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.
The key metric is the Coefficient of Performance (COP) — the ratio of heat output to electrical energy input. A COP of 3.0 means the heat pump delivers 3 units of heat for every unit of electricity consumed. Modern air source heat pumps achieve seasonal COPs (SPF) of 2.5–3.5 in UK conditions. Ground source heat pumps, which extract heat from the more stable ground temperature, achieve SPFs of 3.0–4.5.
The efficiency of a heat pump decreases as the temperature difference between the source and the output increases. This is why heat pumps work best with low-temperature heat distribution systems (underfloor heating at 35–45°C, or oversized radiators) rather than conventional radiators designed for 70–80°C flow temperatures.
Air Source vs Ground Source: Which Is Right for Your Hertfordshire Home?
| Factor | Air Source (ASHP) | Ground Source (GSHP) |
|---|---|---|
| Installation cost | £8,000–£15,000 | £15,000–£25,000 |
| BUS grant | £7,500 | £7,500 |
| Seasonal COP (UK) | 2.5–3.5 | 3.0–4.5 |
| Space required | External unit (1m² footprint) | Large garden (horizontal) or borehole (vertical) |
| Planning permission | Usually not required | May be required for borehole |
| Noise | Low (40–50 dB at 1m) | Very low (internal unit only) |
| Best for | Most Hertfordshire properties | Large rural properties with garden space |
| Maintenance | Annual service (£100–£200) | Annual service (£150–£300) |
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 Your Home Needs Before a Heat Pump Installation
A heat pump is not a drop-in replacement for a gas boiler. For it to work efficiently and cost-effectively, your home needs to meet certain conditions. The most important is adequate insulation. A heat pump delivering heat at 45°C will struggle to maintain comfortable temperatures in a poorly-insulated property — and will run at a low COP, making it expensive to operate. The general rule is: insulate first, then install the heat pump.
The second condition is appropriate heat distribution. Heat pumps work best with underfloor heating (UFH) or large-surface radiators. If your property has conventional radiators sized for a gas boiler, they may need to be upsized — typically by 20–40% in surface area. A heat loss survey will calculate the required radiator size for each room at the lower flow temperature. For new extensions, TCM always recommends UFH as the heat distribution system, as it is compatible with both current gas boilers and future heat pumps. See our House Extension service page for details.
The third condition is a suitable hot water cylinder. Heat pumps heat water more slowly than a gas boiler, so a larger hot water cylinder is typically required — usually 200–300 litres for a family of four. If your property currently has a combi boiler (no cylinder), you will need to install a cylinder as part of the heat pump installation.
Finally, the electrical supply must be adequate. A heat pump requires a single-phase 240V supply with a 32A circuit breaker. Most UK homes have adequate electrical supply, but older properties may need an upgrade. The installer will check the electrical supply as part of the survey.
The Boiler Upgrade Scheme: £7,500 Towards Your Heat Pump
The Boiler Upgrade Scheme (BUS) provides a £7,500 grant towards the cost of an air source or ground source heat pump. The grant is paid directly to the MCS-accredited installer, who deducts it from the installation cost — so you pay the net cost upfront. There is no income threshold for the BUS — it is available to all homeowners in England.
To be eligible for the BUS, your property must have a valid EPC with no outstanding recommendations for loft insulation or cavity wall insulation. If your EPC has these recommendations, you must complete the insulation works first and obtain a new EPC before applying. This is actually a useful prompt — completing the insulation first will improve the efficiency of the heat pump and reduce running costs.
The scheme is funded until March 2028. Applications are made by the MCS-accredited installer on your behalf — you do not need to apply directly. To find an MCS-accredited installer in Hertfordshire, visit the MCS installer finder at mcscertified.com.
For properties that are not eligible for the BUS (e.g., those with outstanding EPC recommendations), the EPC improvement measures described in our EPC guide should be completed first. TCM can carry out the necessary insulation and refurbishment works to bring the property to BUS eligibility.
How TCM Prepares Hertfordshire Homes for Heat Pumps
TCM Building & Maintenance does not install heat pumps directly — but we prepare Hertfordshire homes for heat pump installation by carrying out the necessary insulation, refurbishment, and heating system upgrades. This includes cavity wall insulation, loft insulation top-up, internal wall insulation, underfloor heating installation in extensions, and radiator upgrades.
