Let’s be honest — when you first start looking into heat pumps, the terminology can be confusing.

From COPs and compressors to glycol and ground loops, it’s easy to get a bit lost.

But don’t worry — we’ve got your back.

We’ve pulled together a friendly, fuss-free glossary of common heat pump terms to help you make sense of it all.

Let’s dive in.

A

Ambient Air

The outdoor air that air source heat pumps use to collect heat. Even in freezing conditions, this air contains enough thermal energy to heat your home efficiently without relying on fossil fuels.

Air Source Heat Pump

A heating system that extracts heat from the outdoor air and transfers it indoors to provide space heating and hot water. A practical alternative to gas or oil boilers for well-insulated homes. See this Victorian terrace retrofit example.

B

Brine Loop

A buried loop of pipes filled with water and antifreeze (glycol) that circulates through the ground, collecting heat in ground source systems.

Buffer Cylinder

A short-term heat storage tank that prevents the heat pump from switching on and off too frequently. Used in this barn conversion.

Bivalent System

Where a heat pump works alongside another heat source (usually a boiler). Helpful in colder areas or homes not fully heat-pump ready.

C

Coefficient of Performance (COP)

A way to measure how efficient your heat pump is. A COP of 4 means it generates 4 units of heat for every 1 unit of electricity.

Commissioning

The final setup step where the installer tests and optimises your system.

Cascade

A configuration where multiple heat pumps work together to meet higher demands — often in larger buildings.

Closed Loop

Sealed pipework buried in the ground in a continuous loop for ground source systems.

Condenser

The part of the heat pump where refrigerant condenses into a liquid and releases heat into your system.

D

Defrost Cycle

An automatic process that melts ice from the outdoor coil during cold weather.

Demand

The amount of heat your home needs at any given time. Your heat pump adjusts to match.

E

Expansion Valve

Regulates the amount of refrigerant going into the evaporator to enable heat absorption.

Efficiency

The ability of your heat pump to convert electricity into heat. Higher efficiency = lower bills.

Electrical Supply

Heat pumps need electricity, and the type (single-phase or three-phase) affects your setup.

F

Flow Temperature

The temperature of water flowing through your heating system. Lower = better efficiency.

Fabric First

Improving insulation and airtightness before installing a heat pump makes your system work much more efficiently.

G

Ground Loop

Pipework buried underground that collects heat for a ground source heat pump.

Glycol

Antifreeze used in ground loop systems to stop the fluid freezing in cold weather.

Ground Source Heat Pump

A system that collects heat from the ground via underground pipework. Efficient and consistent year-round. Read about this real installation at Thornbridge Outdoors.

H

Heat Pump

Technology that moves heat from one place to another using electricity — a bit like a fridge, but in reverse. Click here for a more detailed view on how heat pumps work.

Heat Exchanger

Transfers heat from the refrigerant to your home’s heating system (radiators, underfloor, or hot water).

Hybrid System

A hybrid heat pump system combines a heat pump with a boiler for extra flexibility. Check out this hybrid installation at Kilduff House.

Heat Loss Calculation

An heat loss calculation is an assessment to work out how much heat your home loses — essential for choosing the right heat pump system size for your property.

I

Inverter Compressor

A type of compressor that adjusts its speed based on the heating demand. Unlike fixed-speed compressors, it ramps up or down as needed, improving efficiency, reducing wear and tear, and keeping your home more comfortable.

Insulation

Insulation is key to reducing heat loss. Whether it’s in the loft, walls, floors, or windows, good insulation helps your home hold on to warmth — which means your heat pump doesn’t have to work as hard, lowering your energy use and bills.

J

Joules

The basic unit of energy. Heat pumps don’t create energy — they move joules of heat from the air, ground or water into your home. Clever, right?

K

Kilowatts (kW)

A unit of power that measures both the size of your heat pump (its output) and how much electricity it consumes. It’s important for determining what size heat pump your home needs.

K1, K2, K3

These refer to radiator types. K1 = single panel; K2 = double panel with one convector; K3 = triple panel with two convectors. The more panels and convectors, the more heat a radiator can emit — crucial when working with the lower flow temperatures of heat pumps.

L

Low-Temperature Heating

Heating systems that use warm (but not hot) water — ideal for use with heat pumps. These systems are more efficient and work best with underfloor heating or oversized radiators.

Load

The total heating and hot water demand of your home. It’s a key factor in designing your heat pump system so it’s neither underpowered nor oversized.

