ABV: Automatic bypass valve

A valve that automatically opens when other parts of the system are shut. The most common use of this is when TRVs and lockshields are installed, but it can also be used for multi-zone systems. ABVs make sure there is always sufficient flow. 

AFV

Anti-freeze valve. Installed to protect the ASHP and pipework from freezing conditions. At 3ºC they open up and let the heating water out. They will only open if the heat pump is off during freezing conditions, as the heat pump keeps the water at 20ºC when the outside temperature is below 5ºC.

ASHP: Air source heat pump

A unit that transfers heat from outside into heating water using the refrigeration cycle. 

BUS: Boiler Upgrade Scheme

£7,500 grant given to homeowners towards the cost of a new heat pump installation. Run by OFGEM. 

BUH: Back-up heater

An independent electrical heater which supplements a heat pump for space heating and/or DHW. Usually piped in-line on the heat pump pipework. 

CH: Central Heating

The heating system in the property. Normally, just referring to the emitters and pipework, such as rads and UFH. CH is sometimes written on motorised valves to differentiate between CH and HW.

CCTs: Close Coupled Tees

System separation, allowing two parts of a system to circulate at different velocities. 

COP: Coefficient of Performance

Efficiency at that time. Instantaneous ratio of energy produced and energy consumed. 4KW produced, and 1KW consumed would be written like 4.0, which means the system is 400% efficient at that moment. 

CT: Current Transformer (CT clamp)

An instrument that measures current in a cable used (with known voltage) to estimate load power. Typically, a clip-on or a clamp. 

DHW or HW: Domestic Hot Water

The hot water system of a property. Includes the cylinder, hot-water pipework and heating pipework feeding the cylinder. 

EPC: Energy Performance Certificate

Rating of a property’s energy performance by an independent accredited assessor.

EV: Expansion vessel

A tank filled with air or nitrogen. As the water is heated it expands and enters the EV and compresses the air. EVs can be red, blue, white or grey. A red EV is used for heating only. The others can be used for hot water and heating. 

GSHP: Ground Source Heat Pump

A unit, usually placed indoors, which transfers thermal energy from external underground pipework into heating water using the refrigeration cycle.

HVAC: Heating, Ventilation and Air Conditioning

A collective term for trades and equipment. 

HW or DHW: Domestic hot water

The hot water system of a property. Includes the cylinder, hot-water pipework and heating pipework feeding the cylinder. 

LLH: Low-loss header

System separation. A vessel or pipework that allows two parts of a system to circulate at different velocities. 

LS: Lockshield valve

A standard radiator valve which can be set at a lower setting to restrict flow to a radiator, normally used for balancing. 

LWT: Leaving water temperature

Temperature of the water leaving the Outdoor Unit. Usually referred to as Flow.

MCS: Microgeneration Certification Scheme

An independent company overseeing all renewable technology in the UK. They also set the standards for installation.

MVHR: Mechanical ventilation with heat recovery

A system that transfers heat from air that is removed from the property into air that is entering the property.

ODT: Outdoor temperature

The temperature reading from the devices outdoor sensor

OFGEM: Office of Gas and Electricity Markets

Governmental regulatory body supervising the gas and electricity markets. Runs and operates the BUS. 

PHE or PHEX: Plate heat exchanger

A type of equipment which transfers heat from one medium to another. In a heat pump, it's from refrigerant to heating water. In a combi boiler, it’s from heating water to hot water. 

PRV: Pressure relief valve

Sometimes referred to as a blow off. A safety device which allows water to escape when the set pressure is reached.

PRV: Pressure reducing valve

A device that reduces water pressure as it enters a system. These are found on all unvented cylinders and are a type of safety device

PWM: Pulse width modulation (PWM pump)

Some types of heat pumps and boilers can control an external pump using PWM. It is the small wires going to a pump.

Rad

Radiator

R22 / R32 / R410a / R290

Refrigerants are named using a standard system that reflects what type of refrigerant they are and, for synthetic ones, how many carbon atoms are in the molecule. The number isn’t random, it just helps engineers identify the type of gas.

R32: Refrigerant 32

GWP 675. A widely used refrigerant in domestic heat pumps from around 2015 to the present. It has a 68% lower GWP than R410A and offers good efficiency. R32 is mildly flammable (A2L) but has minimal siting restrictions compared to R290. It is expected to be phased out over the coming years as regulations move toward ultra-low GWP refrigerants.

R410a: Refrigerant 410a

GWP 2088. Commonly used in heat pumps between 2003 and 2018. It has zero ozone depletion potential (ODP), unlike older refrigerants such as R22 (ODP 0.055). R410A is non-flammable and is still found in some existing domestic systems and commercial heat pumps. It is already being phased out due to its high GWP.

R290: Refrigerant 290

GWP 3. A natural refrigerant (propane) is increasingly used in modern air-to-water heat pumps. It delivers excellent efficiency with an extremely low environmental impact. R290 is highly flammable (A3), so additional siting and safety clearances are required. It is widely considered a long-term, future-proof refrigerant.

R22: Refrigerant 22

GWP 1810. ODP 0.055. An older refrigerant was historically used in heat pumps and air conditioning systems before modern environmental regulations. R22 damages the ozone layer and has been fully banned in the UK and EU for new equipment and servicing. Included here for historical comparison only.

