Heat pump glossary

ABCDEFHkLMPRSVW

A

Air source heat pump (ASHP)

A heat pump that extracts heat from outdoor air and transfers it to your heating and hot water. The most common type in UK homes.

Air to water heat pump

An air source heat pump that heats water for radiators, underfloor heating and a hot water cylinder. The type covered by the £7,500 grant.

Air to air heat pump

A heat pump that blows warm (or cool) air into rooms through indoor units. It doesn't heat hot water.

B

Boiler Upgrade Scheme (BUS)

The government grant in England and Wales that gives £7,500 towards an air source or ground source heat pump. Claimed by your installer and administered by Ofgem.

Buffer tank

A small water tank that adds volume to the heating system so the heat pump can run longer, steadier cycles. Not every system needs one.

C

COP (coefficient of performance)

How many units of heat a heat pump delivers per unit of electricity at a specific moment. A COP of 4 means 4 kWh of heat from 1 kWh of electricity.

D

Defrost cycle

A short, automatic reversal that melts frost on the outdoor coil in cold, damp weather. You may see steam; it's normal.

E

EPC (Energy Performance Certificate)

A rating of your home's energy efficiency. You need a valid EPC, issued within the last 10 years, to claim the Boiler Upgrade Scheme.

F

Flow temperature

The temperature of water leaving the heat pump to your radiators. Lower flow temperatures (35 to 50°C) mean higher efficiency.

F-gas

Fluorinated refrigerant gases such as R32. Engineers working on split systems with these refrigerants need F-gas certification.

H

Heat loss calculation

A room-by-room survey working out how much heat your home loses on a cold day. It sets the size of the heat pump and any radiator upgrades.

Hot water cylinder

An insulated tank that stores hot water heated by the heat pump. Usually 150 to 300 litres for a family home.

Hybrid heat pump

A heat pump paired with a gas or oil boiler. Hybrids are not eligible for the Boiler Upgrade Scheme.

k

kW (kilowatt)

The output size of a heat pump. Most homes need 5 to 12 kW.

L

Legionella cycle

A periodic high-temperature heat of the hot water cylinder, usually weekly, to prevent bacteria growth.

M

MCS (Microgeneration Certification Scheme)

The UK quality scheme for heat pumps and installers. An MCS-certified installation is required to claim the £7,500 grant.

Monobloc

A heat pump with all refrigerant sealed in the outdoor unit, sending heated water into the house. The most common UK design.

P

Permitted development

Planning rules that allow most air source heat pumps to be installed without applying for planning permission, subject to size, siting and noise limits.

R

R290

Propane, a natural refrigerant with a very low global warming potential. R290 heat pumps can reach higher flow temperatures efficiently.

R32

A common synthetic refrigerant with a lower global warming potential than older refrigerants.

RECC / HIES

Consumer codes for renewable energy installers, offering deposit protection and a route to resolve complaints.

S

SCOP (seasonal coefficient of performance)

A heat pump's average efficiency across a whole heating season. A SCOP of 3.5 means 3.5 units of heat per unit of electricity over the year.

Split system

A heat pump with a refrigerant circuit running between an outdoor unit and an indoor unit (hydrobox).

V

Volumiser

A small vessel that increases system water volume, similar in purpose to a buffer tank.

W

Weather compensation

A control that automatically lowers the flow temperature on milder days and raises it when it's colder, improving efficiency and comfort.