What is a Heat Pump?

What is a Heat Pump? 
(Also known as Reverse Cycle (heating or cooling) Heat Pump, and/or Vapor Compression Refrigeration System (VCRS).)

A Heat Pump is a sealed Direct Expansion (DX) refrigerant Cycle that comprises of a Compressor, an Evaporator a Condenser and an Expansion Device. It uses refrigerants* as a media to transfer energy (heat) from outside to inside (heating) or from inside to outside (cooling). a DX cycle can be:

The DX cycle (also known as Vapor Compression Refrigeration System - VCRS), has been around for over 100 years and is used in all types of refrigeration systems (industrial, commercial or domestic including refrigerators & freezers,) all types of air conditioning systems (Splits, VRF, Roof Tops, all types of Chillers, cars / trains / ships, etc,)

Heat Pumps are a major contributor to Net Zero target

 DX Cycle - Heating                     
                      Vapor Compression Refrigeration System - (VCRS)                        Air to Air Split- Heating 



DX Cycle - Cooling                            
                                                  Vapor Compression Refrigeration System - (VCRS)                                Air to Air Split- Cooling                           


*Why refrigerant?

Refrigerant is a chemical gas (can be single component or multiple components) that changes phase from gas to liquid and visa versa. It is extremely energy efficient and can carry more than 50 times energy per weight than air and 10 times more than water.
It boils at as low as -50°C (water +100°C) and condenses at as high as +140°C. 

These characteristic makes it suitable for most habitable areas of the world, smaller inter-connecting refrigerant pipes, compact and very cost effective.

Refrigerants are a controlled substances due to pressures, tempertures, flammability & toxicity and must only be handled by qualified & certified refrigeration engineers. There 3 main standards that governs the use of refrigerants in the manufacture, design, contruction and installation of systems.  ISO817, ASHRAE 34 International Standards and BS EN 378 British & European Standards)

ISO817 and ASHRAE 34 International Standards:
Defines classes relates to flammability:  

And classes relates to Acute Toxicity  (short exposure due to a leak):

 see also : EPA Refrigerant Safety definitions

BS EN 378
BS EN 378 relates to safety and environmental requirements in the design, manufacturer, construction, installation, operation, maintenance, repair and dsiposal of refrigerating systems and appliances containing refrigerants A1, A2/A2L or A3 class..  

 see also : EPA Refrigerant Safety definitions

DX System Energy Efficency when using refrigerants 

Efficency = Energy transfered / Energy used by the process

The efficiency of cooling (EER- Energy Efficiency Ratio) cycle or heating cycle (COP - Cofficient Of Performance) can range anything between 250% (2.5/1) to 500% (5/1) (that is for every energy unit, [say1 kW] electricity consumed, up to 5 kW can be absorbed from inside (unwanted heat) to be rejected outside, or absorbed from outside (air, water or ground) and transferred to inside to heat occupied space or domestic hot water). These efficencies (EER & COP) are based on set at fixed ambient conditiones.

There are seasonal efficiencies (SEER & SCOP) which are more accuratly relats variable ambient conditions and hours of operation relavant to ambient conditions. For example in the UK (considered as mild weather), the SEER can reach 700% (7/1) while SCOP can be 800% (8/1.)

There are mainly Air to Air (most common and cost effective),  Air to Water, Water to Water, Ground to Air and Ground to Water DX systems. Systems can be a) compact (all components in one box) Mono or b) Single Splits (one indoor and one outdoor) or c) Multi Splits/VRF (multiple indoor units connected to one outdoor unit).

The Indoor units can be a Wall, Floor, Ceiling or Ducted fan coils or a refrigerant to water heat exchanger to provide domestic hot water or medium temperature heating water for under floor heating, radiators of water fan coils. The interconnecting refrigeration pipe work would require specialist approved refrigeration engineers and materials. 

There are no extra safety control systems (provided the refrigerant is A1 class non-flammable), no chimneys, extra builder’s work or electrical supplies. Location of outdoor units can be as far as 100 meters away from indoor units in a Air to Air systems.

The effect of an accidental refrigerant leak in a DX system

The refrigerant in the DX cycle (closed circuit) is a controlled substance in terms of pressure, flammability, toxicity and environmental effect. It circulates around to perform the cycle and must not leak (as system will shut down by built-in controls).

The effect of an accidental refrigerant LEAK has three important consequences that must be taken into account:

  1. Safety of installers, service engineers, operators and USERS (operational & accidental release)
  2. GWP (Global Warming Potentional) (operational & accidental release)
  3. Environmental effect ODP (Ozone Depletion Potential) (accidental release)

International standards (ISO817 & ASHRAE34) classification and BS EN 378 (installation of equipment containing refrigerants) regarding the use of refrigerants that influances local and International safety laws such as H&S 2016 no.1107) and ATEX2014/34/EU 
i) Flammability being A1 Non-Flammable, A2/A2L Flammable or A3 Extremly Flammable & Explosive.
ii) Toxicity (mainly displacement of Oxygen in an occupied space) being B1 Low, B2 Medium and B3 High.
iii) Global Warming Potential (GWP) a.Total greenhouse gas emissions over a specific product life cycle, including energy consumption, b. when accidentaly leaks into the atmosphere.
v) Ozone Depletion Potential (ODP) in the event of release into the atmosphere. (applies to CFC, HCFC not HFC or HFO)

Extra mandatory measures : 
If A2/A2L (flammable) or A3 (highly flammable) class refrigerants are used in DX cycles within air conditioning / Heat Pumps or refrigeration systems including domestic refrigerators & freezers:

  1. Special risk assesment per installation must be undertaken (BS EN378)
  2. Must have 3rd party certification (CE or UKCA)
  3. Strict application use where there are flames or sparks present
  4. Clear Warning labels to Occupiers, Installers, Building owners, Transport authorities, Maintenance engineers, Fire and Building control authorities
  5. Special handling certification (engineers, storage and transport)
  6. Insurance companies must be made aware and can entail extra premiums.


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