The Ultimate Heat Pump Glossary: Key Terms Explained for UK Homeowners

Thinking about installing a heat pump but confused by the jargon? From COPs and SPF to monoblocks and defrost cycles, heat pump terminology can be overwhelming. This guide breaks down the most important heat pump terms so you can make informed decisions about your heating system.

Whether you're researching your first heat pump or trying to understand your existing system better, here's everything you need to know in plain English.

Coefficient of Performance (COP)

What is it? The Coefficient of Performance (COP) measures a heat pump’s instantaneous efficiency — the ratio of heat output to electricity input. For example, a COP of 4 means you get 4 kWh of heat for every 1 kWh of electricity used.

Why it matters: A higher COP means better energy efficiency and lower running costs. In real-world conditions, a COP of 3 to 5 is considered excellent.

Seasonal Coefficient of Performance (SCOP) or Seasonal Performance Factor (SPF)

What is it? SCOP and SPF are year-round averages of your heat pump’s performance. Since outside temperatures and heating demands fluctuate, these seasonal metrics give a more realistic view of how efficient your system is over time.

Why it matters: These figures reflect how well your system performs in actual conditions, not just under lab tests. Look for systems with an SPF above 3.5 for optimal performance.

Air Source Heat Pump

What is it? An air source heat pump extracts heat from the outside air. There are two common types:

• Air-to-water: Delivers heat via radiators or underfloor heating

• Air-to-air: Delivers warm air through fans or ducts (often called air conditioning)

Why it matters: Air source heat pumps are the most popular type in the UK and are suitable for a wide range of property types.

Ground Source Heat Pump

What is it? A ground source heat pump extracts stored solar energy from the ground. This is typically done via:

• Horizontal loops: Laid in trenches in your garden

• Vertical boreholes: Drilled deep into the ground, often used in space-limited areas

Why it matters: Ground source systems tend to have higher efficiency but are more expensive and complex to install.

Monobloc vs Split Heat Pumps

Monobloc: A single, self-contained unit that includes all major components and sits outside the property. Water pipes connect the unit to your indoor heating system.

Split system: Divides components between an outdoor unit (compressor, fan) and an indoor unit (hydrobox). Refrigerant pipes connect the two.

Why it matters: Monobloc systems dominate the UK market due to their simplicity and ease of installation.

Mini Split Systems

What is it? A smaller split heat pump system often used for air-to-air applications, like zoned heating or cooling. One external unit may serve multiple indoor units.

Why it matters: Mini splits are ideal for retrofitting specific rooms or where full central heating replacement isn't needed.

Bivalent Operation

What is it? Bivalent systems use two heat sources, usually a heat pump and a gas boiler. The system switches from one to the other based on outside temperature.

Example: The heat pump runs when it’s mild, but when temperatures drop below 5°C, the boiler takes over.

Why it matters: It allows for gradual transitions to low-carbon heating while keeping reliability high in cold weather.

Co-bivalent Operation

What is it? Similar to bivalent systems, but both heat sources run at the same time, with the boiler supporting the heat pump when extra heat is needed.

Why it matters: Used in complex or historic buildings where higher temperatures are occasionally required.

Defrost Cycle

What is it? Air source heat pumps extract heat from outdoor air. In cold, humid UK winters, moisture can freeze on the outdoor unit, reducing efficiency. A defrost cycle temporarily reverses the system or uses other heat sources to melt the ice.

Why it matters: Efficient defrost cycles are critical for maintaining system performance during winter. Some models handle this far better than others.

Summary Table: Key Heat Pump Terms

Term	What It Means	Why It Matters
COP	Instant efficiency ratio	Higher = better efficiency
SPF / SCOP	Seasonal average efficiency	Reflects real-world performance
Air Source Heat Pump	Uses outside air for heat	Most common in UK homes
Ground Source Heat Pump	Extracts heat from soil or rock	Higher efficiency, more complex install
Monobloc	All-in-one outdoor unit	Simple, compact setup
Split System	Indoor and outdoor components	More flexible installation
Mini Split	Small air-to-air system with zoned control	Ideal for partial upgrades
Bivalent	Swaps between heat pump and boiler	Hybrid approach for cold snaps
Co-bivalent	Heat pump and boiler work together	Extra support in high-demand conditions
Defrost Cycle	Melts frost buildup on outdoor unit	Essential for winter performance