An introduction to commercial heat pumps

In the first of a series on evolving heat pump technologies, Greentherm Managing Director ENDA RUXTON provides a technical overview of air-to-water and water-to-water configurations, exploring capacities, application demands, and low-GWP refrigerant safety regulations.

Heat pumps are essential to phasing out fossil fuels in commercial buildings, hospitals, and district networks. By transferring thermal energy rather than generating it, they deliver far more energy than they consume. This efficiency is measured by the Coefficient of Performance (COP), which typically ranges from 3.0 to 6.0 in commercial systems, meaning three to six units of heat are delivered for every unit of electricity consumed.

Unlike residential units, commercial heat pumps operate on a massive scale, from 20 kW to the megawatt range for district heating or industrial applications requiring complex engineering decisions around compressors, architecture, and strict refrigerant safety compliance. Over the coming issues, I will provide a technical breakdown of air-to-water (AtW) and water-to-water (WtW) configurations, including their refrigerants, temperature capabilities, and hydraulic applications.

Air-to-water heat pumps: Typical output sizes and applications

Commercial AtW heat pumps are manufactured across a wide range of capacities. Small commercial units suitable for single buildings or apartment blocks typically fall in the 20–100 kW range and are commonly available as monobloc units (all components in a single outdoor cabinet) or as split systems (an outdoor evaporator-compressor unit connected by refrigerant pipework to an indoor hydraulic module). Mid-range systems from 100 kW to 1 MW are typically modular, with multiple compressor circuits on a common frame, allowing staged operation to match load variation and improving part-load efficiency. Large-scale air-to-water systems, used in district heating networks and industrial pre-heating applications, can aggregate multiple units to deliver an output exceeding 5 MW.

In terms of applications, AtW heat pumps are well-suited to space heating, domestic hot water preparation, swimming pool heating, and low-to-medium temperature process heating. Modern high-temperature variants, using refrigerants such as propane (R290) or R-744 (CO2), can produce flow temperatures of 80–90°C, extending their applicability to legacy heating systems originally designed for higher temperature operation.

Water-to-water heat pumps: Typical output sizes and applications

WtW heat pumps are manufactured from as small as 15 kW (for commercial ground-source applications) up to several megawatts for large institutional or district heating schemes. They are the preferred technology for hospitals, universities, and large hotels where simultaneous heating and cooling is required, since many WtW systems can be configured in heat recovery mode, rejecting heat from a building’s cooling circuit directly into the heating circuit, achieving extremely high combined COPs.

WtW systems are also used extensively in industrial process applications where stable temperature output and high reliability are paramount. Industrial variants may be designed to produce chilled water as the primary output (with heat recovery), or heating water for process needs, with flow temperatures selectable in the range of 35°C to 90°C, depending on the refrigerant and compressor choice. Although some specific units operate well off 100°C. Primary side input circuits would frequently be in the range of 10°C to 45°C.

Refrigerant Safety Classification and GWP

Refrigerants are classified under the ASHRAE Standard 34 system, which combines a toxicity class (A for lower toxicity, B for higher toxicity) with a flammability class (1 for no flame propagation, 2L for lower flammability, 2 for flammable, 3 for highly flammable). This produces designations such as A1, A2L, A3, and B1, each of which carries different handling, installation, and regulatory implications. The dominant refrigerants used in commercial heat pumps span the full range of safety classifications. The industry is currently mid-transition away from high-GWP HFC refrigerants (such as R-410A and R-134a) in response to the EU F-Gas Regulation and equivalent national legislation, driving adoption of lower-GWP alternatives, many of which are A2L or A3 class.

Table 01 provides an overview of some refrigerants in use across various applications, including their class, GWP, and typical applications.

A1 Class Refrigerants (Non-Flammable, Low Toxicity)

A1 refrigerants present no flammability risk, making them the simplest from an installation safety perspective. R-410A has been the workhorse of commercial heat pumps for two decades, offering good thermodynamic performance and manageable operating pressures, though its high GWP of 2,088 has placed it firmly in the crosshairs of phasedown legislation. R-134a is common in large water-cooled chillers and water-towater systems, though its GWP of 1,430 likewise faces regulatory pressure, where R-744 (carbon dioxide) represent the zero-GWP extremes of A1 classification. CO2 systems operate at supercritical pressures over 70 bar and require specialised high-pressure components, but can deliver flow temperatures approaching 90°C, making them particularly attractive for domestic hot water and district heating.

A2L Class Refrigerants (Mildly Flammable)

A2L refrigerants have a maximum burning velocity of 10 cm/s or less. They will propagate a flame under specific conditions, but are far less hazardous than conventional flammable gases. R-32 (difluoromethane) has become one of the most widely deployed commercial refrigerants due to its GWP of 675 (approximately one-third of R-410A), excellent energy efficiency, and relatively benign flammability characteristics.

