is the electricity supply itself. For how the running costs compare, see heat pump running costs.

Everyday use: sound levels, footprint and emissions

The Shoebox is described as as quiet as an average fridge freezer². An installer working with the Shoebox NX reports it is actually really quiet in operation, which he calls a big bonus for the client⁷. Kensa also states the Shoebox heat pump is ideal for Passivhaus developments due to its low heat and aural outputs and high efficiencies¹¹. Because the unit sits indoors, there is no outdoor fan unit, which is the usual source of heat pump noise complaints; the sound a household lives with is closer to a refrigerator than an air conditioner.

On emissions, the ground source heat pumps produce no NOx, SOx, particulates or CO2 emissions at the point of use¹⁷. That is a local air quality benefit over combustion heating, though the carbon content of the electricity used to run the pump still determines its overall footprint.

Space is the other practical question. The Shoebox is designed for homes without plant rooms: fitted inside airing cupboards or kitchen cabinets², with the Shoebox Twin located in tenants' airing cupboards alongside new domestic hot water cylinders¹². The trade-off is that the cupboard is occupied by the heat pump and, usually, a cylinder or heat battery, so storage space is lost. For more on noise from heat pumps generally, see heat pump noise.

Durability: pumps last 20 to 25 years, ground arrays over 100

Kensa designed each Shoebox heat pump to last between 20 and 25 years⁴. The Shoebox NX is listed with a 25+ year life expectancy and minimal maintenance²⁰, and the Chadwell St Mary project was modelled over 25 years, described as the life of the ground source heat pumps in each flat⁹. The underground pipework, which draws renewable heat from the ground, is expected to perform for over 100 years⁴, which means the most expensive and disruptive part of the installation outlives several generations of the heat pump itself.

Warranty terms are shorter than design life. Kensa's ground source heat pump range carries a 1-Year Out-the-box Warranty, a 4-Year Standard Warranty and an 8-Year Extended Warranty⁸. The gap between an 8-year warranty and a 20 to 25 year design life is normal for the industry, but it means the later years of ownership rest on the maker's stated design life rather than a contractual guarantee.

For a household's long-term planning, the split matters. Replacing the Shoebox unit after two decades is an indoor job comparable to replacing a boiler, while the ground array, the boreholes or slinkies, stays in place and keeps working. Kensa supplies both slinkies and straight pipes for ground arrays¹⁵. For more on the collectors themselves, see ground loops and boreholes.

Cost, VAT and funding

There is no published list price for a supplied and installed Shoebox; costs are quoted after a survey. What the product page does state is that additional funding off installation costs is available, with eligibility conditions applying¹. For social housing landlords, Kensa's turnkey route runs from a bespoke feasibility report, through a fixed price quote factoring in available funding, to detailed design and project management from start to finish²⁰, and Kensa invites landlords to arrange a free feasibility study¹⁰. Kensa has managed entire projects from design to delivery, including borehole drilling, underground pipework and installation⁴.

On VAT, the installation of energy-saving materials qualifies for reduced-rate VAT under official guidance, which uses an example of a £1,000 installation charge to a customer, excluding VAT¹⁹. The position differs in Northern Ireland: if the products cost more than 60% of the total cost of installation, you'll only pay 5% VAT on the labour, and you'll pay the standard 20% VAT on the products²⁴. Because a ground source installation is dominated by labour and ground works rather than equipment, the product-heavy threshold is less likely to bite, but the rule is worth checking on any quote. For grant schemes across the UK, see heat pump grants, and for overall costs, ground source heat pump cost.

The Cornwall based manufacturer of the Shoebox

Kensa is a UK heat pump manufacturer based in Cornwall, and its accessories and associated products are all manufactured in the UK². The company is best known for two things: the Shoebox itself, and the shared ground loop model that has made tower block retrofits possible at scale. Its social housing work spans the length of the country, from the Chadwell St Mary tower blocks in Essex⁹ to Fountainbridge in Edinburgh¹³ and Paisley, and its Heat the Streets project has been studied independently by the Energy Systems Catapult²².

Kensa's role in a project can go beyond supply. It manages projects end to end, and can fund, own and maintain the shared ground array on site, leaving the customer responsible only for the heat pump and internal distribution system¹³. For households, that means the maker behind the product is a continuing British company rather than an imported brand with a local service agent, though the home's dependence on electricity, a supplier and a tariff is unchanged by who manufactures the pump. For the company's wider range, see Kensa ground source heat pumps.