SolaX Thermal R290 heat pumps: models and specifications

SolaX is best known in the UK for solar inverters and batteries, and the Thermal R290 is its air to water heat pump range. It comes in five outdoor sizes, from 8 kW to 16 kW, and it is named for its refrigerant: R290, which is propane. Every model in the range is listed as offering heating, cooling and domestic hot water from the one unit¹.

The headline efficiency figure is a SCOP of up to 5.44 at a 35°C flow temperature, published for the 8 kW outdoor unit². The maker puts the series as a whole at 4.55 to 5.15 SCOP on EN 14825 testing in an average climate³. Those two numbers sit in the same document set and are not the same measurement, which is worth knowing before comparing the range with another brand.

The range is aimed at households that already have, or intend to have, SolaX solar and storage. SolaX describes native integration with its own solar inverters and batteries³, and the outdoor units are listed with PV and ESS compatibility². That is the clearest thing the range offers a household chasing energy independence, and it is also the clearest limit: the integration is with one maker's ecosystem.

The SolaX Thermal R290 range: five sizes from 8 to 16 kW

The range is built around five outdoor capacities: 8 kW, 10 kW, 12 kW and 16 kW¹. Each is an air to water heat pump, meaning it heats water that then runs to radiators, underfloor heating or a cylinder rather than heating air directly. The 16 kW unit is listed with a heating capacity of 16 kW and a heating water temperature range of 25 to 80°C¹.

Two model prefixes appear in the listings. The STU3 units cover 8 kW, 10 kW, 12 kW and 16 kW¹, and the STU1 units cover 8 kW, 10 kW, 12 kW and 16 kW as well⁶. The 10 kW outdoor unit, for example, is the STU3-C10R290³. The material available does not explain what separates the STU1 and STU3 lines, so a household comparing them needs that answer from the installer or the maker.

Sizing is not a matter of picking the biggest number that fits. A heat pump is matched to a property's heat loss, and oversizing costs money and can make a system run less efficiently. The general guidance is that a maximum flow temperature of 55°C is typically recommended for efficiency¹⁰, and the range's own ceiling of 80°C is a capability, not a target. Sizing method is covered in heat pump sizing.

Efficiency: SCOP up to 5.44 at 35°C flow temperature

SCOP is the seasonal efficiency figure: heat delivered over a year divided by electricity used. The highest published figure in the range is 5.44 at a 35°C flow temperature, on the 8 kW outdoor unit². The 10 kW unit is listed at 5.30³, the 12 kW at 5.07⁵, and the 16 kW at 4.90¹. The pattern is consistent: the smaller the unit, the higher the figure, because a smaller heat pump spends more of its time at part load where it works most efficiently.

The maker's own series figure is lower and broader: 4.55 to 5.15 SCOP on EN 14825 testing in an average climate³. EN 14825 is the standard test basis for seasonal performance, so that range is the one to compare against another brand's published SCOP. The 5.44 figure is a single model at a single flow temperature.

Flow temperature is what drives the difference. A heat pump running at 35°C is designed for underfloor heating or generously sized radiators; at 55°C it is working harder for the same heat. The 16 kW unit is listed at 3.83 SCOP at a 55°C flow temperature¹, against 4.90 at 35°C¹. That gap is the whole argument for emitter sizing, and it is covered in radiators and emitters.

The 10 kW unit is listed at both 5.30 and 4.07 SCOP, the 8 kW at both 5.44 and 4.12, the 12 kW at both 5.07 and 3.89, and the 16 kW at both 4.90 and 3.83¹. The lower figures in each pair match the 55°C column, so the two sets are likely different flow temperatures rather than a dispute, but that is not stated. Where a figure matters to a decision, the model's own datasheet is the place to check it.

Specifications at a glance: refrigerant, flow temperature and noise

The range runs on R290, which is propane, and the maker gives a maximum water outlet temperature of 80°C³. The 16 kW unit is listed with a heating water range of 25 to 80°C, a cooling water range of 5 to 25°C, and a domestic hot water range of 25 to 70°C¹. The 8 kW unit carries the same heating and hot water ranges². The operating temperature range is stated as -25°C to 46°C¹, which covers UK winter conditions with margin.

On noise, SolaX states a sound pressure level of 35 to 38 dB(A) at one metre for the series³. That is a maker's figure measured at the unit. The UK legal noise limit for heat pumps is 42 decibels⁶, and that is assessed at the nearest neighbour's property, not at the unit, so the two numbers are not directly comparable. Typical air source heat pumps are around 45 dB(A)¹¹, and the general range when a heat pump is running is 40 to 60 decibels¹². The assessment method is set out in MCS 020, and the calculator used for it takes the heat pump make and model, the installation location, the address and a description of the assessment position¹³.

Heating, cooling and domestic hot water in one unit

Every model in the range is listed as offering heating, cooling and domestic hot water functionality¹. That is a single outdoor unit covering three jobs, which is the same proposition other makers offer: Daikin describes heating, cooling and hot water from one system¹⁴, and CIBSE describes a heat pump as capable of providing heating, cooling and hot water for homes, commercial buildings and industrial applications¹⁵.