RWT: Return water temperature

Temperature of the water as it re-enters the Outdoor Unit. Usually referred to as Return.

SCOP: Seasonal Coefficient of Performance

Efficiency like COP but averaged over a period of time. A performance estimate will use a SCOP over a period of 1 year to give the customer an idea of the efficiency to expect. SCOP can be calculated over any time period (hour, day, week, month year). You could compare a SCOP from a cold week to a mild week for example. 

T&PRV: Temperature and pressure relief valve

A safety device is typically installed on the side of an unvented DHW cylinder. Releases a large amount of water if the water temperature reaches the set point (commonly 95ºC) or set pressure (commonly 7 bar). Sometimes referred to as the blow-off

TRV: Thermostatic radiator valve

An energy-saving device fitted to radiators that reduces the flow to the radiator when the room is approaching the set temperature. 

UFH: Underfloor heating

Pipes are installed in the floor. Water is circulated, and the heat is emitted through the floor. 

WC: Weather compensation

A programme that runs on a heat pump or boiler and automatically adjusts the LWT based on the ODT. As the ODT decreases, the LWT increases. 

Air to air (A2A)

Known in the UK as air conditioning or air con. It is also known as an air-to-air heat pump. Refrigerant is circulated to indoor units, and cooling or heating is blown out using fans. 

Air to water (A2W)

Known in the UK as an air source heat pump (ASHP). It is also known as an air-to-water heat pump. Refrigerant is circulated through a plate heat exchanger, and heat is transferred into water, which is circulated through emitters or to a DHW cylinder.

Brine

When Glycol is mixed with water to lower its freezing set-point the liquid is then referred to as brine. Brine is always used in GSHP underground pipework and sometimes in ASHP heating pipework. 

Buffer

A tank installed on a heating system. A true buffer has three or more connections and provides system separation by allowing two parts of a system to circulate at different velocities. A buffer is the name given to the tank, but a buffer that only has 2 connections is a volumiser (see below). 

Cycling

When a heat source starts and stops. It can be measured as starts per hour. The lower the better for any heat source. An oversized heat source or one fitted with a buffer will have more starts per hour and may even “short cycle” (see below).

Design output

The estimated output of the heat pump at design conditions. Typically -3ºC. 

Emitter

Radiator, convector and/or underfloor heating system

Heat pump ASHP or GSHP

A unit that transfers heat from outside and uses the refrigeration cycle to convert the heat into a usable temperature.

Hysteresis

Normally referred to in K (Kelvin, which is the same as ºC, but used when measuring a difference (∆t) in temperature) The difference or in temperature or ∆t between on and off. A 10K hysteresis on a cylinder would mean if the set point is 50, the water would have to decrease by 10ºC for the heat source to start heating it back up to 50 again. 

Indoor Unit (IDU)

An ASHP component that includes the system components required for the outdoor unit to operate. These sometimes have a pump and a diverter valve. They can also include a PHEX if the system is a split type. Only a small number of ASHPs use an indoor unit. 

Micro-zone or micro-zoning

Most commonly associated with UFH. A micro-zone is when there are lots of “zones” (see below) on a heating system, which when only one or two are on at the same time, give the heat source a very small heating load. Micro-zoning causes “short-cycling” (see below), which reduces efficiency because the heat pump needs at least 10 minutes of run time to reach full efficiency.

Outdoor Unit (ODU)

It is the main ASHP unit. It is sometimes referred to as a condenser (because an air-con ODU is called a condenser). It has a fan, compressor and PHEX. Some units contain the plumbing components, such as the pump and expansion vessel. 

Short-cycling

Terrible for heat pump efficiency and should be avoided at the design stage. Short-cycling is when a heat pump only runs for a few minutes before it has to turn off again due to reaching temperature. A heat pump needs at least 10 minutes run time to reach full efficiency. 

Causes of short cycling include: “micro-zoning” (usually from multiple UFH zones in a property), buffers, by raising the temperature of the return water to the heat pump, and oversized heat pumps.

Setback

A heat pump has an “on” set temperature e.g 20ºC. The setback temperature is the “off” temperature or minimum temperature of the property. Usually we set this 2-3ºC lower than the “on” temperature. This prevents a huge demand on the heat pump when the next time programme comes on.

Thermal store

A large vessel whose only purpose is to store heat ready to use at a later time. A thermal store can be heated with cheap electricity to use at a later time e.g during peak rate, or with solar thermal to use for hot water.

Volumiser

A vessel whose only purpose is to provide more volume in a system. ASHPs need a minimum volume to complete the defrost cycle so a volumiser can provide that. They can also be used to improve the efficiency of an ASHP. A volumiser can be used to negate the effects of micro-zoning.

A volumiser may sometimes be referred to as a buffer because that's the component's name when purchased. What differentiates a buffer from a volumiser is that a volumiser only has two pipes. 

Water Law

Samsung terminology for weather compensation

Weather compensation: 

A programme that runs on a heat pump or boiler and automatically adjusts the LWT based on the ODT. As the ODT decreases, the LWT increases. 

Zone

A property may have more than one heating zone. So we refer to them as zones. The most common type of zones are upstairs and downstairs