Choose by tub volume, outdoor temperature and desired recovery time. Do not compare heat pumps on headline COP alone because COP changes with air and water temperature.
AQUARK Mr.Silence: broad 7–21 kW range
Steam & Oak currently lists AQUARK Mr.Silence models from 7 to 21 kW. The range uses an inverter compressor and twisted-titanium heat exchanger, with published operation down to -10°C ambient.
At warm test conditions, the listed COP range is much higher than under cooler conditions, which illustrates why context matters.
iNHEAT SpaLine 7 kW: heating and cooling
The iNHEAT SpaLine 7 kW is reversible and lists a water temperature range of 10–40°C with operation down to -25°C ambient. It includes Wi-Fi and several operating modes.
Its optimum published water flow is 2 m³/h.
Gecko 5–7.5 kW: integrated spa-system option
Steam & Oak also sells Gecko 5 and 7.5 kW inverter units designed to integrate with Gecko control systems. This can simplify control for compatible spas.
How to compare COP honestly
COP is output heat divided by electrical input, but it changes with outdoor air temperature and target water temperature. A COP quoted at 27°C air is not representative of a cold UK winter.
Compare performance at conditions close to how the spa will actually be used.