For a client in Harpenden with a 1960s detached property, TCM carried out a full pre-heat pump preparation programme: cavity wall insulation (EPS beads), loft insulation top-up to 270mm, and underfloor heating installation in the new rear extension. The client then engaged an MCS-accredited installer for the ASHP, qualifying for the £7,500 BUS grant. The total TCM works cost £14,500. The client's heating carbon emissions reduced by approximately 65%.
If you are considering a heat pump and want to understand what preparation works are needed, read our Retrofit Insulation Guide or contact us for a free survey.
Frequently Asked Questions
Is my Hertfordshire home suitable for a heat pump?+
Most Hertfordshire homes built after 1930 are suitable for an air source heat pump, provided the insulation is adequate and the heat distribution system (radiators or underfloor heating) is appropriately sized. Older properties with poor insulation may need insulation upgrades before a heat pump is cost-effective. A heat loss survey by a qualified MCS-accredited installer will determine suitability and specify the correct heat pump size.
How much does an air source heat pump cost in Hertfordshire?+
An air source heat pump (ASHP) for a three-bedroom semi-detached property in Hertfordshire typically costs £8,000–£15,000 installed, including the heat pump unit, installation, and any necessary upgrades to the heat distribution system. The Boiler Upgrade Scheme (BUS) provides a £7,500 grant, reducing the net cost to £500–£7,500. Ground source heat pumps cost £15,000–£25,000 installed and attract the same £7,500 grant.
Will a heat pump work with my existing radiators?+
Heat pumps work best with underfloor heating or large-surface radiators, because they deliver heat at lower flow temperatures (35–50°C) than a gas boiler (60–80°C). Existing radiators may need to be upsized to deliver the same heat output at lower temperatures. A heat loss survey will identify which radiators need upgrading. In well-insulated properties, existing radiators are often adequate without upsizing.
How much can I save on energy bills with a heat pump in Hertfordshire?+
Savings depend on the efficiency of the heat pump (COP), the current heating system, and energy prices. A modern ASHP with a seasonal COP of 3.0 uses one unit of electricity to deliver three units of heat. At 2026 energy prices (electricity ~28p/kWh, gas ~7p/kWh), the running cost per unit of heat is approximately 9.3p for the heat pump vs 7.8p for a 90% efficient gas boiler — slightly higher. However, with a solar PV system generating cheap electricity, the heat pump running costs can be significantly lower. The carbon savings are substantial: a heat pump reduces heating carbon emissions by 50–70% compared to a gas boiler.
What is the Boiler Upgrade Scheme and how do I apply?+
The Boiler Upgrade Scheme (BUS) provides a £7,500 grant towards the cost of an air source heat pump or a ground source heat pump, and a £5,000 grant for a biomass boiler. The grant is paid directly to the MCS-accredited installer, who deducts it from the installation cost. To apply, you need a valid EPC with no outstanding recommendations for loft or cavity wall insulation. Your installer will apply on your behalf. The scheme is funded until March 2028.
Do I need planning permission for a heat pump in Hertfordshire?+
Air source heat pumps are permitted development in England (no planning permission required) provided they meet certain conditions: the unit must be at least 1 metre from the property boundary, no more than one unit per property, and the unit must not be installed on a wall or roof facing a highway. Ground source heat pumps require a borehole or ground array, which may require planning permission depending on the depth and location. Listed buildings and properties in conservation areas may require planning permission for any heat pump installation.
Related Guides
Retrofit Insulation in Hertfordshire
Insulate first — the essential pre-condition for an efficient heat pump
EPC Rating Guide Hertfordshire
Improve your EPC to qualify for the Boiler Upgrade Scheme
Energy-Efficient Extensions & Loft Conversions
Design your extension to be heat-pump ready from day one
Annual Property Maintenance Checklist
Include heat pump servicing in your annual maintenance plan
Ready to Prepare Your Home for a Heat Pump?
TCM Building & Maintenance surveys Hertfordshire properties and carries out the insulation, refurbishment, and heating upgrades needed to make a heat pump installation cost-effective. Contact us for a free consultation.