Knowledge

Background reading and resources to help you make sense of the database. Learn the fundamentals in the FAQ, compare the refrigerants used in heat pumps, or browse our directory of useful external links.

What Is a Heat Pump?

A heat pump is an electric appliance that heats your home (and often your hot water) by moving heat that already exists outside — in the air, the ground, or a body of water — rather than burning fuel to create heat. It's the same technology as a fridge, just running in reverse.

A fridge pulls heat out of the cold inside of the box and dumps it into your warm kitchen — that's why the back of a fridge feels warm. A heat pump does exactly the same thing, but on a much bigger scale: it pulls heat out of the cold outdoors, however chilly, and pumps it into your warm home. It can do this even when it's close to freezing outside, because there is still usable heat energy in the air down to well below 0 °C.

How it works

A small amount of electricity runs a compressor, which is used to concentrate and move heat rather than generate it directly. That's why a heat pump can deliver several times more heat energy than the electrical energy it consumes — typically 3–4 units of heat for every 1 unit of electricity.

Behind that simple picture is a repeating loop: a cold refrigerant gas absorbs heat from outside (the evaporator), a compressor squeezes it to raise its temperature further, the hot gas gives up its heat to your home's water or air (the condenser), and an expansion valve drops the pressure so the cycle can start again. You don't need to understand this to use the database — it just explains why figures like COP, SCOP and flow temperature (covered in our other guides) matter so much.

The three main types

Browse air source, ground source and water source products in the database.

Heat pump vs gas boiler

A gas boiler burns fuel on-site to generate heat directly; a heat pump uses electricity to move heat that's already there. That fundamental difference shows up across almost every point of comparison:

Figures are typical UK ranges for guidance only — actual performance depends on the specific product, installation and property. See our COP & SCOP guide for how efficiency is measured.

Why "300–400% efficient" sounds strange

A gas boiler can never be more than 100% efficient — it can only release the energy locked in the gas it burns, and some is always lost. A heat pump isn't creating extra energy; it's moving free heat from outside, so the "output" can be several times the electricity "input". The diagram below shows roughly what happens to one unit of input energy in each case.

Key considerations — is a heat pump right for your home?

Heat pumps can heat pretty much any UK home, but a good result depends on getting a few things right beforehand. These are the main factors worth thinking through before you commit:

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Reviewed July 2026. This guide is general information, not professional or installation advice — always get a proper heat loss survey and installer quote for your specific property.

Refrigerants Compared

The refrigerant is the working fluid that lets a heat pump move heat from outside to inside. Which one a unit uses affects its environmental impact, its efficiency and achievable flow temperature, how it must be installed, and — increasingly — whether it can still be sold in the years ahead. This guide compares the refrigerants you will find across the database.

GWP (Global Warming Potential) measures how strongly one kilogram of the gas warms the planet over 100 years compared with one kilogram of CO₂ — so CO₂ is 1, and a refrigerant with a GWP of 2,000 is 2,000 times worse if it leaks. The safety class (ASHRAE) flags flammability and toxicity: A1 non-flammable and low toxicity, A2L mildly flammable, A3 highly flammable, B2L mildly flammable but toxic.

At a glance

GWP figures are 100-year values, rounded. They vary slightly between IPCC assessment reports — the EU moved from AR4 to AR5 values in 2025 — but the relative picture is unchanged.

The families, with pros and cons

The regulations

Fluorinated refrigerants are controlled by F-Gas regulations, which shrink the supply of high-GWP gases over time and ban them in new equipment where cleaner alternatives exist. Natural refrigerants (R290, CO₂, ammonia) and HFOs largely sit outside these limits. The picture differs across the UK.

European Union — Regulation (EU) 2024/573 (also applies in Northern Ireland)

Thresholds and category definitions are simplified here; some categories have safety-related exemptions. Existing installed equipment is unaffected — the bans apply to new products placed on the market.

Great Britain (England, Scotland, Wales)

Great Britain did not adopt the EU's 2024 regulation. It keeps the retained Regulation (EU) 517/2014 — the earlier HFC quota phase-down plus existing rules such as the GWP ≥ 2,500 servicing ban — so the EU's new product bans above do not currently apply in GB. Defra reviewed the rules and launched a consultation on a tighter GB HFC phase-down in November 2025; in May 2026 it confirmed it would not change the phase-down steps for the 2027 period yet, with a fuller response due later in 2026. The direction of travel mirrors the EU — tighter HFC limits and a shift to low-GWP and natural refrigerants — but the precise GB timeline is still being decided.

