Air Source Heat Pump Savings Calculator

Air Source Heat Pump Guide

How Heat Pumps Work?

An air source heat pump (ASHP) extracts heat from outside air and moves it inside — like a refrigerator working in reverse. For every 1 kWh of electricity used, a modern ASHP delivers 2.5–4 kWh of heat, giving a coefficient of performance (COP) of 2.5–4.0. At COP 3 and electricity at 24.5p/kWh, the effective heat cost is 24.5/3 = 8.2p/kWh, compared to gas at 5.5p/kWh ÷ 0.9 boiler efficiency = 6.1p/kWh. Currently, gas heating is often cheaper per unit but this gap is narrowing.

What's the key thing to understand about The Boiler Upgrade Scheme?

The UK Government's Boiler Upgrade Scheme provides a £7,500 grant toward the purchase and installation of an air source heat pump (from April 2024 upwards). This significantly changes the financial case: a £12,000 installation becomes £4,500 net. Applications are made through the installer — not by the homeowner directly. The scheme requires an EPC (Energy Performance Certificate) rated D or below without a recommendation to add cavity wall or loft insulation unless it has already been done.

When Heat Pumps Work Best?

Heat pumps work best with: underfloor heating or large, low-temperature radiators (optimised for 40–50°C flow rather than gas boilers' 60–80°C); good insulation (heat pumps work harder in poorly insulated homes, reducing efficiency); modern double or triple glazing; and a buffer tank for hot water. Properties with solid walls (no cavity insulation) and no underfloor heating typically see worse economics until they are insulated first. The Passivhaus-style of buildings — excellent insulation, minimal heat loss — are where heat pumps perform at their absolute best, since the low heat demand suits the gentler, steadier output a heat pump provides compared to a gas boiler.

What do I need to know about Financial Outlook?

The electricity-to-gas price ratio matters enormously. At current UK prices (electricity ~24p, gas ~5.5p), the price ratio is approximately 4.4:1. For a heat pump to match gas on running costs, it needs a COP above 4.4 — achievable in mild weather but often not in deep winter when heat demand is highest and COP is lower. The case for heat pumps improves as: the electricity price ratio falls (more renewables reduce electricity cost), gas prices rise (North Sea depletion, import dependency), carbon pricing all make heat pumps a more competitive choice over the system's lifetime.

What do I need to know about Noise and Siting Requirements?

Air source heat pumps have an outdoor unit with a fan that draws air across a heat exchanger, and while modern units are considerably quieter than early models, they do produce some operational noise, typically in a similar range to a modern domestic fridge-freezer or quiet air conditioning unit heard from a short distance, though this varies between models and how hard the unit is working at any given time. Under permitted development rules in England, a heat pump can usually be installed without full planning permission provided it meets set criteria, including a minimum distance from the boundary of a neighbouring property, since this distance requirement exists specifically to manage noise impact on neighbours. Good siting practice also means avoiding placing the unit directly beneath a bedroom window, either your own or a neighbour's, and allowing clear space around the unit for airflow, since a heat pump starved of airflow at the intake or outlet has to work harder and can both run less efficiently and generate more noise than a well-sited equivalent. Vibration-isolating mounts and a solid, level base also reduce structure-borne noise transmitted into the building compared with a unit bolted directly and rigidly to a wall.

What should I know about Radiator and Pipework Upgrades for Heat Pumps?

Heat pumps are most efficient, and achieve the best COP figures used in this calculator's savings estimate, when run at a lower flow temperature than a typical gas boiler, often in the region of 35-45°C rather than the 70-80°C a gas system might use. Running a heat pump at this lower temperature means existing radiators, originally sized assuming the higher gas boiler flow temperature, often can't release enough heat into the room at the lower temperature to keep it comfortable, since a radiator's output drops substantially as the temperature difference between the radiator and the room shrinks. This is why many heat pump installations involve replacing some or all radiators with larger ones, or switching to underfloor heating in some rooms, to compensate for the lower flow temperature while still meeting the room's heat loss requirement, rather than being able to directly swap a heat pump onto an unmodified gas-boiler-era heating system and expect the same comfort level. A proper heat pump installation should include a room-by-room heat loss calculation and radiator sizing check as standard, and the cost of any necessary radiator upgrades is a real part of the total installation cost that's worth confirming is included in any quote, rather than appearing as a surprise extra.

How Do Air Source and Ground Source Heat Pumps Compare?

Both air source and ground source heat pumps work on the same basic refrigeration-cycle principle, extracting heat from a low-temperature source and concentrating it to a usable higher temperature for home heating, but they differ significantly in installation requirements, performance, and cost. Air source heat pumps extract heat from outside air using an external unit similar in size to a large air conditioning condenser, require relatively modest installation space and no groundwork beyond the unit's base and pipework connection to the house, and are considerably cheaper to install, which is why they make up the large majority of UK heat pump installations. Ground source heat pumps extract heat from the ground via buried pipework, either horizontal loops requiring a large area of garden to be dug up, or vertical boreholes drilled where garden space is limited, and this groundwork adds substantial installation cost and disruption compared with an air source system. The performance trade-off generally favours ground source: ground temperature stays much more stable year-round than air temperature, typically sitting around 10-12°C regardless of how cold the air gets, which means ground source heat pumps maintain higher efficiency (coefficient of performance) during the coldest winter weather, when air source units have to work hardest to extract heat from near-freezing or sub-zero outside air. For most UK homes, the much lower installation cost and simpler installation process make air source the practical default choice, with ground source generally only justified where garden space allows a straightforward installation and the household values its somewhat better cold-weather performance enough to offset the higher upfront cost.

What do I need to know about How COP Changes with Outdoor Temperature?

A heat pump's coefficient of performance isn't a single fixed number — it drops as the outdoor temperature falls, because the refrigerant cycle has to work harder to extract usable heat from colder air. This is why the 'SCOP' (seasonal COP) used for annual cost estimates is always lower than the best COP a heat pump can hit on a mild day.

Worked example: a home with an annual heat demand of 10,000 kWh and a heat pump achieving a realistic seasonal COP of 3.0 across the whole heating season. Electricity input = 10,000 ÷ 3.0 = 3,333 kWh At an illustrative electricity price of 25p/kWh: cost = 3,333 × £0.25 ≈ £833/year The gap between a heat pump's best-case COP (often quoted in marketing material, measured on a mild day) and its real seasonal COP (what actually shows up on the electricity bill) is one of the most common sources of disappointment with heat pump running costs — a system rated at 'COP 4.0' isn't delivering that figure every hour of the heating season, only on the mildest days, which is exactly why SCOP, not peak COP, is the number that should feed into any annual cost estimate.