Cooling is the part UK households most often overlook. The cooling water temperature range is 5 to 25°C¹, which is the range for chilled water running to fan coils or underfloor pipes. It is not air conditioning: the unit cools water, and the house needs emitters that can use cool water. That distinction is covered in heat pump cooling.

Domestic hot water runs at 25 to 70°C¹. A cylinder is required, and the cylinder is not part of the outdoor unit. The hot water temperature range is the water the heat pump can produce, not the temperature at the tap, which depends on the cylinder and its controls. Cylinder choice is covered in hot water cylinders for heat pumps.

The material available does not describe the Thermal R290 range working alongside a gas boiler or a solar water heater. What it does describe is PV and ESS compatibility² and native integration with SolaX solar inverters and batteries³. For a household weighing a hybrid arrangement instead, hybrid heat pumps covers how a heat pump and boiler share the load.

Three indoor configurations: Hydraulic Integrated, Hydraulic or Control Unit

The range offers three indoor configuration options: a Hydraulic Integrated Unit, a Hydraulic Unit, or a Control Unit². The same three options are listed across the outdoor models and across the indoor units, including the STN1-C01 and STN1-C02 indoor air to water heat pumps¹⁶.

The three options are a spectrum of how much of the system is built into the box. A Hydraulic Integrated Unit carries the hydraulic components inside it, so the installer has less to assemble on site. A Hydraulic Unit carries the hydraulics but is a separate indoor box. A Control Unit is the controls and the electronics, with the hydraulic parts supplied separately. Which one suits a property depends on space, on whether there is a cylinder, and on how the installer prefers to build the system.

This matters for a household in two ways. First, it changes what has to be found room for inside the house, which is often the tightest constraint in a retrofit. Second, it changes the parts count, and therefore the labour in a quote. The material available does not give dimensions or weights for the indoor units, so those come from the installer.

Integration with SolaX solar inverters and batteries

This is the part of the range that speaks most directly to energy independence. SolaX describes native integration with SolaX solar inverters and batteries³, and the outdoor units are listed with PV and ESS compatibility². In plain terms, the heat pump is designed to be part of the same system as the panels and the battery, rather than a separate appliance bolted on beside them.

The maker's claim on what that delivers is a potential reduction in running costs of 40 to 60% for systems that maximise self-consumption³. That is a maker's figure, not an independent measurement, and it depends on the size of the array, the battery and the household's pattern of use. It should be read as what the maker states rather than as a prediction for a particular house.

The communications side is what makes the integration work. The outdoor units are listed with Modbus TCP/RTU and SG Ready compatibility¹. SG Ready is the interface that lets a heat pump respond to a signal, for example to run harder when there is surplus generation. SolaX also describes a SolaX Cloud app with remote monitoring and over-the-air updates³, and the 10 kW unit is listed with smart app control and remote monitoring³.

The dependence is worth stating plainly. A heat pump that schedules around solar generation and a battery is still drawing from the grid when the sun is not shining and the battery is empty, and the app and remote monitoring depend on a broadband connection and the maker's cloud service. SolaX batteries are sold via UK installers¹⁸. The wider question of whether to pair storage with a heat pump is covered in solar and battery with a heat pump.

Scaling up: parallel connection of up to 8 units

The range is designed to be connected in parallel, which is how a larger property or a small commercial building is served without moving to a different product line. The material available does not give the maximum number of units that can be paralleled, so that figure has to come from the maker or the installer.

What the listings do show is that the range spans 8 kW to 16 kW in single units¹, and that the same three indoor configuration options apply across the range². A parallel arrangement is a cascade: several outdoor units feeding one system, with the controls deciding how many run at once. That is how a system keeps its part-load efficiency instead of running one large unit at a low output.

For a household, the practical questions are whether the electricity supply can carry the total, and whether there is space for more than one outdoor unit. The published power supply for the 10 kW and 16 kW outdoor units is 380 to 415 V, 3N, 50 Hz¹, which is three-phase. That is a significant point for a typical UK home, where single-phase 230 V is the norm, and it is covered in electricity supply for a heat pump. SolaX's own three-phase hybrid inverter is listed with three-phase unbalanced output support¹⁹, which is a separate product but relevant to a household planning a three-phase system.

Certification, testing and UK availability

The range is listed as Quiet Mark Certified and available for the UK market, with the caveat that products may only be available in specific regions¹. That caveat is worth taking at face value: a model being certified for the UK does not mean every installer stocks it or that it is held in a local warehouse.

On certification, the material available does not state an MCS certificate number for the Thermal R290 range. MCS 007 is the certification scheme for heat pumps²¹, and checking whether a heat pump has an MCS certificate is part of the standard pre-installation checklist²³. A heat pump project completion pack is expected to include details of the manufacturer's MCS certification²⁴. That is the document to ask for, because MCS certification is what makes a system eligible for the Boiler Upgrade Scheme and for some warranty and insurance routes.

Heat pumps that integrate with solar photovoltaic systems are eligible under the Boiler Upgrade Scheme²⁵, which matters here because PV integration is the range's main selling point. The scheme's rules and the grant amounts are covered in heat pump grants and heat pump cost.