R-454B and R-452B are HFO/HFC blends designed as near-drop-in replacements for R-410A with GWPs in the range of 466–676. R-1234yf and R-1234ze(E) are hydrofluoroolefins (HFOs) with GWPs of less than 10, increasingly used in new large commercial and industrial heat pump designs.

Installation of A2L systems requires attention to ventilation, leak detection, and equipment placement, though the requirements are less onerous than for A3 systems. Most commercial equipment manufacturers have moved or are moving their standard product ranges to A2L refrigerants.

A3 Class Refrigerants (Highly Flammable)

A3 refrigerants, including R-290 (propane) and R-600a (isobutane), are highly flammable hydrocarbons. Their thermodynamic properties are exceptional, with R-290, in particular, having a low boiling point, excellent heat transfer characteristics, zero ODP, and a GWP of just 3, making it highly attractive from both an efficiency and environmental standpoint.

Commercial heat pump products using R-290 are now offered by several major manufacturers, and the technology is considered mature for units up to 50 kW.

Larger R-290 systems are entering the market in cascade or distributed configurations to manage charge sizes. Flow temperatures with R-290 can reach up to 80°C in purpose-designed high-temperature variants.

In the European Union and the UK, the use of flammable refrigerants in commercial heat pumps is governed by a combination of the following regulations

– Machinery Directive (2006/42/ EC), the ATEX Directive 2014/34/EU (equipment for use in potentially explosive atmospheres)

– EN 378 (Refrigerating Systems and Heat Pumps Safety and Environmental Requirements)

– EN 60079 series standards for explosive atmospheres.

For A3 refrigerants, the following requirements are particularly significant.

Maximum refrigerant charge limits

EN 378 Part 1 specifies maximum permissible refrigerant charge limits based on occupancy category and room volume. For A3 class refrigerants, the practical charge limit in occupied spaces is extremely restrictive; the Lower Flammability Limit (LFL) of propane is approximately 88 g/mÑ, and EN 378 limits the charge such that an accidental full release would not produce a concentration exceeding 25% of the LFL in the smallest room through which refrigerant could migrate. In practice, this restricts A3 systems in indoor plant rooms to charge sizes typically below 1.5 kg without additional engineered controls.

Commercial R-290 heat pumps are often designed with factory-sealed refrigerant circuits and minimised charge quantities. Many manufacturers have engineered units in the 20–50 kW range with charges of 1.2–2.5 kg using improved heat exchanger designs and optimised circuit geometry. For larger systems, cascade arrangements can be used to keep each individual A3 circuit within permissible charge limits while delivering higher total output.

Zone Classification and ATEX Equipment Requirements

Under ATEX Directive 2014/34/EU, any area where a flammable atmosphere may exist must be classified into explosion zones. For A3 refrigerant plant rooms or enclosures:

– Zone 1: Areas where a flammable atmosphere is likely to occur in normal operation typically within 1 metre of refrigerant connection points, relief valve discharge points, and flexible hose connections.

– Zone 2: Areas where a flammable atmosphere is not likely in normal operation, but may occur in abnormal conditions, the general space within a plant room housing an A3 system where a significant leak could accumulate.

– Zone 0 classification (continuous presence of flammable atmosphere) is generally avoided by design; no operational component should create a permanent source of refrigerant release.

All electrical equipment installed within classified zones must comply with ATEX equipment categories appropriate to the zone: Category 2G (suitable for Zone 1) or Category 3G (suitable for Zone 2). This includes motors, fans, control panels, junction boxes, sensors, and luminaires.

Ventilation requirements

Adequate ventilation of plant rooms containing A3 refrigerant equipment is mandatory. EN 378 and associated guidance require that mechanical ventilation systems achieve sufficient air changes to dilute any accidental refrigerant release below the Lower Flammable Limit (LFL). For propane, the typical design requirement is 8–12 air changes per hour under normal operation, with the capability to increase to emergency ventilation rates when triggered by refrigerant detection.

Ventilation extract points must be positioned at low level, as both propane and isobutane are heavier than air and will accumulate at floor level. Inlet air should be introduced at a high level to create a downward displacement flow pattern. Emergency ventilation systems must be classified for operation in ATEX Zone 2 at a minimum, and ventilation fan motors must be ATEX-rated or positioned outside the hazardous area with non-sparking impellers and antistatic ducting.