Northern Ireland

Under the Windsor Framework, Northern Ireland continues to follow the EU F-Gas Regulation, so the EU timeline above applies there.

All of this sits under the global Kigali Amendment to the Montreal Protocol, which commits developed countries to cutting HFC use to a small fraction of historic levels over the coming decades.

Where it's heading

The shrinking quota makes high-GWP gases such as R410A and R134a steadily scarcer and more expensive, pushing the whole market toward low-GWP and natural refrigerants. In practice:

• Residential heat pumps are converging on R290 (propane). It already dominates new monobloc ranges, has a GWP of 3, sits outside the phase-down and supports the higher flow temperatures UK retrofits often need — the most future-proof choice.
• R32 remains common today but its 675 GWP exceeds the EU's 150 limit for small monoblocs from 2027, so EU and Northern Ireland units are moving to R290. R32 is likely to persist longer in GB and in larger or split systems.
• Commercial and industrial systems are shifting to R290, CO₂, ammonia and HFOs (R1234ze, R1233zd, R515B) for large and high-temperature duties.

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Reviewed June 2026. Regulations change and the GB rules are under review — always confirm the current position before making decisions. This guide is general information, not regulatory or professional advice.

Understanding COP & SCOP

COP and SCOP both describe how efficiently a heat pump turns electricity into heat — but they answer different questions, and neither figure means anything without the conditions it was measured at. This guide explains what each one is, how to read the test conditions shown throughout this database, and how to compare models fairly.

What COP measures

COP (Coefficient of Performance) is the heat delivered divided by the electricity consumed at one specific operating point. A COP of 4 means the unit produces 4 kW of heat for every 1 kW of electricity it draws — in other words, 400% efficient. It is an instantaneous snapshot, taken with the source and output temperatures held fixed.

Why the conditions are everything

A COP only has meaning alongside the two temperatures it was measured at, written as source/output. For an air source heat pump, A7/W35 means 7°C outdoor air going in and 35°C water leaving to the heating system. Change either temperature and the COP changes — often dramatically. The same machine might post a COP of 5.0 at A7/W35 and 2.7 at A7/W55; both are true.

How COP changes with conditions

Two levers move the number: a colder source lowers COP (the unit has to work harder to gather heat), and a higher flow temperature lowers it (it has to lift the heat further). The figures below are illustrative ranges for a typical modern air source unit, not a specification — they show the shape of the effect.

The flow-temperature effect is the one you have most control over once a system is installed — see the Flow temperature & efficiency guide.

What SCOP measures

SCOP (Seasonal Coefficient of Performance) averages efficiency across a whole heating season. The EN 14825 standard, often used by manufacturers, sets very specific test procedures for estimating SCOP. Rather than a single operating point, it weights many points across a full year of outdoor temperatures and part-load conditions for a reference climate. That makes it a far better predictor of real annual running cost than any single COP.

SCOP carries conditions too. The two that matter are the application flow temperature and the climate. A complete figure therefore reads something like "SCOP 4.6 (W35, average climate)". The familiar ErP energy label (A+++ to G) is derived directly from SCOP.

What the EN 14825 SCOP test assumes

Because SCOP is a calculation across a season rather than a single measurement, EN 14825 fixes a detailed set of assumptions so every manufacturer's figure is built the same way. Knowing them explains both how the number is reached and why a lab SCOP won't exactly match your own bills.

A reference climate, not your weather. Efficiency is weighted across a standard heating season for one of three defined climates. The Average profile is mandatory and is the one behind the ErP energy label; Warmer and Colder are optional. Each sets a reference outdoor design temperature (the coldest design point) and a total number of heating hours.

Within the chosen climate, the rest of the framework is:

• A spread of temperatures, not one. The season is divided into temperature "bins", each with a number of hours and a matching heating demand that rises as it gets colder. COP is tested at part-load points of +12, +7, +2 and −7°C plus the design temperature (−10°C for Average; a −15°C point is added for Colder), then averaged by how many hours fall in each bin.
• Mostly part-load running. Most of the season is mild, so the unit spends most hours well below full output. The method therefore rewards good part-load and modulating efficiency — where inverter-driven units do well.
• A backup heater below the bivalent point. Below a declared outdoor temperature the heat pump can't meet demand alone, and a supplementary (usually electric) heater covers the shortfall at lower efficiency — and that penalty is counted in.
• A defined flow-temperature application. The test is run for a low-temperature (35°C) and/or medium-temperature (55°C) application — which is why a SCOP only means something alongside its W35 or W55 basis.
• Auxiliary electricity counts. The headline SCOP also includes power drawn in thermostat-off, standby, off and crankcase-heater modes. You may also see SCOP_on (active mode only) and SCOP_net (excluding auxiliary modes and any backup heater).

