How do heat pumps work? Mesh Energy explains

Doug Johnson

How do heat pumps work? Mesh Energy explains

In the first of our Mesh Energy Explains series of blog posts, we’re looking at just how heat pumps work; specifically, how energy is harvested and transported into the home using little more than an electrical supply. Those of us working in the renewable technology sector often forget that sometimes we need to return to basics and explain some of the fundamentals to those just entering into the field or researching for their personal provision. So, here goes…


A brief history of heat pumps and the basics

In the 19th Century, William Thompson, otherwise known as Lord Kelvin (of temperature scale fame), pioneered the concept of the heat pump using a refrigerant cycle to prove that heat energy could be used to cool. In fact, he foresaw use in air conditioning and refrigeration.


In the 1940s, Robert C. Webber is credited with creating the first modern day heat pump and we haven’t looked back since. Today’s heat pumps work in exactly the same way; with developments in efficient compressor technology and refrigerant gases incrementally improving operational efficiency of the units.


Heat pumps are often described as “fridges working in reverse”. Energy in fridges is extracted from the enclosed interior of the fridge, keeping your food cool and using a tiny amount of electrical energy to power a small compressor. This extracted heat is “processed” and ultimately expelled at the back of the fridge via an element.


A heat pump uses an identical principle and strips low-grade energy from the natural environment using the process of gas evaporation. Once compressed by the compressor cycle, it is during its condensation phase that high temperature energy is exchanged into your under floor, radiator or domestic hot water system.


The diagram below shows the basic cycle.

Diagram of a vapor-compression cycle with blue evaporator, red condenser, compressor, and expansion valve

As a general rule, heat pumps as we know them today use three distinct mediums to harvest energy: earth/ground, air and water. Being pedantic, it is the water moisture in all of these mediums that helps with energy transfer, but for clarity we will stick with these three.


So, it is the changing heat source that helps the evaporation phase of the refrigerant cycle that gives heat pumps their name.


Ground source heat pumps

Ground source heat pumps (also known as geothermal heat pumps) use two practical methods for harvesting energy.


The first method is horizontally buried pipework in the ground which has a glycol/water mixture pumped around it to stop it freezing. The pipes are buried at approximately 1.5m below the ground level and the ambient heat of the earth (nominally 8ºC-12ºC) provides energy to the liquid flowing through the buried pipes. The fluid, which warms up just a few degrees in its journey in the ground, heads back to the heat pump. The water that heads back to the heat pump from the ground collector is often only around 5ºC but this small temperature is passed through a plate heat exchanger and with a significant flow rate, boil the gases in the ground source heat pump refrigerant circuit and impart large quantities of thermal energy to the home using an electric compressor.


Alternatively, the second method uses vertical boreholes (circa 100m) which can be drilled and which require significantly less ground area. These still rely on the ground’s low level thermal energy to warm the fluid but, because of the depth, much more stable ground temperatures are found, and the efficiency of the system improves.



Typical ground source heat pump efficiencies year-round are around 400%.


Air source heat pumps

Air source heat pumps use an external fan unit to draw air across the evaporator unit, which exits at the back of the unit cooler than it entered. Because of varying seasonal air temperatures, these heat pumps are less efficient than their ground source cousins. As a rule, the colder the outdoor air gets, the harder the system has to work to strip energy from the air and elevate it to heat the home.


Air is sucked into air source fan units using a single fan (or fans) and refrigerant pipes strip heat from the air. The air exits the fan units at a cooler temperature. You may be surprised to hear that air source heat pumps can successfully extract energy from the air down to well below -20ºC. Because the refrigerant in the heat pump boils at such a low temperature, even air well below freezing temperatures can give up its heat.


Due to energy in the air affecting outputs from air source units, the efficiencies of these systems typically vary from 450% efficient in the summer and 250% efficient in the winter months. This means that even in the winter one unit of electricity can supply 2.5 units of heat to the home using ‘freezing’ cold air and some clever physics!

3D diagram of an object orbiting a platform with red, blue, and gray curved arrows

Water source heat pumps

Water source heat pumps are so called as they are used in lakes, ponds and streams to harvest energy.