Refrigerant Detection and Alarm Systems

Continuous refrigerant detection is required in plant rooms housing A3 systems. Fixed gas detection systems must be provided with sensors positioned at a low level, calibrated to the target refrigerant. Detectors must be certified to ATEX Category 1G or 2G, depending on their location, and must be subject to a documented calibration and maintenance programme. The detection system should be independent of the heat pump control system to ensure operation even in the event of a heat pump fault.

Compressor technologies – the heart of the heat pump

The compressor is the heart of any heat pump system, and the choice of compressor technology has profound implications for efficiency, noise, reliability, capacity range, and maintenance requirements. Commercial heat pumps utilise four principal compressor types: scroll, screw, reciprocating, and centrifugal. Each has distinct characteristics that make it better suited to particular applications and capacity ranges.

Scroll Compressors

Scroll compressors are the dominant technology in commercial heat pumps from 5 kW to approximately 150–200 kW per compressor stage. They operate through the orbiting of an involute spiral against a fixed scroll, progressively reducing the volume of refrigerant gas pockets from the periphery to the centre, compressing the gas to condenser pressure. Scroll compressors are renowned for their smooth operation, with very few moving parts and no reciprocating forces, resulting in low vibration and quiet operation. Noise levels from modern inverter-driven scroll compressors in an ATW heat pump are typically in the range of 55–65 dB(A) at 1 metre, making them suitable for urban installations.

Variable-speed (inverter-driven) scroll compressors are now standard in quality commercial products, allowing modulation of heating capacity from approximately 25% to 100% of rated output. This part-load capability is critical for seasonal efficiency (SCOP), since heating loads vary enormously with outdoor temperature. The COP at part load with inverter scroll compressors is typically higher than at full load, as the pressure ratio and motor losses reduce at lower speeds. Scroll compressors are generally considered highly reliable, with service intervals of 20,000–40,000 hours and Mean Time Before Failure (MTBF) records well established across millions of field installations. Disadvantages include a relatively limited pressure ratio per stage, which restricts their use in high-temperature lift applications without compound or cascade arrangements.

Screw Compressors

Twin-screw compressors are the preferred technology for commercial and industrial heat pumps in the 200 kW to 2 MW range per circuit. They consist of two helical rotors one male and one female that mesh together within a high tolerance housing, trapping and compressing refrigerant as the rotors turn. Screw compressors are capable of high-pressure ratios and can handle a wide range of operating conditions including high condensing temperatures, making them well suited to high-temperature heat pumps.

Noise and vibration from screw compressors are higher than from scroll units, with typical sound power levels of 75–85 dB(A), necessitating acoustic enclosures or isolated plant rooms in noise-sensitive locations. Capacity modulation is achieved through a slide valve mechanism allowing turndown to around 25–30% of full load. The efficiency of screw compressors at part load via slide valve unloading is generally lower than inverter-driven scroll technology, though variable-speed screw compressors are increasingly available. Maintenance requirements are more intensive than for scrolls, due to complexity and interval checks.

Reciprocating (Piston) Compressors

Reciprocating compressors, though now largely supplanted by scroll technology in new commercial heat pump designs, remain in service in a significant installed base of older equipment and continue to be specified in certain industrial applications where robustness and field-serviceability are prioritised over noise or efficiency. They operate through the reciprocating motion of pistons within cylinders, with suction and discharge valves controlling gas flow. Semi-hermetic reciprocating compressors are particularly valued in industrial settings because they can be fully overhauled on site valves, pistons, rings, and bearings are all individually replaceable, offering a level of repairability that hermetic scroll and screw units cannot match.

The drawbacks of reciprocating compressors in heat pump applications include higher vibration and noise (typically 70–80 dB(A)), lower volumetric efficiency due to clearance volume, and capacity modulation limited to coarse step unloading via cylinder bypassing. Part-load COP is generally inferior to inverter scroll or variable-speed screw alternatives. They remain relevant in refrigeration-derived industrial heat pumps and in applications requiring multi-stage compression with liquid injection intercooling for high-temperature lifts.

Centrifugal Compressors

Centrifugal (turbo) compressors use high-speed impellers to impart velocity to refrigerant vapour, converting kinetic energy to pressure through a diffuser. They are uniquely suited to very large capacity applications typically above 500 kW per stage for refrigerants such as R-134a or R-1234ze. Their primary advantages are exceptionally smooth, vibration-free operation, high reliability with minimal moving parts (one or two rotating assemblies), and excellent efficiency at design conditions.

The Achilles’ heel of centrifugal compressors in heat pump applications is surge an unstable operating condition that occurs when the pressure ratio exceeds the stable operating range of the impeller at a given flow rate. Surge can cause severe mechanical damage and limits the turndown ratio, typically to around 40–50% without capacity control devices such as inlet guide vanes or variable diffuser geometry.

Comparative Summary