What it deliberately leaves out is your building. EN 14825 assumes a standardised climate and load, not your home's heat loss, controls or installation quality. A figure measured in a real installation is called the Seasonal Performance Factor (SPF), and it can sit either side of the rated SCOP.

COP or SCOP — which should you use?

COP is a snapshot at one condition; SCOP looks at the whole year. To judge how a unit will perform — and cost to run — over a year, SCOP at the flow temperature you intend to use is the more useful number. COP is best for understanding behaviour at a particular point, such as a cold snap (A-7/W35) or heating domestic hot water at a high flow temperature.

Comparing fairly

• Match the conditions. Compare A7/W35 with A7/W35, or SCOP W35 with SCOP W35 — never across different tests.
• Don't compare across source types. A ground- or water-source COP (B0/W35, W10/W35) looks higher than an air-source one because the source is warmer and steadier — it isn't directly comparable.
• Be wary of an easy headline. A high COP quoted at an unusually mild condition flatters the unit.
• Read the spread. A strong A7/W35 figure with a much weaker W55 one tells you the unit is happiest at low flow temperatures.
• "Not stated" means caution. If the test basis isn't recorded, you can't reliably compare it.

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Reviewed June 2026. This guide is general information to help interpret the data, not professional or design advice; always confirm performance figures against the manufacturer's datasheet.

Flow temperature & efficiency

Flow temperature — how hot the water is that a heat pump sends to your radiators or underfloor heating — is the single biggest factor you control that affects efficiency. This guide explains why lower is better, the trade-off with your heat emitters, and how to read the flow-temperature data in this database.

What "flow temperature" means

The flow temperature (or leaving water temperature) is how hot the heat pump heats the water it sends out to the heating system. It is the "W" number in a rating such as A7/W35, where 35 is a 35°C flow. The water gives up its heat to the rooms and returns cooler — the return temperature.

Why lower flow temperature means higher efficiency

A heat pump's efficiency is governed by the "lift": the gap between the source temperature and the flow temperature. The smaller that gap, the less work the compressor does, and the higher the COP and SCOP. This is fundamental thermodynamics, not a quirk of any one model.

As a rule of thumb, every 1°C you can shave off the flow temperature improves efficiency by roughly 2–2.5%. Dropping from a boiler-style 55°C to 35°C can lift seasonal efficiency substantially — often taking a SCOP from around 3 to comfortably above 4. The numbers below are indicative, to show the shape of the relationship.

The trade-off: your heat emitters

The catch is that a lower flow temperature means each radiator gives out less heat, so you need more emitting surface to warm the room. Underfloor heating is ideal — a huge surface area running happily at 30–40°C. Existing radiators sized for a 70–80°C gas boiler are often undersized for 35–45°C operation and may need upsizing. Matching emitters to a low flow temperature is the central design task of a heat pump installation, and the main reason a proper heat-loss survey matters.

Weather compensation

Modern heat pumps use weather compensation: they raise the flow temperature only as the weather gets colder, and keep it as low as possible the rest of the time. Leaving this enabled — rather than fixing a high flow temperature year-round — is one of the easiest ways to protect efficiency, and it is how the low seasonal figures are actually achieved in practice.

Hot water is the exception

Domestic hot water has to be stored hotter — typically around 50–60°C, partly to manage legionella — so the heat pump briefly runs at a high flow temperature to reheat the cylinder, at lower efficiency. That is normal and unavoidable; what matters is that space heating stays at a low flow temperature the rest of the time. High-temperature R290 units can reach the hot-water temperatures directly without an electric backup heater.

Maximum flow temperature & high-temperature heat pumps

Every model has a maximum flow temperature (shown here as the upper end of its flow range). High-temperature heat pumps — usually running R290 (propane) — reach 70–75°C, which lets them replace a boiler on existing radiators with little or no upgrading. That is genuinely useful for difficult retrofits, but running permanently hot sacrifices efficiency. Treat a high ceiling as headroom for the coldest days, not an everyday target.

Reading the data here

• The flow temperature range is listed for each model; the upper figure is the hottest it can deliver.
• The COP/SCOP conditions tell you the flow temperature each efficiency figure was measured at — an A7/W35 COP and an A7/W55 COP describe very different operation.
• Design for the lowest flow temperature your emitters allow, then pick units that are efficient at that temperature.

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Reviewed June 2026. This guide is general information, not professional or design advice; a heat-loss survey by a qualified installer is the basis for any real system design.