“Open loop” and “closed loop” systems are used. Open loop systems can be likened to a hoover sucking up water and using this direct energy to transfer to the evaporator via a plate heat exchanger. The closed loop system is basically a ground source system with loops or panels buried or pinned under water to harvest energy from the body of water. There are many advantages regarding the amount of energy that can be harvested from pure bodies of water, especially streams and rivers, as flow rates are high and ensure an almost endless supply of energy to the heat pump.


Due to the high levels of energy in the water, water source heat pump efficiencies can exceed 500%.


Still have questions? Speak to us!

So, there you go. A whirlwind introduction to the magical, yet practical basics behind the increasingly common heat pump!


If you still have any questions about the wondrous world of heat pumps and the part they could play in your low-energy home, please don't hesitate to get in touch with the Mesh team today.

SHARE THIS POST WITH YOUR NETWORK

17 October 2026
Sustainability, impact and integrity form the basis of Mesh's culture where creativity and fun are still very present. As part of this ethos, each year we hold a series of volunteering days for our team. 11 of our colleagues from as far afield as Portugal, Exeter and Bristol recently joined the Surrey Society to work with rangers at West Horsley Place – a 400-acre estate near Guildford – and help maintain some of their ancient woodland. The team lent a hand with ‘coppicing’. This practice involves felling a tree to create a living stump called a ‘stool’ which supports regrowth into new, fast-growing poles or rods. Regular cutting of the stools can extend the life of a tree for centuries, provide sustainable materials for a range of uses, and encourages biodiversity. Doug Johnson, Founding Director of Mesh: "We had a fantastic day in the fresh air which was followed by our monthly team meeting in the beautiful grounds and a social dinner afterwards. Thank you to the Surrey Society for giving us the opportunity to volunteer on another of their projects. This independent charity does some great work to encourage people to explore the outstanding landscape in Surrey. This fits so well with Mesh's sustainability values." 
12 October 2026
Mesh has been announced as a runner up for Energy Consultancy of the Year at the National Energy Efficiency Awards. This latest accolade follows our success in winning Regional Energy Efficiency Consultancy of the Year at the South East Energy Efficiency Awards. According to Doug Johnson, Founding Director of Mesh: "As one of the smaller, specialist consultancies, this was a significant achievement for such a prestigious award and in a highly competitive category. The awards event was a fantastic celebration of innovation and achievement in improving energy efficiency across the built environment. Our teams are committed to excellence in both building performance analysis and building services design – delivering practical solutions and strategies for our clients to accelerate decarbonisation and improve the energy efficiency of new and existing homes and buildings. It is wonderful to have our team's talent and hard work recognised with this national accolade from the industry." 
7 October 2026
Adapting heritage buildings for modern use creates a particular engineering challenge – how do you deliver comfort and energy performance, within the constraints of a historic building? Doug Johnson, Mesh Founding Director of building performance specialists Mesh, shares his insight from a recent project. When we talk about successful heritage transformations, the conversation usually focuses on the architectural design and the restoration process. Less visible but equally important, is the role of building services engineering. Change of use breathes new life into historic buildings. However, achieving year-round comfort for visitors and occupiers of galleries, offices, hospitality or retail can be challenging, not least when occupancy levels fluctuate. Historic buildings are expected to deliver the same standards of comfort, environmental control and operational performance as modern buildings, yet they were never designed to accommodate contemporary MEP systems. Getting that balance right requires different thinking. A Grade II listed Georgian townhouse in London's Mayfair was recently transformed into a space for contemporary art. This provides a useful illustration of the principles involved and how the challenges were addressed. Dating back to the 1730s, the six-storey building in Mayfair was once home to the Burlington Fine Arts Club. It has now been sensitively converted into a suite of contemporary art galleries for Sadie Coles HQ. The objective was not simply to install heating and cooling. It was to create a comfortable, flexible, efficient and futureproof internal environment whilst ensuring that the engineering services remained almost entirely invisible. The Importance of Designing Around Occupancy A key consideration for intelligent building services design is looking at the predicted occupancy levels and whether these will vary. Museums, galleries, visitor attractions, civic buildings and cultural venues can all move rapidly from quiet periods