How Heat Pumps Work: The Basics
A heat pump works on the same principle as a refrigerator — but in reverse. It extracts heat from a low-temperature source (the outside air, the ground, or a body of water) and upgrades it to a higher temperature suitable for space heating and hot water. The process uses a refrigerant circuit: the refrigerant absorbs heat from the source, is compressed (raising its temperature), transfers heat to the heating system, and then expands (lowering its temperature) before the cycle repeats.
A key performance concept is the Coefficient of Performance (COP): heat output divided by electrical input at specified operating conditions. Seasonal performance depends on the installed system, source temperature, flow temperatures, controls and heat demand; a generic COP or SPF range should not be treated as the result for one property.
Heat-pump performance is affected by the temperature lift between the source and the heating system. Emitters, flow temperature, controls and the building heat loss therefore need to be designed as one system rather than selected from a universal radiator or underfloor-heating rule.
Air Source vs Ground Source: Which Is Right for Your Hertfordshire Home?
| Factor | Air Source (ASHP) | Ground Source (GSHP) |
|---|---|---|
| Installation budget | Project-specific installer quotation | Project-specific installer quotation |
| BUS evidence | Check current technology/category eligibility | Check current technology/category eligibility |
| Seasonal performance | System-design dependent | System-design dependent |
| Space / source | Outdoor unit plus internal-system requirements | Ground array, borehole or water-source arrangement plus internal-system requirements |
| Planning / permits | Check current PD conditions and property restrictions | Check planning plus any environmental-permit requirements |
| Noise / amenity | Siting and current planning/MCS noise requirements matter | External source arrangement and plant design are project-specific |
| Best fit | Depends on heat demand, siting, emitters and design | Depends on site, source arrangement, heat demand and design |
| Maintenance | Follow installer/manufacturer requirements | Follow installer/manufacturer requirements |
For most Hertfordshire homeowners — particularly those in semi-detached or terraced properties in towns like St Albans, Watford, Harpenden, and Hertford — an air source heat pump is the practical choice. Ground source heat pumps are better suited to larger rural properties with sufficient garden space for a horizontal ground array, or properties where a borehole is feasible.
What to Assess Before a Heat Pump Installation
Heat demand and fabric: room-by-room heat loss and the building fabric affect the capacity and emitter design. Fabric upgrades can be useful in some properties, but “insulate first” is not a complete system design rule for every building.
Emitters and flow temperature: existing radiators or underfloor heating should be assessed against the proposed system conditions. There is no universal percentage by which radiators must be increased.
Hot water: cylinder and hot-water requirements depend on occupancy, demand, available space and the selected heat-pump/system design. Do not assume one cylinder size for every household.
Electrical supply and controls: electrical capacity, circuits, controls and any other building-services changes should be assessed for the actual heat pump and property by the appropriate competent installer or electrician.
Siting, noise and planning: outdoor-unit siting for an air-source system must be assessed against the current permitted-development limits/conditions, MCS planning standards and any property-specific planning restrictions.
Boiler Upgrade Scheme: Check the Current Category and Eligibility
As of September 2026, GOV.UK lists £7,500 for eligible air-to-water heat pumps and ground-source heat pumps, £2,500 for eligible air-to-air heat pumps, and different values for some eligible off-gas-grid replacements. Grant categories and values can change, so use the current GOV.UK grant page rather than an old headline figure.
BUS eligibility is not established by this article, by an EPC band alone or by a product appearing on a list. Property, technology and installation requirements apply, and the BUS installer must be MCS certified for the relevant technology. See the current eligibility guidance and Ofgem installer guidance.
Treat any grant as conditional until the responsible MCS-certified installer has confirmed the current route and the application is accepted. TCM's building-service scope does not establish BUS installer status or grant eligibility.
TCM's Role in a Heat-Pump-Related Building Project
Heat pumps are within TCM's owner-confirmed building-related scope, but this page does not establish that TCM itself is the MCS-certified heat-pump installer or BUS applicant. Installation availability, installer credentials and the delivery model must be confirmed for the individual project.
TCM can undertake or coordinate building work that may be relevant to a wider heating project—such as refurbishment, fabric work, floor alterations or other agreed building interfaces—subject to the service scope and appropriately competent specialist responsibilities.
The current Experience Inputs Register contains no approved TCM heat-pump case, grant outcome, installation cost dataset or carbon/bill-saving result for publication. Genuine cases can be added only when source, methodology and reuse permission are approved.
For related building-fabric questions, see the Retrofit Insulation Guide. For heat-pump system design or BUS eligibility, use an appropriately competent/MCS-certified installer.
Frequently Asked Questions
Is my Hertfordshire home suitable for a heat pump?+
Suitability depends on the property's heat loss, fabric, emitter sizing, hot-water demand, available space, electrical supply, noise/siting constraints and the proposed system design. Property age or postcode does not establish suitability. Use a competent heat-pump specialist or MCS-certified installer where scheme eligibility or system design matters.
How much does a heat pump cost?+
There is no approved TCM-wide installation price on this page. Cost depends on technology, capacity, emitters, cylinder, electrical work, ground array or boreholes where relevant, access and the installer quotation. If BUS funding is relevant, check the current official grant category and eligibility before treating a grant as part of the budget.
Will a heat pump work with my existing radiators?+
Possibly, but emitter suitability depends on room-by-room heat loss, required flow temperature and the output of the existing radiators at the proposed system conditions. Some systems can use existing emitters; others need larger radiators, underfloor heating or other changes. This requires project-specific system design rather than a universal upsizing percentage.
Will a heat pump reduce my energy bills?+
Not automatically. Running cost depends on seasonal system efficiency, heat demand, electricity and alternative-fuel prices, tariff, controls and how the property is used. COP and SCOP are design/performance metrics, not a guaranteed bill-saving percentage for one home.
What is the Boiler Upgrade Scheme?+
The Boiler Upgrade Scheme provides grants for eligible low-carbon heating installations in England and Wales. Current grant values vary by technology and, for some off-gas-grid replacements, by the heating system being replaced. BUS applications are made through an MCS-certified installer and eligibility has several property, technology and installation requirements. Check current GOV.UK and Ofgem guidance before budgeting the grant.
Do I need planning permission for a heat pump in England?+
Domestic air-source heat pumps can be permitted development when all current limits and conditions are met, including the applicable MCS planning standard. Ground- and water-source systems are also often permitted development, but listed buildings, designated land, planning conditions, Article 4 directions, siting and environmental-permit requirements can change the route. Check the current Planning Portal guidance for the actual proposal.
Related Guides
Retrofit Insulation in Hertfordshire
Assess building fabric and heat loss as part of project-specific system design
EPC Rating Guide Hertfordshire
EPC context and current Boiler Upgrade Scheme eligibility guidance
Energy-Efficient Extensions & Loft Conversions
Design your extension to be heat-pump ready from day one
Annual Property Maintenance Checklist
Follow the servicing requirements for the installed system and provider
Ready to Prepare Your Home for a Heat Pump?
Use the contact page to check which TCM enquiry route is currently verified for building or refurbishment work associated with a heating upgrade. Heat-pump system design, MCS installer responsibility and BUS eligibility remain on the appropriate specialist route.