Legionella Control

Legionella bacteria can grow in stored hot water. Many heat pump systems include regular hot water “pasteurisation” cycles to heat the water to a safe temperature and kill off any bacteria.

M

MCS Certification

A government-backed quality standard for heat pump systems and installers. It’s required if you want to access financial support like the Boiler Upgrade Scheme.

Monitoring

Some systems come with tools to track your heat pump’s performance over time. This helps ensure it’s working efficiently and flags up any potential issues early. See how monitoring helped in this real home.

N

Net Zero

The UK aims to achieve net zero carbon emissions by 2050. Switching to a heat pump is one way you can support this national target and reduce your own carbon footprint.

Night Setback

A smart control feature that lowers your heating slightly at night to save energy, without compromising on comfort when you wake up.

O

Output Capacity

How much heat your pump can generate, measured in kW. It needs to match your home’s heat demand to keep you warm in all conditions.

Open Loop

A type of ground source heat pump that uses groundwater from a well or borehole to extract heat, then returns it after use. Efficient, but only suitable for properties with good access to groundwater.

P

Passive Heating

Using the natural warmth from sunlight or the steady underground temperature to help heat your home. Heat pumps work well with passive design strategies.

Planning Permission

In most cases, domestic heat pump installations don’t require planning permission. But for larger systems, listed buildings, or certain areas, it’s always worth checking with your local authority.

Q

Quiet Operation

Modern heat pumps run quietly — often quieter than your fridge! This makes them fine for residential neighbourhoods.

R

Refrigerant

The liquid/gas that flows through your heat pump, carrying heat from one place to another. It changes state to absorb and release energy efficiently.

R290

A type of natural refrigerant (propane) that has a low global warming potential. It’s increasingly used in newer heat pump models.

Retrofit

Installing a heat pump into an existing property (rather than a new build). It often requires adjustments to insulation, radiators, or pipework — but the results can be fantastic.

S

SCOP (Seasonal Coefficient of Performance)

A measure of your heat pump’s efficiency across an entire heating season. A higher SCOP means lower energy bills and carbon emissions. Learn more about SCOP here.

Secondary Circuit

This is the system that carries heat from your heat pump to your radiators, underfloor heating, or hot water cylinder.

Short Cycling

When a heat pump turns on and off too often. It reduces efficiency and can wear out components faster. Proper design and buffer tanks help prevent this.

Smart Controls

Thermostats and control systems that learn your habits, optimise your heating patterns, and let you manage your heat pump remotely.

T

Thermal Store

A hot water cylinder that stores energy from your heat pump for later use. Useful for balancing demand and ensuring hot water availability.

Thermal Mass

The ability of materials like stone, concrete, or brick to absorb, store and slowly release heat. Great for keeping indoor temperatures stable.

U

Underfloor heating

Underfloor heating systems warms the floor’s surface, providing a more even and wider distributed heat source than radiators alone do.

U-Values

A number that tells you how well your home’s components (like walls or windows) retain heat. Lower U-values mean better insulation.

V

Vertical Borehole

Vertical borehole drilling proves deep, narrow holes in the ground for install a ground source heat pump in places with limited horizontal space.

Variable Speed Compressor

A compressor that adjusts its output based on real-time heating needs. It’s more efficient and extends the lifespan of the system.

W

Weather Compensation curve

A weather compensation curve (also known as a heat curve) is a control system that adjusts the heat pump’s output in response to the outdoor temperature. It keeps your home comfortable while saving energy.

Water Source Heat Pump

A type of system that extracts heat from a lake, river, or borehole water source. Highly efficient, but site-dependent.

X

eXergy

(Yes, we know it’s a stretch — but we’re sticking with it!) Exergy is the portion of energy that can do useful work. Heat pumps are great at converting low-grade heat into effective indoor heating.

Y

Yearly Savings

The reduction in energy costs you can expect from using a heat pump compared to conventional systems. Explore what you can expect.

Z

Zero Carbon

Heating your home without creating emissions. When powered by renewable electricity, heat pumps are a vital step towards a net-zero future.

Zoning

Dividing your home into separate heating areas so each can be controlled independently. It helps improve comfort and reduce energy use.

We hope this glossary helped make the heat pump world a little less daunting and a lot more doable.

At IMS Heat Pumps, we’re all about keeping things simple, smart, and supportive — so whether you’re deep into your home upgrade or just exploring your options, we’re here to guide you every step of the way.

And remember, there’s no such thing as a silly question (not even “What on earth is eXergy?” — we covered that too!).

Got a term we missed? Or a project in mind? Give us a shout — we’d love to chat.