of low occupancy to large events with significant increases in heat gain and ventilation requirements. Rather than designing for a single occupancy assumption, modelling multiple use scenarios provides a much clearer understanding of how a building needs to perform throughout the year. By analysing different occupancy patterns, environmental conditions and operational requirements, MEP systems can be sized more accurately to avoid both underperforming and over-engineered solutions which increase energy consumption. In heritage projects, this approach is particularly valuable because space for plant and services is often limited. Every piece of equipment needs to justify its inclusion. Early Co-ordination and Integration with the Architectural Design Heritage and listed buildings rarely offer generous service routes. Unlike new-build projects, where risers, ceiling voids and plant spaces can be designed in from the outset, historic buildings often require services to be threaded through a series of constrained voids. This challenge becomes even greater in tall, narrow buildings such as townhouses, where plant may need to serve multiple floors through restricted vertical routes. The lesson is clear – building services engineering has to be designed in from the outset. Early collaboration between architects, engineers and conservation specialists is essential to establish whether the proposed systems are feasible, where equipment can be located and how services can be integrated within the building fabric. The earlier this co-ordination takes place, the greater the opportunity to avoid compromise, reduce project risk and optimise energy efficiency. Increasing Capacity for Heating and Cooling in a Heritage Setting A common response to increasing heating or cooling demands is simply to add more equipment. In many heritage settings, that option is not possible. Planning restrictions, minimising visual impact and noise levels, and conservation requirements often limit the number and location of external plant units. Roofs, courtyards and rear elevations frequently offer little flexibility which demands a more intelligent approach to system design. On the Sadie Coles gallery project, instead of increasing the number of external units, a zoned strategy was developed which matched available plant capacity to individual rooms based on anticipated occupancy patterns and the predicted demand for heating and cooling. This allowed the available capacity to be used with greater energy efficiency whilst maintaining compliance with planning constraints. The broader lesson is that building performance is not solely determined by the quantity of equipment installed. Understanding how spaces are used often unlocks opportunities to achieve enhanced performance within the available infrastructure. Maintaining a Clean Aesthetic The most effective building services design is often the least visible whilst meeting performance requirements. This is particularly true in heritage settings where restored architectural features, historic detailing and carefully considered interiors should remain the focus. Achieving this requires close collaboration with the wider design team. Diffusers, grilles, fan coil units, controls and service routes must be carefully integrated into the architecture rather than be imposed upon it. In gallery environments, there is an additional consideration – the services engineering must not distract from the exhibits themselves, either visually or acoustically. This demands a level of co-ordination and design discipline that goes beyond compliance. Passive Design Matters Any building should be as energy efficient as possible and passive measures often provide some of the most effective opportunities to improve comfort whilst minimising energy consumption. In the Mayfair gallery, a glazed roof lantern was both a beautiful architectural feature and a potential source of overheating. Rather than relying solely on active cooling, actuated roof lights were incorporated into the glazing above to allow warm air to be purged naturally from the space below. Combined with natural ventilation through restored sash windows, this approach reduced cooling demand whilst improving thermal comfort. These principles are not new, but remain highly relevant. Understanding airflow, solar gain and natural ventilation can often deliver significant benefits alongside the design of mechanical systems. Futureproofing Heritage Assets Buildings that have survived for centuries have often done so because they have adapted over time. The services strategy should support that continued evolution. Futureproofing does not necessarily require major additional investment. Sometimes it simply means incorporating connection points, spare capacity or flexible infrastructure that allows future adaptations to be made more easily. In the case of the gallery project, provision was included for supplementary low-level electric heating should the building use change. Such measures may never be required, but they provide resilience and flexibility that can extend the useful life of the building with the least disruption possible. Services Engineering to Support Heritage Buildings Successful heritage refurbishment is not about choosing between conservation and performance. It is about achieving both. The most effective building services solutions are those that support the architectural and client vision, improve comfort, reduce energy consumption and remain largely unseen by the people using the building. When done well, engineering becomes part of the fabric of the building rather than an addition to it. That is ultimately what intelligent building services design should achieve – modern performance delivered in a way that respects the past whilst preparing buildings for the future. The Project Team: Client: Sadie Coles HQ Lead contractor (design and delivery): Work Ltd Architectural partner: Giles Reid Architects MEP design / building performance: Mesh About Mesh Mesh offers a range of design and engineering services to help organisations in the public and private sectors to improve the energy efficiency of new and existing buildings, housing and estates. This includes support for architects; planning; thermal modelling and overheating analyses; carbon reduction strategies; feasibility studies for renewable technologies; MEP design, and regulatory compliance.
Mesh is working on the first phase of an estate rejuventation programme at Waterperry Gardens
7 October 2026
Building performance specialist Mesh is providing a range of multi-disciplinary services for a new build scheme and estate rejuvenation programme at Waterperry House and Gardens, including building performance and embodied carbon analysis, BREEAM assessments and MEP services design and engineering.
7 October 2026
Mesh has been appointed to work on three new care home projects – in Cambridge, Barnstaple and in the Exmoor National Park – providing assistance with regulatory compliance, analysis to mitigate overheating, energy strategy development, and low-carbon services design, engineering, and specification. There has been a significant increase in demand for Mesh's building performance services for care home projects this year. There are a number of drivers: Running costs - there is a clear need to reduce operational costs in care homes given global uncertainty around energy prices. Comfort - It is vitally important to maintain the comfort of residents and vulnerable people, and particularly to reduce the risk of overheating in light of climate change and rising temperatures in the summer months. Performance - New build care homes and extensions have to perform over time rather than simply meet minimum requirements for statutory compliance. Sustainable living - In a competitive care home market, more sustainable living accommodation means lower bills for residents and enhanced wellbeing.  Dylan Ross of Mesh: “Care homes are occupied 24/7 so building performance and energy efficiency really matters. We are seeing more care home developers and operators look beyond regulatory compliance and at how their buildings will actually perform over time – in terms of energy use, thermal comfort and maintenance implications. Our role is to bring building performance and services design together using evidence-led solutions to help inform decision making and deliver buildings for social care that maximise comfort for residents and minimise energy consumption, carbon emissions and running costs for care home operators.” Mesh specialises in improving the energy efficiency of both new and existing buildings, and works with care home operators, developers and architects.
Building performance specialist Mesh has developed an energy strategy for Hampshire Fire Service
7 October 2026
Building performance specialist Mesh has worked with Hampshire Fire & Rescue Service to improve the energy efficiency of its building estate and support decarbonisation.
 Jen Wallace, Managing Director of building performance specialist Mesh
12 September 2026
Building performance and M&E engineering specialist Mesh has appointed Jen Wallace as Managing Director
Mesh - finalist for the Energy Efficiency Awards National Consultancy of the Year Award
15 August 2026
Building performance specialists Mesh have been announced as a finalist for National Consultancy of the Year at the Energy Efficiency Awards
A listed heritage building where Mesh advised on the energy strategy to improve efficiency
15 August 2026
An article from energy specialists Mesh advising owners of historic buildings and listed homes on the measures to improve energy efficiency and decarbonisation.
10 August 2026
As part of our sustainability and netzero commitments, we contribute to the Climate Action Workforce – a service provided by Ecologi that allows businesses to fund certified carbon reduction and tree-planting projects on behalf of their employees. This helps us to engage our teams in collective environmental impact, supporting global reforestation, habitat restoration, and certified carbon avoidance initiatives. In the past month, we have contributed to: - Agroforestry in Kenya - The provision of fuel-efficient cooking stoves in Uganda - Protection of the Matavén forest in eastern Colombia. Since Mesh joined the Ecologi scheme, we are proud to have supported 85 global projects , each carefully vetted for quality and impact. If you would be interested in measuring and reducing your business carbon footprint and contributing to global carbon reduction schemes, do check logout Ecologi . 

Subsribe to memo from mesh